Staff Publications

Staff Publications

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    'Staff publications' is the digital repository of Wageningen University & Research

    'Staff publications' contains references to publications authored by Wageningen University staff from 1976 onward.

    Publications authored by the staff of the Research Institutes are available from 1995 onwards.

    Full text documents are added when available. The database is updated daily and currently holds about 240,000 items, of which 72,000 in open access.

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    Microfluidic methods to study emulsion formation
    Muijlwijk, Kelly - \ 2017
    Wageningen University. Promotor(en): C.G.P.H. Schroën, co-promotor(en): C.C. Berton-Carabin. - Wageningen : Wageningen University - ISBN 9789463430715 - 169
    emulsions - microfluidics - food emulsions - droplets - adsorption - colloidal properties - emulsies - microfluidics - voedselemulsies - druppels - adsorptie - colloïdale eigenschappen

    Emulsions are dispersions of one liquid in another that are commonly used in various products, and methods such as high-pressure homogenisers and colloid mills are used to form emulsions. The size and size distribution of emulsion droplets are important for the final product properties and thus need to be controlled. Rapid coalescence of droplets during emulsification increases droplet size and widens the size distribution, and therefore needs to be prevented.

    To increase stability of emulsions, emulsifiers are added to adsorb at the oil-water interface before droplets collide. The time allowed for emulsifier adsorption is typically in the range of sub-milliseconds to seconds and to optimise emulsification processes, emulsifier adsorption and coalescence stability need to be measured in this time-scale, for which the microfluidic methods described in this thesis were developed.

    Chapter 2 provides an overview of existing literature on cross-flow microfluidic emulsification. The effects of various parameters such as microfluidic design, shear forces, and interfacial tension forces on droplet formation and the resulting droplet size are discussed, as well as the use of microfluidics to produce food-grade emulsions. Based on this evaluation, the methods to elucidate interfacial tension and coalescence stability are chosen, and these are presented in the next chapters.

    To measure emulsifier adsorption in the sub-millisecond time-scale, a tensiometric method was developed using a cross-flow microfluidic Y-junction, which is described in Chapter 3. This method is based on the relation between droplet size and interfacial tension at the moment of droplet formation, which is referred to as the acting interfacial tension. The acting interfacial tension of a system with hexadecane as the dispersed phase and sodium dodecylsulfate (SDS, a model surfactant) solutions as the continuous phase was successfully measured for droplet formation times ranging from 0.4 to 9.4 milliseconds and with high expansion rates (100-2000 s-1). Comparison of these results with data from a drop tensiometer (a conventional, static, and supra-second time-scale method) indicates that mass transport in the microfluidic Y-junction is fast and probably not limited by diffusion.

    Emulsifier mass transport conditions were further investigated in Chapter 4. The continuous phase viscosity and velocity were systematically varied and the effect on the acting interfacial tension in presence of water-soluble SDS was measured. We found that the acting interfacial tension was independent of the continuous phase viscosity, but was inversely dependent on continuous phase velocity. Both aspects led us to conclude that convective emulsifier transport in the continuous phase determines the acting interfacial tension in the Y-junction. When using oil-soluble surfactant Span 20 (dissolved in hexadecane), the acting interfacial tension also decreased with increasing continuous phase velocity, and we therefore concluded that convection also dominated mass transport of emulsifiers dissolved in the to-be-dispersed phase.

    The Y-junction method was used in Chapter 5 to elucidate the effect of the dispersed phase viscosity on adsorption of the food-grade emulsifiers Tween 20 (dissolved in the continuous water phase) and Span 20 (dissolved in the dispersed oil phase). A reduction in dispersed phase viscosity sped up adsorption of Tween 20, probably because the shorter hydrocarbon made intercalation of the hydrophobic surfactant tail at the interface easier. Dispersed phase viscosity had an even greater effect on adsorption of Span 20 because convective transport towards the interface was increased.

    Next to interfacial tension, also coalescence can be measured with microfluidics and a microfluidic collision channel was used in Chapter 6 to measure emulsion coalescence stability shortly after droplet formation under flow. Coalescence of emulsions stabilised with proteins was measured at various concentrations, pH values, and adsorption times. We found that protein concentrations just below the concentration needed for monolayer surface coverage may be used effectively. β-lactoglobulin-stabilised emulsions were most stable. Emulsions stabilised with whey protein isolate (with as main component β-lactoglobulin), were less stable and when these proteins were oxidised, this led to reduced stability, therewith indicating that also the oxidative state of proteins needs to be considered in emulsion formulation.

    The relevance of our work for microfluidic research and industrial emulsification processes is discussed in Chapter 7. Microfluidic devices can be used to study emulsion formation and stability under conditions relevant to industrial emulsification processes; at short time-scales and with convective mass transport. In this thesis we used various food-grade ingredients, and with that application in that field has come closer. We expect that the findings on emulsions can also be applied on foams. With the discussed microfluidic devices different aspects that are important for emulsion formation can be decoupled: for example interfacial tension during droplet formation and emulsion coalescence stability. Furthermore, microfluidic methods are available to for example gain insight in emulsion interface mobility and emulsion storage stability, and we envision that all these microfluidic methods will lead to faster ingredient screening, lower ingredient usage, and more energy efficient emulsion production.

    Double emulsions as fat replacers : linking emulsion design to stability and sensory perception
    Oppermann, Anika - \ 2017
    Wageningen University. Promotor(en): Kees de Graaf, co-promotor(en): Markus Stieger; Elke Scholten. - Wageningen : Wageningen University - ISBN 9789463430722 - 186
    fats - fat - sensory sciences - sensory evaluation - emulsions - perception - gelation - vetten - vet - sensorische wetenschappen - sensorische evaluatie - emulsies - perceptie - gelering

    The use of double (w1/o/w2) emulsions, in which part of the oil is replaced by small water droplets, is a promising strategy to reduce oil content in food products. For successful applications, (1) significant levels of fat reduction (i.e. significant amounts of water inside the oil droplets) have to be achieved, (2) double emulsions have to be stable against conditions encountered during processing and storage, and (3) the mouthfeel and sensory perception have to be similar to that of full-fat equivalents. With the present work, significant progress was made in understanding the complex relations between double emulsion design, achievable levels of fat reduction, emulsion stability and sensory perception. We show that through careful emulsion design, stable double emulsions with high levels of fat reduction (up to 50%) can be obtained while maintaining fat-related sensory properties, making double emulsions a promising approach for the development of fat-reduced food products.

    Fibrillar structures in mixed systems
    Peng, Jinfeng - \ 2016
    Wageningen University. Promotor(en): Erik van der Linden, co-promotor(en): Paul Venema; K.P. Velikov. - Wageningen : Wageningen University - ISBN 9789462578265 - 284
    cellulose - bacteria - fibres - protein isolates - whey - mixtures - emulsions - mechanical properties - cellulose - bacteriën - vezels - eiwitisolaten - wei - mengsels - emulsies - mechanische eigenschappen

    Fibrillar structures are important structuring elements for food products. Understanding the behaviour of fibrillar structures in complex food systems is essential for successful industrial applications. This thesis presents the behaviour of two different fibrillar structures, i.e. whey protein isolate (WPI) fibrils and bacterial cellulose (BC) microfibrils in mixtures under various conditions. The WPI fibrils are prepared from WPI and the BC microfibrils are extracted from commercial available ‘Nata de Coco’ by high-energy de-agglomeration. In Chapter 1, a general introduction is given, where we introduce two different fibrillar structures that were studied in this thesis. Also, the aim and the outline of the thesis are presented. In Chapter 2, 3, 4 and 5, the behaviour of mixtures containing WPI and BC microfibrils under different conditions are investigated. By varying the concentration ratios, pH, NaCl concentration and further applying heating treatment, their physico-chemical properties in mixed solutions, mixed solutions after heating and further heat-induced mixed gels are investigated and characterized at both pH 2 and pH 7. In general, both mixing WPI and BC microfibrils without heating and subsequently applying heating treatment lead to stable and homogeneous mixtures at pH 7, as long as BC microfibril concentration is above a critical value. Microscopic images showed that the WPI aggregates and BC microfibrils co-existed in the system. WPI denatured and aggregated in the mixture in the same way as when it is heated alone. Upon gelation, the WPI and BC microfibrils form a duplex gel consisting of two independent and homogeneous networks spanning the whole system. At pH 2, the WPI and BC microfibrils also form stable and homogeneous mixtures in the liquid state, both before and after heating. Microscopic images showed two fibrillar structures that are uniformly and independently present. Upon gelation at higher WPI concentration, a bi-fibrillar gel is formed consisting of a WPI fibrilllar gel and BC microfibrillar gel that co-exist. In Chapter 6 and 7, the behaviour of WPI fibrils at pH 2 in dispersions containing spheres, i.e. emulsions and polystyrene latex dispersions are studied. When WPI and spheres are both positively charged (i.e. WPI-stabilized emulsion), we observed depletion flocculation and depletion stabilization when the WPI fibril concentration increases. When WPI and the spheres are oppositely charged (i.e. polystyrene latex dispersions), bridging flocculation and steric/electrostatic stabilization were observed at low WPI fibril concentration, followed by depletion flocculation and depletion stabilization upon increasing WPI fibril concentrations. In Chapter 8 the stability of emulsions at pH 2 in the presence of only BC microfibrils and in the presence of both BC microfibrils and WPI fibrils was studied. When only BC microfibrils added at a sufficiently high concentration, the emulsions are stabilized by the presence of a yield stress as generated by the BC network. When both WPI fibrils and BC microfibrils are added to the emulsions, the networks they form behave in the same way, as when they are added to the emulsions separately. The WPI fibrils induced depletion flocculation and stabilization of the emulsions, despite the presence of the BC microfibrils. However, at high enough BC microfibril concentrations, the emulsions can be stabilized against depletion flocculation as induced by the WPI fibrils. The competition between stabilization and/or de-stabilization induced by the BC microfibrils and the WPI fibrils can lead to emulsions with different microstructures and rheological properties. A general discussion on the results obtained in this thesis is presented in Chapter 9, which includes recommendations for further research and concluding remarks.

    Upscaling microstructured emulsification devices
    Sahin, S. - \ 2016
    Wageningen University. Promotor(en): Karin Schroen. - Wageningen : Wageningen University - ISBN 9789462577527 - 127
    emulsification - emulsions - droplets - emulgering - emulsies - druppels

    Emulsions, which are dispersions of two immiscible liquids (e.g. oil and water), are part of our daily life through many products that we use such as milk, mayonnaise, salad dressings, ice cream, lotions, shampoos, medicines, wall paints, etc. Many quality attributes of these products such as stability, texture, colour, visual and sensorial perception are affected by droplet size and size distribution.

    Conventional emulsification technologies such as high pressure homogenizers have poor control on droplet size distribution, they are energy intensive and not suited for fragile multiple emulsions. In the last decades, alternative emulsification concepts that employ microengineered structures have been developed. They can produce uniform droplets of a specific size using orders of magnitude less energy, and are suitable for multiple emulsions.

    However, most of these techniques generate one droplet at a time and the productivity of a single droplet generation unit is very low. To reach significant throughput, many units need to be run in parallel, which is far from trivial especially for the production of droplets below 10 micrometres. In this regard, EDGE (Edge-based Droplet GEneration) devices are better suited for upscaling since they can generate multiple uniform droplets simultaneously from one droplet formation unit.

    Unlike standard upscaling in industry, the characteristic dimension remains the same for microstructured (EDGE) devices, and issues related to upscaling were found to be linked to (sub-) micrometre scale (e.g. wettability and flow geometry). In EDGE emulsification, the contact surfaces need to be wetted well by the continuous phase, and in chapter 2 we show that the interactions of the liquids and surfactants with all available surfaces/interfaces influence wettability. In general, oils that have strong interaction with the surface can only be emulsified successfully in combination with surfactants that bind strongly to the surface. Also the pressure range in which droplets can be produced is greatly influenced by these interactions, e.g. proteins showed much wider pressure stability and an order of magnitude higher productivity, therewith also showing that EDGE emulsification is well suited for food-grade emulsions; that is as long as an appropriate combination of construction material and emulsion components is used.

    Also the geometry of the EDGE devices can be used to increase productivity. Previous research indicated that higher resistance on the plateau can improve the pressure stability, which inspired us to redesign the droplet formation units and place regularly spaced micron-sized partitions on the main plateaus, as reported in chapter 3. The micro-plateaus were positioned such that the number of droplet formation points was increased compared to regular EDGE, and it was found that the additional flow resistance resulted in remarkably wide pressure range while supplying oil to all micro-plateaus that were equally active, thereby leading to two orders of magnitude higher droplet productivity. Interestingly, at high pressures a second wide range generating approximately three times larger uniform droplets was discovered.

    In chapter 3, only one partitioned EDGE geometry was investigated for hexadecane and 0.5% SDS solution; therefore, in chapter 4 the underlying droplet formation mechanisms was investigated further by systematically varying the geometry of the micro-plateaus and the viscosity of the liquids. It was found that the micro-plateau geometry greatly influenced emulsification behaviour. The second regime, in which large droplets were formed, was only observed for narrow micro-plateaus, suggesting that a certain minimum flow resistance is needed for the second regime to occur. In the first regime, in which small droplets were formed, droplet size was dependent on the viscosity ratio of the liquids, in a similar way to that found for regular EDGE devices.

    The partitioned EDGE devices were upscaled in chapter 5 with 75000 micro-plateaus. This first upscaled device, the so-called multi-EDGE, was used to produce monodisperse hexadecane droplets of ~10 micrometres at 0.3 m3 m-2 h-1 (80% micro-plateau activation). As expected, the differences in plateau geometry (due to technical limitations) compared to the devices reported in chapter 3 led to an order of magnitude lower productivity. Nonetheless, the initial results were promising, and provided clear leads to improve the productivity further. Last but not least, with the current multi-EDGE device enough product can be made to conduct rheology and stability tests for truly monodisperse emulsions.

    In chapter 6, we studied a different microstructured device, the packed bed premix emulsification equipment, and showed that food-grade double emulsions (containing 5% v/v primary emulsion) can be refined at high throughputs, typically in the range of 100‑800 m3 m-2 h-1, while keeping their encapsulation yield above 90%. Droplet size reduction was similar to that found for single emulsions; the refined droplets were smaller than the pore sizes of the packed bed, and no marked fouling was observed under the conditions tested. Further, the process was robust and reproducible, making the technique a genuine option for double emulsion production.

    In the last chapter, chapter 7, we compare microstructured emulsification techniques on various aspects, and explain how the findings of this thesis help mitigate the identified bottlenecks (e.g. wettability, parallelization, productivity) that prevent upscaling of the technology. Finally, we conclude with an outlook on upscaling and discuss the aspects related to possible applications of the technology in the future.

    Oral coatings: a study on the formation, clearance and perception
    Camacho, S. - \ 2015
    Wageningen University. Promotor(en): Kees de Graaf, co-promotor(en): Markus Stieger; F. van de Velde. - Wageningen : Wageningen University - ISBN 9789462575653 - 223
    afdeklagen - eiwitten - orale toediening - tong - mond - smering - emulsies - in vivo experimenten - sensorische evaluatie - perceptie - dynamica - zoetheid - fluorescentie - coatings - proteins - oral administration - tongue - mouth - lubrication - emulsions - in vivo experimentation - sensory evaluation - perception - dynamics - sweetness - fluorescence

    Oral coatings are residues of food and beverages that coat the oral mucosa after consumption. Several studies have reported on the lubrication properties in mouth, and the after-feel and after-taste impact of oral coatings. Further, oral coatings have been suggested to influence subsequent taste perception. Although it is well known that oral coatings can influence sensory perception, there was little information available on the chemical composition and physical properties of oral coatings. As such, the aim of this thesis was to understand which factors influence the composition of oral coatings and their sensory perception.

    This study started with the development of an appropriate calibration method for an already described methodology to quantify oil oral coatings: in vivo fluorescence. Further, the samples studied were shifted from pure oil (used on previous studies) to a more realistic food beverage: o/w emulsions. Pig´s tongues are known to be a good model of human tongue. As such, Chapter 2 used pig´s tongues on the calibration of the method, to mimic the fluorescence in mouth of oil coatings. On chapter 2, Confocal Scanning Laser Microscopy images showed that stable o/w emulsions (1-20% (w/w)) stabilised by Na-caseinate created individual oil droplets on the surface of the pigs tongue, as such a new descriptor for oil coatings was developed. Oil fraction, i.e. mass of oil per surface area of the tongue, was shown to be higher on the back compared to the front anterior part of the tongue. This is thought to be due to the morphology of the tongue and abrasion of the oil coating owed to the rubbing with the palate. Further, in vivo measurements showed that oil fraction deposited on the tongue increased linearly with oil content of o/w emulsions. Coating clearance from the tongue was a fast process with around 60% of the oil being removed on the first 45s. After-feel perception (Fatty Film and Flavour Intensity) was shown to be semi-logarithmic related to oil fraction on the tongue.

    Chapter 3, further investigated different properties of 10% (w/w) o/w emulsions that influence the oil fraction deposited on the tongue, its clearance and after-feel perception. Three different properties were studied: protein type, protein content and viscosity of the o/w emulsions. To study the influence of protein type, two different proteins which behave differently in-mouth were studied: Na-caseinate - creates emulsions which do not flocculate under in mouth conditions, and lysozyme – creates emulsions which flocculate under in mouth conditions. To study the influence of protein content, three concentrations of Na-caseinate and lysozyme were used (0.2, 3, 5.8% (w/w) all in excess to stabilize the water/oil interface). To study the influence of viscosity of o/w emulsions, three o/w emulsions stabilized with 3% (w/w) Na-caseinate were thickened with varying concentrations of xanthan gum (0-0.5%) (w/w).

    Generally, the irreversible flocculation of lysozyme stabilized emulsions with saliva did not create a significant difference on oil deposition compared to emulsions stabilized with Na-caseinate, immediately after expectoration of the emulsions. Nevertheless, lysozyme stabilised emulsions caused slower oil clearance from the tongue surface compared to emulsions stabilized with Na-caseinate. Protein content had a negative relation with oil fraction on the tongue for lysozyme stabilized emulsions and no relation for Na-caseinate stabilized emulsions. The presence of thickener decreased deposition of oil on tongue, although viscosity differences (i.e., thickener content) did not affect oil fraction. After-feel perception of creaminess and fatty-film was strongly influenced by the presence of thickener likely due to lubrication in-mouth, i.e., the higher the concentration of thickener in the emulsions the stronger was the perception. Oral coatings perception was further influenced by the protein used in the emulsions, with Na-caseinate stabilised emulsions creating coatings with higher perception on creaminess and fatty-film.

    Chapter 2 and chapter 3 provided knowledge on the deposition and clearance of oil coatings, but little was known on the formation of oil coatings. Chapter 4 focused on the formation of oil coatings formed by Na-caseinate stabilised o/w emulsions (1-20% (w/w)). The formation of oil coatings was a rapid process, where the maximum oil deposition was achieved at normal drinking behaviour (~3s). Further, in Chapter 4 we investigated the hypothesis often referred on literature, in which oil coatings form a physical barrier which prevents tastants to reach the taste buds, and thus create a reduction on taste perception. It was concluded that oil coatings formed by emulsions within one sip did not affect subsequent sweetness perception of sucrose solutions. We suggested that the oil droplets deposited on the tongue (as seen on chapter 2) did not form a hydrophobic barrier that is sufficient to reduce the accessibility of sucrose to the taste buds and consequently does not suppress taste perception.

    Previous chapters focused on oral coatings formed by liquid o/w emulsions, however studies describing oral coatings formed by semi-solids and solids are scarce. As such, chapter 5 focused on the formation, clearance and sensory perception of fat coatings from emulsion-filled gels. Four emulsion-filled gelatin gels varying in fat content and type of emulsifier (whey protein isolate - created fat droplets bound to matrix; tween 20 - created fat droplets unbound to matrix) were studied. As in for oil coatings formed by liquid o/w emulsions, fat coatings formed by emulsion-filled gels reach their maximum deposition in the first seconds of mastication. This suggests that the first bites are the most relevant for the formation of fat coatings on the tongue. Further, fat fraction deposited on tongue increased when oral processing time of the gels increased. This trend was clearer for gels with higher fat content (15%) compared to gels with lower fat content (5%). Fatty perception increased with increasing mastication time, and decreased after expectoration with increasing clearance time. Fat fraction deposited on tongue and fatty perception are higher in gels with unbound droplets compared to bound droplets, as well as in gels with 15% fat compared to 5% fat.

    To elucidate the role of protein on oral coatings, Chapter 6 focused on the development of a method to quantify protein in the oral coatings. Further, Chapter 6 studied the influence of protein content, in-mouth protein behaviour (lysozyme - protein which creates flocs with saliva vs. Na-Caseinate - protein which does not create flocs with saliva) and presence of thickener on the formation of protein oral coatings and sensory perception of protein coatings. Protein coatings were collected from the front and middle part of the anterior tongue using cotton swabs after subjects orally processed protein solutions for different time periods. Protein concentration of the coating (mass protein/mass coating) was quantified with the Lowry method. Similarly to oil/fat coatings, results show protein coatings are formed rapidly, reaching maximum deposition on the first seconds of the samples´ oral processing. Further, different protein in mouth-behaviour (Na-caseinate vs. lysozyme) did not create differences on protein deposition on the tongue. Presence of xanthan-gum in the processed samples decreased protein deposition on the tongue, compared to when samples without xanthan-gum were processed. The perception of protein coatings was strongly influenced by the viscosity and protein used in the samples. Higher viscosity of the samples lead to higher intensity on creaminess and thickness. Lysozyme samples created coatings with high sweetness and astringent intensity, which is related to the molecular structure of the protein.

    Changes in the viscosity of beverages can cause changes in thickness perception. The changes in thickness perception can be accompanied by differences in other sensory properties, such as sweetness and creaminess which might be undesirable when reformulating beverages or developing new products. Knowledge on the differences by which viscosity of beverages can be modified to create a difference in sensory perception is currently lacking. Chapter 7 focus on the determination of the Just Noticeable Difference (the minimal difference that can be detected between two stimuli) for thickness perception of beverages. Oral thickness sensitivity (K=0.26) was found to be comparable to literature values for kinesthetic food firmness and spreadability, creaminess, sourness and bitterness perception.

    The aim of this thesis was to determine and characterize factors influencing oral coatings and their sensory perception. For this purpose, reliable methods to quantify oil and protein deposited on the tongue had to be developed to later study the macronutrients deposition. Further, the influence of stimulus properties on the formation and clearance dynamics of oral coatings and their impact on sensory perception were investigated.

    New tools in modulating Maillard reaction from model systems to food
    Troise, A.D. - \ 2015
    Wageningen University. Promotor(en): Vincenzo Fogliano, co-promotor(en): Claire Berton-Carabin; P. Vitaglione. - Wageningen : Wageningen University - ISBN 9789462575455 - 129
    maillard-reactie - maillard-reactieproducten - modulatie - controle - inkapselen - olijfolie - melk - emulsies - modellen - voedsel - gereedschappen - maillard reaction - maillard reaction products - modulation - control - encapsulation - olive oil - milk - emulsions - models - food - tools
    New tools in modulating Maillard reaction from model systems to food

    The Maillard reaction (MR) supervises the final quality of foods and occupies a prominent place in food science. The first stable compounds, the Amadori rearrangement products (APs) and Heyns rearrangement products (HPs), represent the key molecules from which a myriad of reactions takes place and each of them contributes to the formation of Maillard reaction end-products (MRPs) or advanced glycation end products (AGEs).

    Several papers have dealt with the control of the MR in foods ranging from the thermal loading reduction, to the use of alternative process technologies, reactants impact or enzymes, as well as to the monitoring of the end-products formation by multiresponse modeling. The strategies used up to now aim at common goals: the reduction of potentially toxic compounds and the promotion of desired molecules formation as well as flavor, aroma, color and texture attributes. In other words the ultimate target is the promotion of food quality by tuning the MR.

    This thesis introduces four alternative strategies that are able to control the final extent of the MR in foods.

    The possibility to segregate reactants by encapsulating some minor components and thus delaying the MR was highlighted in Chapter 2. The encapsulation of sodium chloride, ascorbic acid, PUFA and iron inside hydrophobic capsules was used as a possible example: the core material release over the time delayed the reaction rates.

    The results obtained through the treatment with the enzyme fructosamine oxidase (Faox) I and II which is able to deglycate free Amadori products and capitalize the local unfolding of lysine peptide bound residues were reported in Chapter 3. Data showed that Faox can reduce the formation ofNε-(Carboxymethyl)-L-lysine and bound hydroxymethylfurfural in model system and in low lactose milk.

    The effects obtained with the addition of spray-dried olive oil mill wastewaters in milk was illustrated in Chapter 4. This ingredient acts as a source of phenylethanoids, which can trap a-hydroxycarbonyls and a-dicarbonyls and can form adducts with amino groups after the oxidation of phenolic rings into quinone. The use of this functional ingredient before milk thermal treatment resulted in a reduction of off-flavor, reactive carbonyls species and bound MRPs.

    The possibilities offered by the location of MR reactants in microemulsion was investigated in Chapter 5. The oil/water partition coefficient of amino acids played a key role in the formation of Amadori compounds. The anchoring effect of tricaprylin and Tween 20 toward aliphatic amino acids in microemulsion systems was evaluated and compared to a control aqueous solution of amino acids and glucose. Results confirmed the hypothesis: the higher the partition coefficient the lower the formation of aliphatic amino acids Amadori compounds.

    All of the four proposed strategies involved location and interaction of reagents, reactants, intermediates and final products. As a result each strategy depicted a specific route for the control of the final extent of the MR. Many steps are still necessary to scale up these methodologies into the food production chain, however new ways for obtaining foods of superior quality have been paved.

    Complex coacervates and microgels for emulsions : robust, responsive, reversible
    Monteillet, H.J.M. - \ 2015
    Wageningen University. Promotor(en): Frans Leermakers, co-promotor(en): Mieke Kleijn; Joris Sprakel. - Wageningen : Wageningen University - ISBN 9789462574526 - 147
    emulsies - gels - stabilisatie - elektrolyten - scheiding - emulsions - gels - stabilization - electrolytes - separation

    The use of ionic liquids (ILs) as replacement of organic solvents in liquid-liquid extractions has shown great promise due to their low volatility, flammability and toxicity, tunable solvency to a wide variety of extractable compounds and mild- ness to delicate compounds such as biomolecules for pharmaceutical applications. However, the efficiency of extractions using ionic liquids is limited as the inher- ently high viscosity of ILs slows down the mass transfer. Increasing the interfacial area between the immiscible phases is an efficient way to increase the efficiency of liquid extractions; typically done by formulating emulsions, dispersions of fluid droplets suspended in a second immiscible continuous phase. While strategies to formulate stable emulsions from conventional apolar solvents, such as aliphatic or halogenated oils, in water are abundant, the peculiar properties of ionic liquids requires the exploration of new strategies to formulate stable emulsions; for exam- ple, common surfactant stabilization leads to rapid Ostwald ripening due to the inherent water solubility of many ionic liquids. Moreover, while the intended ionic liquid-in-water emulsions must be stable at operating temperatures for prolonged times, it should be possible to break the emulsion on-demand to recover the ex- tracted product. Also, the interfacial layer used for stabilization should not hinder the transfer of the intended product to the droplet phase. To increase the sus- tainability of extraction processes, recovery of both ionic liquid and stabilizer for re-use in a subsequent extraction step is highly desired. Aimed to establish new ways of stabilizing emulsions in general, and ionic liquid emulsions in specific, this thesis describes investigations into two novel stabilizers: interfacial electrostatic complexes and soft colloidal microgels.

    In Part I, we focussed on how oppositely charged polyelectrolytes interact and form complexes across an oil-water interface. In Chapter 2, we demonstrated a new method for emulsion stabilization, in which electrostatic complexes formed across a liquid interface between two polyelectrolytes, one dissolved in the aqueous phase, the other in the oil phase. Using tensiometry we followed the polyelectrolyte adsorption at the oil-water interface; while the presence of either polyelectrolyte alone leads to interfacial depletion, the presence of both species leads to strong adsorption at the interface. This was further confirmed using confocal fluorescence microscopy where the colocalization of both species at the interface was observed; the strong overlap of peak intensities at the interface suggests a strongly intermixed layer. Using this approach, we prepared stable emulsions, which could be reversibly broken and reformed by simple pH and salt triggers. Interestingly, oil-in-water but also water-in-oil emulsions could be produced. This is the first demonstration of using selective associative phase separation to stabilize a segregating system.

    The experimental results triggered questions on the nature of the interfacial layer, which was too thin to be ascertained in detail using microscopy. Therefore,

    we turned to self-consistent field (SCF) modelling to develop a deeper understand- ing of the structure and thermodynamics of this interfacially-templated complex- ation, as presented in Chapter 3. In analogy with our experiments, we use the Scheutjens-Fleer lattice method to consider mixtures of two solvents, an anionic oil- soluble polyelectrolyte, a cationic water-soluble polyelectrolyte, their counterions and additional indifferent monomeric electrolyte. We first considered a two-phase system with only one polyelectrolyte and salt. We found that the polyelectrolyte adsorption depends on its concentration. For polyelectrolyte concentrations lower than the salt concentration, the polyelectrolyte is depleted from the oil-water in- terface while for polyelectrolyte concentrations higher than the salt concentration, the polyelectrolyte adsorbs at this interface. This transition from depletion to ad- sorption originates from a competition between small ion and macroion adsorption, governed by the overall ionic strength. Upon introducing a second polyelectrolyte in the immiscible second solvent, a new phase spontaneously formed at the inter- face between oil and water. Surprisingly, our calculations showed that ion release entropy is not the driving force for complexation, as it often is in bulk complex coacervation; co-assembly is governed by enthalpic contributions. This is due to the solvent-selectivity of the polyelectrolytes in this scenario, which leads to low solvent content in the coacervate layer, hence close approach of the opposite charges resulting in a relatively large Coulombic enthalpy. Finally, we examined systems with asymmetric composition of the two polyelectrolytes within the same theoret- ical approach. This revealed an unusual pseudo-partial wetting scenario, due to interactions occurring at different length scales. When the electrostatic interactions are short ranged, the microscopically thin wetting film transitions to a mesoscopic thin film. However, charges built up on either side of the coacervate layer restrict the growth of the film to macroscopic dimensions. In our experiments we observe that the coacervate layer becomes turbid over time, suggesting structures on op- tical length scales, much larger than the typical dimensions of the polymer coils. This may be explained by the pseudo-partial wetting scenario due to the coexis- tence of a mesoscopic film with interfacial liquid droplets nucleating due to thermal fluctuations.

    In the second part of this thesis, Part II, we studied the adsorption and or- ganization of colloidal microgels at a variety of liquid interfaces. These soft and deformable hydrogel colloids have gained a lot of interest in recent years due to their excellent ability to stabilize emulsions. As a result of their polymeric nature and osmotic equilibrium with the bulk solution, microgels exhibit an interesting duality between colloidal properties and polymeric behaviour. Microscopic research into their interfacial behaviour is often made difficult as they offer little refractive index contrast to the continuous phase and covalent attachment of fluorophores is known

    to drastically alter their interactions. To overcome this problem, in Chapter 4 we introduce composite microgels, in which a solid fluorescent core is embedded in the centre of a soft and tunable hydrogel shell, thereby decoupling the imaging features of these microgels with the tunability of their softness, size, solvent-responsivity and interactions. We surprisingly find that while these microgels adsorb sponta- neously, without any energy barrier which is usual for the Pickering adsorption of micron-sized colloids, their anchoring at the liquid interface is irreversibly strong. Due to the high adsorption energy, saturated interfacial layers of these microgels show mild compression of the particles, increasing their packing density at the cost of elastic deformation. Moreover, we showed that these particles are able to stabilize a wide variety of oil-water interfaces and due to their spontaneous adsorp- tion allow the fabrication of Pickering droplets using microfluidics, which is usually hindered by the adsorption barrier for solid particles.

    In Chapter 5, we arrive at the ultimate aim of this thesis, i.e. to provide proof- of-concept for a fully sustainable extraction process based on IL-in-water emulsions. We first show how microgels are able to create emulsions of a wide variety of ILs in water and prevent their Ostwald ripening, resulting in extended stability at room temperature. Upon heating and applying centrifugal compression, the emulsion can be rapidly broken, with all of the microgels returning the aqueous phase which can then be re-used in a secondary extraction step. Finally, we demonstrated that through the use of a paramagnetic ionic liquid, the concentration and breaking step can be performed without energy input with a simple permanent magnet, rendering the process sustainable from start to end.

    Finally, in Chapter 6, we studied the adsorption and conformation of these composite microgels at solid-liquid interfaces. We first demonstrate how conven- tional sample preparation for studying microgels at solid interfaces, often involving a drying step, induces strong sample artefacts. We therefore developed a method to study the adsorption and conformation of microgels in-situ using liquid-state confocal and atomic force microscopy. Our results showed how the packing density for particle adsorption is governed by particle-particle repulsion, as adsorption en- ergies are typically very high. Using Quantitative Nanomechanical Mapping, the spatially-resolved mechanical analysis of surfaces using atomic force microscopy, we find that the degree of spreading of microgels during adsorption at a solid interface is governed by adsorption energy and particle softness as expected. This leads us to conclude that the unique properties of microgels at interfaces results from a subtle interplay between adsorption energy and internal elasticity.

    Understanding and manipulating coalescence in dense emulsions
    Feng, H. - \ 2013
    Wageningen University. Promotor(en): Martien Cohen Stuart; Jasper van der Gucht, co-promotor(en): Joris Sprakel. - Wageningen : Wageningen UR - ISBN 9789461737373 - 113
    emulsies - natuurlijke droging - emulgering - afdeklagen - film - emulsions - natural drying - emulsification - coatings - film

    Coatings and paints play a significant role in daily life; they prolong the lifetime of materials by offering protection against, for example, corrosion, weathering or fouling, and literally add color to our lives. Due to their widespread use, their environmental consequences have become focus of increasingly strict regulations and public awareness. There has been a strong effort to replace traditional solvent-based coatings with waterborne coatings to reduce or eliminate the volatile organic compounds (VOC) that traditionally formed the main component of paints. A pronounced shift from solvent-based to water-based systems has already taken place for decorative (consumer) coatings. However, for more demanding applications in industry, the replacement of solvent-based paints with greener waterborne formulations still has a long way to go, due to their lower performance in terms of both mechanical, durability and aesthetic aspects. The development of waterborne coatings with the same or better performance than solvent-borne systems is thus an important step towards the further vanishing of VOC-rich coatings. Ultimately, the final aimis to replace all solvent-borne coatings with VOC-free paint formulations.

    Waterborne paints form a very promising candidate, yet several key aspects of their properties during storage, handling and during their lifetime as a coating, remain poorly understood. Waterborne coatings are complex multiphase systems, containing a wide variety of dissolved and dispersed components in the common aqueous continuous phase. During the drying of the paint, after application, this complex mixture must undergo a phase inversion to achieve a homogeneous film of the resinfrom its initial dispersed state. While this state governs the structure, and thus final properties of the coating film, its complexity precludes a deep understanding to date. This is due to the complexity of the drying and phase inversion process, which is governed by a seemingly immense number of chemical and physical parameters.

    We therefore adopted a simplification approach, minimizing the number of parameters to obtain a first-pass insight into the phase inversion process. We started by directly visualizing how coalescence occurs in a drying 2D emulsion film, both on the single-particle scale, with confocal microscopy, and by macroscopic imaging. Based on these observations, we built a hydrodynamic model that explains some of the key governing parameters in the film formation process. Furthermore, we explored the possibilities to manipulate phase inversion and coalescence, by developing new thermoresponsive surfactants. These new strategies allow us to obtain new insights into this complex problem.

    Understanding coalescence in dense emulsions

    The first part of this thesis focusses on understanding how coalescence and phase inversion occurs in a drying emulsion film, through direct quantitative imaging. Our observations at different length scales are unified in a hydrodynamic model to arrive at a microscopic understanding of this complex macroscopic phenomenon.

    In Chapter 2 we observed two distinctmodes of phase inversion in surfactant-stabilized o/wemulsions exposed to aunidirectional drying stress. Coalescence occurs either through a nucleation-and-growth mechanism, where coalesced pockets form and grow randomly throughout the sample, or through a coalescence front that propagates into the sample from the drying end. The way in which coalescence occurs is determined by a balance between the established pressure profile across the film and the local critical disjoining pressure in the emulsion. For very stable emulsions, narrow plateau borders can develop, leading to steep pressure gradients; the actual pressure only exceeds the critical pressure in a narrow zone around the drying front and front coalescence results. The opposite occurs for unstable emulsions; only shallow pressure profiles develop before coalescence commences throughout the bulk of the sample. Moreover, we find that surfactant concentration plays a significant role through its effect on the critical disjoining pressure atwhich coalescence occurs. This, to our knowledge, is the first observation and explanation of different modes of coalescence dynamics in dense emulsion films.

    In chapter 3 we present a hydrodynamic model for the water flow in a jammed emulsion, subjected to a unidirectional drying stress. Water flows through the Plateau borders towards the drying end, driven by gradients in the capillary pressure. Our model predicts the pressure gradients that arise, and allows us to explain the different modes of coalescence observed experimentally in chapter 2. From these results, we estimate the boundaries (critical pressure and evaporation rate) between bulk and front coalescence. We explore the parameter space of our hydrodynamic model, to further investigate the key factors involved in film formation. We show that, those two distinct coalescence behaviors can be obtained within the same model by varying the critical disjoining pressure. Furthermore, we get a ‘coalescence modes phase diagram’ to show where and how the coalescence transit from one to other.

    Manipulating coalescence in dense emulsions

    In Chapter 4 we showthe successful synthesis of well-defined thermoresponsive surfactants through Atom Transfer Radical Polymerisation (ATRP) using a alkyl-functional initiator. These surfactantscan be used to stabilise emulsions for over four months at room temperature, below the collapse transition of the hydrophilic block of the surfactant, yet can be triggered to break the emulsion within minutes when the sample is heated to above 40 °C. This on-demand coalescence is mediated by desorption of the surfactants from parts of the surface, as evidenced by surface tension measurements and direct microscopic observations of the droplets surface. Our results suggest that these well-defined thermoresponsive surfactants form an interesting platform to study droplet coalescence and triggered phase inversion in emulsion systems. Moreover, the ability to break a very stable emulsion on demand has industrial relevance for several applications, such as in film formation of waterborne emulsion paints and the recovery of products during emulsion-based extraction and reaction processes.

    In Chapter 5 we reported on a new approach to study coalescence in dense thermoresponsive emulsions using a microfluidic-based microcentrifugation method in which a constant external field can be applied. We have shown that both thermodynamic and kinetic properties can be measured through automated image analysis, and that the temperature-responsivity of the surfactants can be used to trigger different modes of coalescence on demand. These results form further proof that our conclusions in Chapters 3&4 regarding the nature of the transition from front to bulk coalescence are valid; also here we observe that changing the critical disjoining pressure, through changing the temperature, can induce a spontaneous switch in coalescence mode. This new approach forms a stepping stone for further investigations into the governing mechanisms that dominate phase inversion and film formation.

    Using the knowledge and methods developed in this thesis, new avenues for studying film formation have been opened. Our work focussed on highly idealised emulsions, real coating systems exhibit some complicating factors, such as viscoelasticity of the latex droplets, and even chemical reactions between different species of droplets, interactions with several surface active species and solid pigment particles. Moreover, the length scales in real paints are a few orders of magnitude smaller, requiring the development of new methodologies suitable for these length scales. These topics will be subject for future study, and are required to fully understand and control the properties of water-based coatings.

    Atomization of dilute oil-in-water emulsions during application of crop protection products
    Hilz, E. - \ 2013
    Wageningen University. Promotor(en): Martien Cohen Stuart; Frans Leermakers, co-promotor(en): A.W.P. Vermeer. - S.l. : s.n. - ISBN 9789461735416 - 199
    drift - spuiten - druppelstudies - verstuiving - druppelgrootte - formuleringen - pesticiden - emulsies - drift - spraying - droplet studies - atomization - droplet size - formulations - pesticides - emulsions

    Crop protection products are usually applied as sprays. These spray droplets have a certain size distribution. Fine droplets are often required to achieve a good coverage of the plant and to guarantee the biological efficacy of an agrochemical product. At the same time very fine droplets in spray are not desirable. Due to their low mass and velocity, these droplets can be carried from the application site by crosswind and e.g. can contaminate surface water. Droplet drift can be minimized by reducing the number of very fine droplets in spray. Dilute emulsions produce coarser sprays compared to water when atomized through a standard flat fan nozzle. For this reason dilute emulsions can reduce drift risk.

    The mechanism of spray formation of dilute emulsions has been investigated in this thesis. The proposed mechanism also describes spray formation in more complex mixtures of dilute emulsions with surfactants or polymers.

    Premix emulsification systems
    Nazir, A. - \ 2013
    Wageningen University. Promotor(en): Karin Schroen; Remko Boom. - S.l. : s.n. - ISBN 9789461735157 - 151
    emulgering - emulgeren - emulsies - kunstmatige membranen - vervuiling door afzetting - poriëngrootte - zeven - emulsification - emulsifying - emulsions - artificial membranes - fouling - pore size - sieves

    Emulsions are dispersions of two (or more) immiscible liquids (e.g., oil and water), and are widely used in various industries including food, cosmetics, pharmaceutics, etc. Premix membrane emulsification is an interesting technique for the controlled production of small and uniformly sized droplets. In this process, a coarse emulsion (premix) is first prepared which is then passed under mild pressure through a (microporous) membrane. Due to its high throughputs, the premix emulsification is a promising process, however, the internal fouling is the most important drawback. The research descripted in this thesis aimed to better understand the fundamental limitations in this process, and to find a solution to this, by either reducing fouling by using well-defined membranes, or by making use of a dynamic membrane in the form of packed bed. Furthermore, new designs for continuous operation for emulsification and foam formation are presented. Based on the obtained results, the proposed technologies in this thesis are expected to have significant industrial application.

    Physico-chemical and techno-functional properties of proteins isolated from the green microalgae Tetraselmis sp.
    Schwenzfeier, A. - \ 2013
    Wageningen University. Promotor(en): Harry Gruppen, co-promotor(en): Peter Wierenga; Michel Eppink. - S.l. : s.n. - ISBN 9789461734532 - 132
    algen - eiwitten - functionele eigenschappen - schuimen - emulgeren - emulsies - algae - proteins - functional properties - foaming - emulsifying - emulsions

    In this thesis, the mild isolation of an algae soluble protein isolate (ASPI) and the characterisation of its techno-functional properties are described. The ASPI was isolated from the green microalgae Tetraselmis sp. by beadmilling and subsequent anion exchange adsorption. The isolate obtained contained 59 ± 7% (w/w) protein and 20 ± 6% (w/w) carbohydrates, the latter composed for approximately one fourth of uronic acids (4.8 ± 0.4% [w/w]). In the pH range 5.5 – 6.5, in which currently used legumin seed protein isolates (e.g. soy) show low solubility, ASPI retained high solubility independent of ionic strength. In the soluble pH range, the foam stability of ASPI is superior to the foam stabilities of whey protein isolate (WPI) and egg white albumin (EWA). At pH 7, ASPI stabilized foams are 1.7 times more stable than WPI stabilized foams. Further fractionation of APSI results in foams even 3 times more stable than WPI stabilized foams. In addition, emulsions stabilised with ASPI are stable against droplet aggregation around pH 5 at low ionic strength, while emulsions stabilised by WPI are not stable at this pH. The stability of ASPI emulsions at this pH is attributed to the co-adsorption of the charged polysaccharide fraction present in ASPI. The role of the charged polysaccharides on stabilisation of the emulsions was confirmed by fractionating ASPI into protein-rich and charged polysaccharide-rich fractions. The combination of charged polysaccharides and proteins in ASPI results in good techno-functional properties that are between that of pure proteins and that of the naturally occurring protein-polysaccharide hybrid gum arabic (GA). It is concluded that ASPI represents an attractive substitute for currently used high-value food protein isolates. Due to the combination of the positive interfacial properties of its protein fraction with the broad pH stability of its charged polysaccharide fraction, ASPI possesses the positive attributes of two types of techno-functional ingredients.

    Understanding flow-induced particle migration for improved microfiltration
    Dinther, A.M.C. van - \ 2012
    Wageningen University. Promotor(en): Remko Boom, co-promotor(en): Karin Schroen. - S.l. : s.n. - ISBN 9789461733498 - 207
    microfluidics - filtratie - migratie - deeltjes - stroming - suspensies - emulsies - membranen - microfluidics - filtration - migration - particles - flow - suspensions - emulsions - membranes

    Membrane microfiltration processes are used in for example the food, biotechnology, chemical and pharmaceutical industry, and more generally in e.g. wastewater treatment. Microfiltration is mostly used to separate components that are greatly different in size, e.g. micro-organisms from water, but rarely to fractionate components that are of similar size. This latter option would be interesting for many applications, since it would lead to enriched starting materials and possibly new products, but is hampered by accumulation of components in and on the membrane due to size exclusion by the pores. This leads to flux reduction and increased retention of components in time, basically the accumulated layer determines which components can pass the membrane (see Figure 1).

    Figure 1. Schematic representation of a cross-flow microfiltration process with a decrease in permeate flux over the length of the membrane due to pore blocking and particle adsorption in the pores and on the membrane walls.
    Most research focusses on accumulation mechanisms (concentration polarization, cake formation and adsorption) and concepts targeted at controlling particle accumulation. One example is back-pulsing, but this only gives a short term solution leading to extensive cleaning procedures given the way membranes are currently operated in practice. Clearly it would be beneficial if accumulation could be prevented, and through that, more stable operation could be achieved.

    This thesis presents how flow-induced particle migration can be used for stable membrane flux and retention of components in time. The particle migration mechanisms that are considered in this thesis, shear-induced diffusion, inertial lift, and fluid skimming, act on particles that are typically between 0.1 and 10 micron. They induce separation of components in the fluid moving (larger) particles away from the membrane, therewith facilitating separation; basically pore size no longer determines particle permeation. In the thesis it will be shown that these effects improve processing of dilute suspensions and make processing of highly concentrated systems possible, which is beyond the scope of current microfiltration processes.

    Before the design of these processes, methods to measure velocity and concentration profiles in microfluidic devices are described, compared and evaluated. The small dimensions of these devices will cause particles to migrate; as is used later in the thesis to facilitate segregation and separation. A drawback of the small dimensions is that they make measurement of velocity and concentration gradients difficult. Based on our evaluation, Nuclear Magnetic Resonance (NMR) and Confocal Scanning Laser Microscopy (CSLM), although expensive, are the most promising techniques to investigate flowing suspensions in microfluidic devices, where one may be preferred over the other depending on the size, concentration and nature of the suspension, the dimensions of the channel, and the information that has to be obtained.

    CSLM is used to study the behaviour of suspensions, between 9 and 38 volume%, at the particle level. Under Poiseuille flow in a closed microchannel, shear-induced diffusion causes migration in these suspensions. Under all measured process conditions, particles segregate on size within an entrance length of around 1000 times the channel height. Mostly, the larger particles migrate to the middle of the channel, while the small particles have high concentrations near the walls. This indicates that the small particles could be collected from their position close to the wall and that this principle can be applied to microfiltration (see Figure 2).

    Figure 2. Schematic representation of a cross-flow microfiltration process with a constant permeate flux over the length of the membrane due to shear-induced particle migration in combination with the use of a closed entrance length and large pores.
    Microfiltration of emulsions proves that the small particles can be removed without accumulation of particles in and on the membrane, as long as the process conditions are chosen appropriate. The membrane cross-flow module consists of a closed channel to allow particles to migrate due to shear-induced diffusion followed by a membrane with 20 micron pores, being much larger than the particles, where fractions of these emulsions can be removed. The emulsions consist of small droplets (~2.0 micron) and large droplets (~5.5 micron), with total concentrations between 10 and 47% and different ratios between small and large particles. As expected, the size of the emulsion droplets in the permeate is a function of trans-membrane pressure, membrane design and oil volume fraction (i.e., of the total, the small and the large particles). The guidelines for appropriate process conditions are described and application of the right process conditions leads to very high selectivity. This means that the permeate only consists of small droplets, and on top of that their concentration is higher than in the original emulsion. Especially at high droplet concentration (which is known to cause severe fouling in regular membrane filtration), these effects are occurring as a result of shear-induced diffusion. If only small particles are targeted in the permeate, the module can be operated at fluxes of 40 L/(h•m²); if fractionation is targeted the fluxes can be maintained considerably higher (2-10 fold higher).

    Separation of concentrated suspensions is currently done by dilution and since the process based on shear-induced diffusion works well at low velocities and high concentrations, industrial application could have major benefits in terms of energy and water use. An outlook is given on how current industrial processes can be designed and improved in terms of energy consumption by making use of particle migration. It is shown that return of investment of installation of these new membrane modules is short compared to the membrane life time, due to high energy savings. In order to reach this, it will be necessary to take unconventional process conditions that target particle migration and membrane designs as a starting point.

    Besides concentrated suspensions, also dilute suspensions benefit from particle migration. Migration phenomena can induce fractionation of yeast cells from water in dilute suspensions, using micro-engineered membranes having pores that are typically five times larger than the cells. The observed effects are similar to fluid skimming (in combination with inertial lift), and the separation performance can be linked to the ratio between cross-flow and trans-membrane flux, which is captured in a dimensionless number that can predict size of transmitted cells. For sufficiently high cross-flow velocity, the particles pass the pore and become part of the retentate; the separation factor can simply be changed by changing the ratio between cross-flow velocity and trans-membrane flux. Since the membranes have very large pores, fouling does not play a role and constant high trans-membrane flux values of 200–2200 L/(h•m2) are reached for trans-membrane pressures ranging from 0.02 to 0.4 bar.

    In conclusion, particle migration can improve (membrane) separation processes and even has the potential to lead to totally new separation processes. Particle migration can be advantageous in both dilute as well as concentrated systems, leading to reduced fouling, reduced energy and water consumption and a reduction in waste. This can all be achieved at production capacity similar or better than currently available in microfiltration processes.

    Microtechnologie voor minder vette mayonaise
    Dijke, K.C. van - \ 2010
    Chemie Magazine 2010 (2010)1. - ISSN 1572-2996 - p. 13 - 13.
    voedseltechnologie - emulsies - emulgeren - vetemulsies - vetarme producten - microtechnieken - voedselemulsies - voeding en gezondheid - food technology - emulsions - emulsifying - fat emulsions - low fat products - microtechniques - food emulsions - nutrition and health
    De Wageningse onderzoeker dr. ir. Koen van Dijke ontwikkelde een nieuw microtechnologisch concept voor de vorming van emulsies: EDGE (Edge based Droplet Generation). Het maakt de emulsificatie beter beheersbaar en energie-efficiënter. Bovendien kunnen er dubbelemulsies mee gemaakt worden voor een romige maar toch vetarme mayonaise
    Emulsification in microfluidic Y- and T-junctions
    Steegmans, M.L.J. - \ 2009
    Wageningen University. Promotor(en): Remko Boom, co-promotor(en): Karin Schroen. - [S.l. : S.n. - ISBN 9789085854579 - 150
    emulsies - emulgering - emulgeren - druppelgrootte - emulsions - emulsification - emulsifying - droplet size
    On a daily basis, we encounter many emulsion-based products such as butter or sun cream, which consist of oil droplets in water, or water droplets in oil. Traditionally, these emulsions are produced with systems that allow a high throughput, but yield a broad droplet size distribution. Therefore, the industry is interested in emulsification techniques that give more monodisperse emulsions, such as emulsification with microfluidic devices, i.e. defined geometries with channel diameters in the order of several to hundreds of micrometers.

    The goal of this thesis was to develop a new microfluidic emulsification technique that has the potential to be scaled up for the production of large volumes of monodisperse emulsion. We chose to study shear-driven microfluidic devices, i.e. T- and Y-junctions, due to their high productivity per junction and their potential for mass-parallelisation. However, reliable application of these junctions is only possible when the droplet-formation mechanism and droplet size determining parameters are fully understood. Therefore, we took a single junction both as a starting and as a focal point of this thesis.

    The thesis starts off by indicating and quantifying the parameters that determine droplet size in microfluidic T-junctions. In literature, (monodisperse) emulsification at T-junctions is studied for a broad range of channel dimensions, flow rates, and materials. However, it is not yet clear which parameters determine the droplet size. Therefore, in Chapter 2 statistical analysis is used to quantitatively relate droplet size data from various literature sources. For T-junctions it is found that emulsion droplet size of drops, discs, and plugs can be described by a two-step model consisting of a droplet growth and a droplet detachment step. This suggests that an emulsion droplet grows until a certain volume is reached, after which it starts to detach. The channel dimensions determine droplet growth, while the continuous- and the disperse-phase flow rate determine the abating time (i.e. the fast decrease of the neck resulting in detachment).

    In the remainder of this thesis microfluidic (flat) Y-junctions are discussed; they resemble T-junctions, but are hardly studied in literature. In Chapter 3, emulsification of hexadecane in various ethanol-water mixtures at different process conditions, i.e. flow rates and static interfacial tensions, is experimentally investigated. We focus on droplet formation at the Y-junction or downstream without the incipient droplet blocking the downstream channel (i.e. the dripping and the jetting regime). For Y-junctions, the droplet size is described with a force balance between the interfacial tension force and the shear force at the point where the incipient droplet is kept to the bulk by a neck. It is found that the droplet size at Y-junctions is determined by the interfacial tension, the channel dimensions, and the viscosity and flow rate of the continuous phase; but not by the flow rate of the disperse phase. This makes operation of Y-junctions intrinsically easier than T-junctions, for which the flow rates of both phases need to be (accurately) controlled.

    Where Chapter 3 concentrates on process conditions, in Chapter 4 the effect of (Y-) junction design on the droplet size is investigated. In five different Y-junction geometries and one T-junction with a depth of 5 m, hexadecane is emulsified in ethanol-water mixtures at a given static interfacial tension and at various process conditions, e.g. flow rates. For the various Y-junctions, no effect on droplet size is observed from the junction angle and the length(s) and/or the width(s) of the microchannel(s). In contrast, significant differences are observed between T- and Y-junctions.

    In Chapter 5, the force balance, found in Chapter 3, is extended by including the effect of the viscosity of the disperse phase and a broader range of viscosities and/or flow rates of the continuous phase. The force balance is mainly adapted by rewriting the shear force from the drag force on a sphere to the drag force on the cross-sectional area of the squeezed incipient droplet (head). It is found that the emulsion droplet size at Y-junctions is determined by the interfacial tension, the channel dimensions, the viscosity, density, and flow rate of the continuous phase, and the resistance with the wall. The influence of the viscosity of the disperse phase and the viscosity ratio were found negligible, just as the disperse-phase flow rate.

    The first five chapters show that droplet size at microfluidic Y-junctions is strongly influenced by the interfacial tension and therefore it is important to quantify its value under dynamic conditions. Traditional tensiometric techniques do not allow interfacial tension measurement under the conditions applied in Y-junctions: high shear and droplet formation in less than milliseconds. Therefore, in Chapter 6, (monodisperse) emulsification at microfluidic Y-junctions is proposed as a new tensiometric technique. A calibration curve is derived for hexadecane in various ethanol-water mixtures with a range of static interfacial tensions. Subsequently, this curve is used to estimate the apparent dynamic interfacial tension for solutions with the surfactants SDS or Synperonic PEF108. The apparent dynamic interfacial tension is found to be determined by the flow rates of the continuous and disperse phase, the surfactant and its concentration. In addition, we showed that surfactant transport in Y-junctions is dominated by convection.

    In Chapter 7, the thesis is concluded by comparing emulsification with microfluidic Y-junctions to other shear-driven microfluidic geometries with cross-flow membrane emulsification as a benchmark technology. Especially, the negligible effect of the flow rate and the viscosity of the disperse phase on the droplet size makes microfluidic Y-junctions unique. To illustrate the large-scale feasibility of microfluidic Y-junctions, typical emulsification device volumes and required areas to process 1 m3h-1 of disperse phase were calculated. The requirements are found to be comparable to values obtained from literature for membranes and microsieves. The energy input of the current microfluidic Y-junction design is comparable to traditional emulsification techniques, but since there is room for optimisation, we are hopeful that these values may well be reduced.

    Emulsification with microstructured systems : process principles
    Zwan, E.A. van der - \ 2008
    Wageningen University. Promotor(en): Remko Boom, co-promotor(en): Karin Schroen. - [S.l.] : S.n. - ISBN 9789085049234 - 119
    emulgering - emulgeren - emulsies - membranen - microporiën - filterbedden - druppelgrootte - kunstmatige membranen - emulsification - emulsifying - emulsions - membranes - micropores - filter beds - droplet size - artificial membranes
    The aim of this thesis is to elucidate the underlying processes and mechanisms that determine the droplet size of emulsions produced with microstructured systems, such as premix microstructure homogenization and microchannel emulsification. The ultimate goal is to describe these methods based on detailed knowledge on droplet break-up and droplet formation mechanisms. This includes, amongst others, the influence of viscosity of the (to-be) dispersed and continuous phase, interfacial tension, velocity, and the geometry of the system on droplet break-up and formation. This was done both computationally and experimentally. The insight that was generated was translated into several design rules that can be used for optimization.
    The fate of fat: tribology, adhesion and fat perception of food emulsions
    Dresselhuis, D.M. - \ 2008
    Wageningen University. Promotor(en): Martien Cohen Stuart, co-promotor(en): E.H.A. de Hoog; G.A. van Aken. - [S.l.] : S.n. - ISBN 9789085048657 - 152
    voedingsvet - vetten - emulsies - perceptie - tribologie - frictie - smering - adhesie - sensorische evaluatie - mond - orale biologie - dietary fat - fats - emulsions - perception - tribology - friction - lubrication - adhesion - sensory evaluation - mouth - oral biology
    Met de toename van het aantal mensen met overgewicht neemt ook de vraag naar producten die minder vet bevatten, maar nog steeds wel een lekker smaak hebben toe. Om zulke producten te maken is het heel belangrijk te weten wat er tijdens consumptie met het product gebeurt en hoe mensen eigenlijk vet waarnemen. We hebben ons in ons onderzoek vooral gericht op de bijdrage van vet op wat we noemen het mondgevoel en hoe fysische chemische processen in de mond dit kunnen beïnvloeden. Allereerst hebben we vastgesteld dat mensen als ze tijdens en na het consumeren van een vloeibaar product een lage frictie waarnemen tussen tong en verhemelte, ze dit associëren met vet en romigheid. In het tweede deel van het onderzoek hebben we aanwijzingen gevonden hoe de capaciteit van levensmiddelen-emulsies om een smerende laag op de tong te vormen (en dus de frictie in de mond te verlagen), afhangt van de kans dat de emulsiedruppels plakken (adhesie) en kunnen spreiden op de tong. Met deze kennis over plakken en spreiden van emulsies op de tong kunnen we nu emulsies proberen te maken waarbij we meer efficiënt gebruik maken van de hoeveelheid vet in het product en dus mogelijkheden scheppen om het vetgehalte te verlagen zonder aan smaak in te boeten.
    When emulsions meet saliva : a physical-chemical, biochemical and sensory study
    Silletti, E. - \ 2008
    Wageningen University. Promotor(en): Willem Norde, co-promotor(en): G.A. van Aken; Monique Vingerhoeds. - S.l. : s.n. - ISBN 9789085048213 - 243
    emulsies - eigenschappen - sensorische evaluatie - speeksel - uitvlokking - lysozym - eiwitexpressieanalyse - emulsions - properties - sensory evaluation - saliva - flocculation - lysozyme - proteomics
    Keywords: Emulsion, flocculation, bridging, saliva, salivary protein, salivary peptides, lysozyme, -lactoglobulin, complex formation, LC-MS, SELDI-TOF-MS, proteomics.

    Upon consumption food emulsions undergo various structural and compositional changes in the mouth. One of these changes is that mixing of an emulsion with saliva induces droplet flocculation
    In the study described in this thesis we investigated the influence of saliva on emulsions properties, the mechanism of flocculation and the role in sensory perception. Firstly, we started with evaluating the effect of parameters related to emulsions on flocculation (i.e. differently charged surfactants and proteins such as -lactoglobulin and lysozyme used as emulsifiers and oil-volume fraction). Among the obtained results, we observed that the sign and the density of the charge on the surface of the droplets determine the (ir-)reversibility of flocculation upon dilution with water and shearing. Secondly, the effect of saliva-related parameters was analyzed. Among other aspects, it appeared that an increase in salivary protein concentration increased emulsion flocculation, and that extensive flocculation is typically found for unstimulated saliva. This approach shows that both emulsion and saliva properties affect the flocculation behavior of emulsions/saliva mixtures.
    To investigate the nature of the flocculation, we characterized the salivary protein composition in both the continuous phase of the emulsion/saliva mixture and on the emulsion droplets. Different physical-chemical and biochemical techniques were used. For this approach, we focused on -lactoglobulin and lysozyme stabilized emulsions, which flocculated reversibly and irreversibly, respectively, upon mixing with saliva. A large number of salivary proteins and peptides in the molecular mass (Mr) range between 0.8 kDa and 100 kDa and the salivary mucins MUC5B and MUC7 (Mr > 200 kDa) associated with emulsion droplets of the emulsions. The results also indicate that the emulsifying protein at the oil-water interface determines which salivary components associate with the droplets in the flocs. A hypothesis is formulated that emulsion flocculation is mainly driven by a complex formation involving specific interactions and electrostatic attraction between salivary peptides/proteins and the emulsifying proteins at the droplets surface.
    The importance of the saliva-induced droplet flocculation was demonstrated with a sensory paneling study. Emulsions stabilized by whey protein isolate, (predominantly composed of -lactoglobulin) showed reversible flocculation and were perceived as creamy. In contrast, emulsions stabilised by lysozyme showed irreversible flocculation and were perceived as dry, rough and astringent.
    To conclude, this thesis shows that saliva-induced emulsion flocculation is driven mainly by association of salivary peptides and proteins to the droplets surface. Because of this, flocculation is determined by the composition of the droplet interface as well as the composition of the saliva, and can be controlled by variation of emulsion parameters (charge, pH, ionic strength). This interaction between emulsions and saliva may help to improve our understanding an control the sensory perception of emulsions.

    Food gels filled with emulsion droplets : linking large deformation properties to sensory perception
    Sala, G. - \ 2007
    Wageningen University. Promotor(en): Martien Cohen Stuart, co-promotor(en): G.A. van Aken; F. van de Velde. - [S.l.] : S.n. - ISBN 9789085048329 - 235
    gelering - emulsies - druppels - mechanische eigenschappen - sensorische evaluatie - gelation - emulsions - droplets - mechanical properties - sensory evaluation
    Key words: polymer gels, particle gels, emulsion, large deformation, friction, sensory This thesis reports studies on the large deformation and lubrication properties of emulsion-filled gels and the way these properties are related to the sensory perception of the gels. The design of the studies included polymer and particle gels containing oil droplets of which the interaction with the gel matrix was varied, resulting in droplets either bound or unbound to the matrix. The unique combination of gel matrices and droplet-matrix interactions allowed to obtain a representative overview of the effect of the oil droplets on the properties studied. The molecular properties of the gel matrices determined the way the large deformation properties of the gels depended on the deformation speed. Polymer gels showed a predominantly elastic behaviour. Particle gels showed a more viscoelastic behavior. The effect of the oil content on the Young’s modulus of the gels was modulated by the droplet-matrix interactions, in agreement with existing theories. Bound droplets increased the Young’s modulus of the filled gels, whereas unbound droplets decreased it. Oil droplets embedded in the gel matrix acted as stress concentration nuclei. They also increased energy dissipation due to friction between structural elements of the gel (oil droplets and gel matrix). Stress concentration resulted in a decrease of the fracture strain for all gels and in a decrease of the fracture stress for polymer gels. For gels with non-aggregated bound droplets, a reduction in oil droplet size had the same effect on their rheological properties as an increase in oil volume fraction. The lubrication properties of the gels strongly depended on both the molecular and functional properties of the gel matrix and the oil content. For each type of gel matrix, the lubrication behaviour was affected by the ‘apparent viscosity’ of the broken gels, which in turn depended on the droplet-matrix interactions. The sensory perception of emulsion-filled gels appeared to be dominated by the properties of the gel matrix and by the oil content. Polymer gels were perceived as more melting, whereas particle gels were perceived as more rough. With increasing oil content both types of gels became more creamy and spreadable. The increase in spreadability and part of the increase in creaminess could be explained with the effect of the oil droplets on the breakdown properties of the gels. Since for all gels the scores for creaminess increased with increasing oil content, the release of oil droplets during oral processing could not completely explain the perception of oil-related sensory attributes. It is therefore concluded that the perception of these attributes is mediated by the lubrication properties of the broken gel. The large deformation and lubrication behaviour of the gels were the most important parameters related to sensory perception. Both parameters were affected by the droplet-matrix interaction. As a matter of fact, the droplet-matrix interaction affected the fracture behaviour of the filled gels, which was related to their spreadability, and the ‘apparent viscosity’ of the broken gels, which controlled the lubrication properties of these systems.
    Structure-rheology relations in sodium caseinate containing systems
    Ruis, H.G.M. - \ 2007
    Wageningen University. Promotor(en): Erik van der Linden, co-promotor(en): Paul Venema. - [S.l.] : S.n. - ISBN 9789085046486 - 125
    natriumcaseïnaat - reologische eigenschappen - afschuifkracht - gelering - emulsies - structuur - verzuring - spectroscopie - licht - verstrooiing - sodium caseinate - rheological properties - shear - gelation - emulsions - structure - acidification - spectroscopy - light - scattering
    The general aim of the work described in this thesis was to investigate structure-rheologyrelations for dairy related products, focusing on model systems containing sodium caseinate. The acid inducedgelationof sodium caseinate, of sodium caseinate stabilized emulsions, and the effect of shear on the structure formation was characterized. Special attention was given to the sol-gel transition point, which was defined by a frequency independent loss tangent. It was shown that the sol-gel transition point is completely controlled by the pH and the temperature, independent of the concentration sodium caseinate or the applied shear rate. Considering sodium caseinate solutions, increase of the temperature of acidification caused a decrease of the critical pH forgelationand a more dense gel structure. The formed gels were not in thermodynamicequilibrium,however, due to the slow kinetics of the system they were stable on the time scale of the experiment. At the gel point we have strong indications that the structure can not be characterized by a single fractal dimension. During the acid inducedgelationof sodium caseinate stabilized emulsions a single sol-gel transition was observed. Addition of an excess of sodium caseinate to the emulsion resulted in two sol-gel transitions upon acidification. Application of shear during the acidification of the emulsions showed a decreasing radius of the aggregates formed at thegelpointwith increasing shear rate. The aggregates formed becamemore densedue to the application of shear while the network that was formed by the aggregates became less compact. No shear induced alignment was observed of emulsion droplets dispersed in water or ina sodiumcaseinatesolution, while emulsion droplets dispersed in axanthansolution did align in a shear field. Addition of sodium inhibited the string formation of the emulsion droplets
    Interfacial properties of water-in-water emulsions and their effect on dynamical behavior
    Scholten, E. - \ 2006
    Wageningen University. Promotor(en): Erik van der Linden, co-promotor(en): Leonard Sagis. - Wageningen : - ISBN 9789085043669 - 148
    emulsies - grensvlak - oppervlaktespanning - dynamica - gelatine - dextraan - arabische gom - emulsions - interface - surface tension - dynamics - gelatin - dextran - gum arabic
    Keywords: biopolymer mixtures, water-in-water emulsions, phase separation, interfaces, tension, bending rigidity, permeability, droplet deformation, morphology.

    The objective of this work was to investigate interfacial properties of biopolymer-based water-in-water emulsions, and to determine the effect of these interfacial properties on the kinetics of phase separation and the deformation behavior of emulsions droplets in shear flow. Since the experimental determination of interfacial properties, such as interfacial thickness and bending rigidity is difficult, we have developed a model that determines these parameters from the experimentally accessible interfacial tension and the interaction potential of the dissolved biopolymers. From the results we could conclude that the thickness of these water/water interfaces is much larger than for oil/water interfaces. The bending rigidities for these interfaces were found to be very large compared to those of water/oil interfaces. The permeability of these interfaces was tested with the spinning drop and the droplet relaxation method. These water/water interfaces were found to be permeable to all ingredients in the system at long time scales (spinning drop experiments) and permeable to water for short time scales (droplet relaxation after cessation of a flow field). This permeability was incorporated into the description of the droplet relaxation time, from which the interfacial tension and the permeability can be deduced simultaneously. Due to the permeability, both the spinning drop method and the droplet relaxation method (without contribution of permeability) cannot be used to measure the interfacial tension accurately. Furthermore, both bending rigidity and permeability were incorporated into the description of coarsening ofbicontinuousstructures during phase separation. We found four different regimes for coarsening depending on whether the process is dominated by interfacial tension, bending rigidity or permeability.
    Membrane emulsification: droplet formation and effects of interfacial tension
    Graaf, S. van der - \ 2006
    Wageningen University. Promotor(en): Remko Boom, co-promotor(en): Karin Schroen; Ruud van der Sman. - Wageningen : s.n. - ISBN 9789085043485 - 159
    emulgering - emulsies - membranen - druppels - druppelstudies - oppervlaktespanning - simulatiemodellen - computersimulatie - kunstmatige membranen - emulsification - emulsions - membranes - droplets - droplet studies - surface tension - simulation models - computer simulation - artificial membranes
    Membrane emulsification is a relatively new technique to produce emulsions. In this method the oil phase is pushed through a membrane, a sieve with very small holes, and forms droplets in the water phase at the other side of the membrane. The most important advantage of this technique is that all the formed droplets have the same size and the emulsion has therefore a better quality. In this research, the influence of surfactants on the droplet formation and detachment process has been studied with the help of experiments and computer simulations.The results showed that both the concentration surfactant and the velocity of the oil phase flowing through the membrane influence the droplet size.
    Minerale olie met pyrethroide helpt tegen virus in Zantedeschia
    Leeuwen, P.J. van; Derks, A.F.L.M. - \ 2004
    BloembollenVisie 2004 (2004)36. - ISSN 1571-5558 - p. 24 - 25.
    bloembollen - zantedeschia - plantenvirussen - epidemiologie - insecten - chemische bestrijding - opbrengsten - emulsies - oliën - pyrethroïden - ornamental bulbs - zantedeschia - plant viruses - epidemiology - insects - chemical control - yields - emulsions - oils - pyrethroids
    Gewasbescherming tegen virus in Zantedeschia: onderzoek door PPO-Bloembollen naar het effect van bespuitingen met minerale olie en pyrethroide op virusaantasting en opbrengst
    Emulsion droplet spreading at air/water interfaces: mechanisms and relevance to the whipping of cream
    Hotrum, N.E. - \ 2004
    Wageningen University. Promotor(en): Martien Cohen Stuart, co-promotor(en): G.A. van Aken; Ton van Vliet. - [S.I.] : S.n. - ISBN 9789085040255 - 123
    emulsies - strooien - oppervlaktespanning - druppels - geslagen room - slagroom - emulsions - spreading - surface tension - droplets - whipped cream - whipping cream
    Keywords:emulsion, spreading coefficient, surface tension, emulsifier, whipped cream, dairy foam, partial coalescence

    In this thesis, the interaction between emulsion droplets and expanding air/water interfaces was investigated. The objective was to deepen our knowledge concerning the physical processes that take place at the expanding air surfaces that form during aeration of emulsions. Emulsions can become aerated as a result of various processing operations, for example, stirring or pouring. Moreover, emulsions may be aerated with the intention of producing an aerated food product such as whipped cream or ice cream. Emulsion droplet/air interaction can have important consequences for emulsion stability. For example, emulsion droplet spreading at the air/water interface can initiate a collective oil spreading mechanism, resulting in the spreading of many oil droplets. This may lead to coalescence of the emulsion droplets. The tendency for an oil droplet to spread at an expanding air/water interface depends on the values of the dynamic interfacial tensions at the air/water, oil/water and oil/air interfaces. This can be expressed in terms of a dynamic spreading coefficient; when the spreading coefficient is positive, oil spreads out of the droplets. Experimental results confirmed that oil indeed only spreads out of emulsion droplets if the dynamic spreading coefficient is positive. The tendency for an emulsion droplet to spread at the air/water interface could be controlled by manipulating the surface expansion rate, the protein type and concentration, and type and concentration of emulsifier in the emulsion. The presence of crystalline fat, although relevant to the stability of emulsions exposed to shear, was not found to influence the spreading behaviour of emulsion droplets at the air/water interface. The results of the emulsion droplet spreading experiments lead to the development of a model that describes the whipping time of cream in terms of the proportion of the air bubble surface for which the spreading coefficient is positive. Experimental results for the whipping of model creams could be well explained by this model.
    Microstructure, rheology and demixing in emulsions flocculated by polysaccharides
    Blijdenstein, T.B.J. - \ 2004
    Wageningen University. Promotor(en): Erik van der Linden, co-promotor(en): G.A. van Aken; Ton van Vliet. - Wageningen : S.n. - ISBN 9789058089588 - 121
    emulsies - uitvlokking - polysacchariden - reologie - structuur - emulsions - flocculation - polysaccharides - rheology - structure
    keywords: Emulsion, b-lactoglobulin, polysaccharides, salt, sucrose, depletion, bridging, percolation, microstructure, micro-rheology, rheology, demixing, creaming, network compression. Abstract In this thesis, a study is presented on gravity-induced demixing behaviour of oil-in-water emulsions, stabilised by b-lactoglobulin and flocculated by various polysaccharides. Flocculation by polysaccharides mainly results in formation of emulsion droplet networks and can proceed via depletion and via bridging. Structural and rheological properties of these different networks were investigated and compared on a micro-and macroscopic level. These properties were related to the demixing behaviour of the emulsions. For emulsion droplet networks, gravity-induced compression of the network leads to separation of a serum layer. For depletion-induced networks, the initial rate of demixing by network compression is usually low and at high polysaccharide concentrations, usually a delay-time is observed before substantial demixing occurs. This delay-time scales with the permeability of the network, the viscosity and the density difference between oil and water. Once demixing has started, the network quickly collapses until the emulsion droplets are packed rather closely together. In bridging-flocculated emulsions, the initial demixing rate is higher, but more water was retained at longer times. The effects of protein, sugar and salts on demixing of depletion-flocculated networks were investigated as well. Protein affected the rate of flocculation and counteracted network formation. Sucrose affected the demixing rate via the viscosity and density of the aqueous phase, but it did not affect the droplet-droplet interactions. Salt affected the electrostatic droplet-droplet interactions. As a result, depletion-flocculation by dextran was inhibited at low salt concentrations. Addition of Ca2+ ions led to a decrease in repulsion between the protein layers, resulting in stronger droplet-droplet bonds, reinforcing a droplet network and retarding network compression.
    Physico-chemical and functional properties of sunflower proteins
    Gonzalez-Perez, S. - \ 2003
    Wageningen University. Promotor(en): Fons Voragen, co-promotor(en): Harry Gruppen; A.L.J. Vereijken. - [S.l.] : S.n. - ISBN 9789058089045 - 145
    zonnebloemeiwit - fysicochemische eigenschappen - denaturatie - oplosbaarheid - schuimen - schuim - emulgeren - emulsies - sunflower protein - physicochemical properties - denaturation - solubility - foaming - foams - emulsifying - emulsions
    Keywords: Sunflower protein, Helianthusannuus ,helianthinin, albumins, solubility, structure,denaturation, pH, temperature, ionic strength,phenoliccompounds,chlorogenicacid, foams, emulsions, functionality

    The research described in this thesis deals with the relation between specific sunflower proteins, their structure and their functional properties as a function of extrinsic factors as pH, ionic strength and temperature.

    Sunflower protein isolate (SI) devoid ofchlorogenicacid (CGA), the mainphenoliccompound present, was obtained withoutdenaturationof the proteins. Sunflower proteins were found to be composed of two main protein fractions: 2S albumins or sunflower albumins (SFAs) andhelianthinin. Subsequently, these protein fractions werebiochemicallyand structurally characterized under conditions relevant to food processing.

    Depending on pH, ionic strength, temperature and protein concentration,helianthininoccurs in the 15-18S (high molecular weight aggregate), 11 S (hexamer), 7S (trimer) or 2-3S (monomer) form. Dissociation into 7S from 11S gradually increased with increasing pH from 5.8 to 9.0. Enhancing the ionicstrengthresulted in stabilization of the 11S form. Heating and lowering the pH resulted in dissociation into themonomericform ofhelianthinin. The 11S and 7S form ofhelianthinindiffer in their secondary structure, tertiary structure, and thermal stability. With respect to solubilityas a function of pH,helianthininshows a bell shaped curve with a minimum at approximately pH 5.0 at low ionic strength. At high ionic strength,helianthininis almost insoluble at pH< 5.0.

    The second main sunflower fraction,SFAs, revealed to be very stable against pH changes (pH 3.0 to 9.0) and heat treatment (up to 100°C), and their solubility was only marginally affected by pH and ionic strength. The solubility of the SI as a function of pH seems to be dominated by that ofhelianthinin.

    Foam and emulsion properties of the sunflower isolate as well as those of purifiedhelianthinin,SFAsand combinations thereof were studied at various pH values and ionic strengths, and after heat treatment. Sunflower proteins were shown to form stable emulsions, with the exception ofSFAsat alkaline and neutral pH values. Increasing amount ofSFAsimpaired the emulsifying properties. Regarding foam properties, less foam could be formed fromhelianthininthan fromSFAs, but foam prepared withhelianthininwas more stable againstOstwaldripening and drainage than foam prepared withSFAs. Increasing amounts ofSFAshad a positive effect on foam volume and a negative one on foam stability and drainage. It was found that treatments that increase conformational flexibility improve the emulsion and foam properties of sunflower proteins.
    Failure behaviour of adsorbed protein Layers: consequences for emulsion and foam stability
    Vliet, T. van; Aken, G.A. van; Bos, M.A. ; Martin, A.H. - \ 2003
    In: Food Colloids, Biopolymers and Materials. - Cambridge UK : Royal Society of Chemistry - ISBN 9780854048717 - p. 176 - 191.
    schuim - emulsies - eiwitten - reologische eigenschappen - foams - emulsions - proteins - rheological properties
    Entering and spreading of protein-stabilized emulsion droplets at the expanding air-water interface
    Hotrum, N.E. ; Cohen Stuart, M.A. ; Vliet, T. van; Aken, G.A. van - \ 2003
    In: Food Colloids, Biopolymers and Materials / Dickinson, E., van Vliet, T., Cambridge : Royal Society of Chemistry - ISBN 9780854048717 - p. 192 - 199.
    emulsies - schuim - schuimen - eiwitten - caseïnaten - grensvlak - mechanische eigenschappen - emulsions - foams - foaming - proteins - caseinates - interface - mechanical properties
    Colloids and interfaces in life sciences
    Norde, W. - \ 2003
    New York; Basel : Marcel Dekker - ISBN 9780824709969 - 433
    colloïden - colloïdale eigenschappen - grensvlak - oppervlaktespanning - emulsies - schuim - reologische eigenschappen - studieboeken - oppervlaktechemie - colloids - colloidal properties - interface - surface tension - emulsions - foams - rheological properties - textbooks - surface chemistry
    Emulsies uit membranen
    Boom, R.M. ; Gijsbertsen-Abrahamse, A.J. ; Schroën, C.G.P.H. - \ 2003
    Voedingsmiddelentechnologie 36 (2003)14/15. - ISSN 0042-7934 - p. 14 - 16.
    emulgeren - emulsies - membranen - energie - besparingen - conferenties - voedselindustrie - voedseltechnologie - emulgering - emulsifying - emulsions - membranes - energy - savings - conferences - food industry - food technology - emulsification
    Membraanemulgeren is een veelbelovende jonge techniek waarbij met weinig energie een betere productkwaliteit wordt verkregen. Op 4 juni werd door onderzoeksschool VLAG in Wageningen een themadag georganiseerd over het betreffende onderwerp. De aanleiding to deze dag was de promotie van ir. A. Gijsbertsen - Abrahamse. Zij promoveerde als eerste in Nederland op deze nieuwe techniek
    Membrane emulsification: process principles
    Gijsbertsen-Abrahamse, A.J. - \ 2003
    Wageningen University. Promotor(en): Remko Boom, co-promotor(en): A. van der Padt. - [S.I.] : S.n. - ISBN 9789058088451 - 104
    emulsies - emulgeren - membranen - druppelstudies - computationele vloeistofdynamica - emulsions - emulsifying - membranes - droplet studies - computational fluid dynamics
    With membrane emulsification in principle monodisperse emulsions can be produced, requiring a relatively low energy density which implies that the shear stress exerted on the ingredients is low. In membrane emulsification the to-be-dispersed phase is pressed through the membrane pores; under certain conditions droplets are formed at the membrane surface. In cross-flow membrane emulsification the droplets are detached by the continuous phase flowing across the membrane surface. A limiting factor for emulsion production on a commercial scale will be a low disperse phase flux. Better knowledge of how membrane parameters affect the disperse phase flux would enable the targeted development of membranes, optimal for the process of cross-flow membrane emulsification for a given application. Therefore, the objective of this research is to gain a fundamental understanding of the mechanism of droplet formation at the membrane surface and of the flow of the disperse phase through the membrane as a function of the membrane characteristics.
    Droplet formation was studied at a microscopic level with computational fluid dynamics (CFD) simulations and by microscopic experiments of droplet formation at a very thin microsieve with uniform pores. Since these membranes are extremely well defined, they are a good model system for detailed study. Results from both simulations and experiments indicate that to prevent coalescence and steric hindrance of droplets, the membrane porosity should be very low. Steric hindrance resulted in polydisperse emulsions and led to coupling of droplet detachment from neighboring pores. Furthermore, although the pores all had the same diameter, the number of pores at which droplets were formed only increased gradually with increasing transmembrane pressure. This effect was further studied with a scaled-up analogon and could be modeled by taking the resistance of the pores and the resistance of a membrane substructure into account. This model is compared with a model for flow through an isotropic membrane with interconnected uniform pores and extended to describe flow through a membrane with a pore size distribution. This model is used to show that in most cases the estimation of a membrane pore size distribution by using the liquid displacement method is not correct. Just as in membrane emulsification, pores become active at higher transmembrane pressures than expected. Finally, the effects of several membrane parameters on membrane emulsification performance are summarized. As an example, the membrane area required for a typical industrial application is estimated using the models mentioned above, for different types of membranes.
    Biochemical and functional characterisation of casein and whey protein hydrolysates : a study on the correlations between biochemical and functional properties using multivariate data analysis
    Ven, C. van der - \ 2002
    Wageningen University. Promotor(en): A.G.J. Voragen; H. Gruppen; D.B.A. de Bont. - S.l. : S.n. - ISBN 9789058086532 - 155
    melkeiwitten - caseïnehydrolysaat - wei-eiwit - eiwithydrolysaten - peptiden - multivariate analyse - vloeistofchromatografie met omgekeerde fase - gelfiltratiechromatografie - infraroodspectroscopie - molecuulgewicht - emulsies - schuim - oplosbaarheid - bitterheid - milk proteins - casein hydrolysate - whey protein - protein hydrolysates - peptides - multivariate analysis - reverse phase liquid chromatography - gel filtration chromatography - infrared spectroscopy - molecular weight - emulsions - foams - solubility - bitterness

    Whey protein and sodium caseinate were hydrolysed with commercially available enzyme preparations. The resulting hydrolysates were characterised using several analytical characterisation methods and by determination of several functional properties. Subsequently, correlations between the biochemical characteristics themselves and between biochemical and functional properties were studied using multivariate regression analysis.

    Biochemical characteristics of hydrolysates were determined using unifactorial methods like the degree of hydrolysis, and by multifactorial methods, i.e . reversed phase (RPC) and size exclusion chromatography (SEC), and Fourier transform infrared (FTIR) spectroscopy. FTIR spectroscopy appeared to discriminate most effectively between hydrolysates made from different protein sources and classes of proteolytic enzymes, followed by RPC and SEC.

    Emulsion and foam properties of hydrolysates were similar or inferior to those of the parental proteins. Casein hydrolysates generally showed better emulsion and foam forming ability than whey protein hydrolysates. Foam forming ability of whey protein hydrolysates was correlated to the molecular weight distribution (MWD) of the peptides, showing that especially peptides with MW of 3-5 kDa contributed to foam forming ability.

    Concerning prevention of emulsion instability due to coalescence it was shown that peptides with a molecular weight larger than 2 kDa are needed. Foam stabilising ability of casein hydrolysates also depended on the MWD of hydrolysates, but higher molecular weight peptides, i.e. larger than 7 kDa, were needed to obtain good foam stability.

    The ability of the three multifactorial characterisation methods (SEC, RPC, FTIR spectroscopy) to predict functional properties was investigated. It appeared that SEC profiles were able to predict emulsion and foam stability of all hydrolysates, as well as foam forming ability, Angiotensin Converting Enzyme (ACE) inhibiting ability and bitterness of whey hydrolysates. RPC profiles were also able to predict these properties and additionally predicted solubility and bitterness of casein hydrolysates. FTIR spectra were best suited to predict a variety of hydrolysate properties, since apart from the before-mentioned properties, the spectra can also be used to predict emulsion forming ability and to improve prediction of bitterness of hydrolysates.

    Finally, the influence of hydrolysis process conditions on ACE inhibiting ability of whey hydrolysates was investigated, showing that ACE inhibiting activity could be optimised by using process optimisation techniques like experimental design and response surface optimisation.

    Verlagen van het stofgehalte in varkensstallen door periodiek vernevelen van een olie-emulsie
    Roelofs, P.F.M.M. ; Binnendijk, G.P. - \ 2001
    Lelystad : Praktijkonderzoek Veehouderij (Rapport / Praktijkonderzoek Veehouderij 208) - 26
    varkenshouderij - varkensstallen - stof - stofbestrijding - raapzaadolie - spuiten - emulsies - kostenanalyse - pig farming - pig housing - dust - dust control - rapeseed oil - spraying - emulsions - cost analysis
    Op het Varkensproefbedrijf in Rosmalen is tussen maart 1995 en december 1999 (met onderbrekingen) onderzoek gedaan naar het effect van het onder hoge druk vernevelen van een olie-emulsie. Hierbij werd gebruik gemaakt van het KEW Dust binding system
    Formation and stability of emulsions made with proteins and peptides
    Smulders, P.E.A. - \ 2000
    Agricultural University. Promotor(en): P. Walstra. - S.l. : S.n. - ISBN 9789058083135 - 143
    emulsies - formatie - stabiliteit - caseïne - lactalbumine - lysozym - ovalbumine - peptiden - emulsions - formation - stability - casein - lactalbumin - lysozyme - ovalbumin - peptides

    The formation and stabilization of oil-in-water emulsions using well-defined and well-characterized proteins and peptides was studied in order to elucidate the relation between their molecular and functional properties. The emulsions were formed with a high-pressure homogenizer. To study the effect of the homogenizer scale on the emulsion properties, emulsions were prepared with a laboratory and a small industrial homogenizer. The flow in the industrial homogenizer was shown to be turbulent. In the laboratory homogenizer, droplet break-up was found to occur in a bounded laminar type of flow, resulting in a poor operating efficiency. The effect of the flow type on the emulsion properties, however, appeared to be small, if the number of passes through the laboratory homogenizer was sufficiently high.

    Proteins appeared to have good emulsion forming properties as long as protein aggregation was absent. In those cases, the recoalescence rate during homogenization was found to be similar and only small differences in the droplet size of emulsions could be determined. The surface excess of the emulsion droplets appeared to be governed by the conformational stability and the aggregated state of the proteins. Globular proteins with a high conformational stability yielded relatively low surface excesses, while a flexible random coil protein, likeβ-casein, yielded a relatively high surface excess. Protein aggregation may be due to physicochemical conditions and surface or heat denaturation. If protein aggregates were present, the emulsion droplets were also often aggregated. The droplet size, surface excess, and rate of recoalescence of these aggregated emulsions were usually found to be relatively high.

    The emulsion forming properties ofβ-casein peptides appeared to be comparable or superior to those of intact proteins. Amphiphilic peptides without the hydrophobic C-terminal domain ofβ-casein yielded a relatively low surface excess, likely due to strong electrostatic interactions between the highly charged groups of the N-terminal end. The surface excess of emulsions made with hydrophobic peptides with a removed N-terminal domain was comparable to those of emulsions made with intactβ-casein. The peptides were due to their relatively small molar mass more readily desorbed from the oil/water interface than intact proteins.

    The coalescence stability of emulsions made with proteins was high even at low protein concentrations and appeared to be mainly determined by the surface excess of the droplets. The emulsion stabilizing properties ofβ-casein peptides were inferior to those of intact proteins probably due to their relatively low molar mass. Comparison of the stability of emulsions made with amphiphilic peptides with an intact or partially removed N-terminal domain showed that this domain was of great importance for providing stability against coalescence. The coalescence stability of emulsions made with hydrophobic peptides was relatively high, which was attributed to the high surface excess of the droplets. The electrostatic and steric interactions appeared to be of great importance for stabilizing emulsions made with peptides against coalescence as was indicated by the effect of changes in pH and ionic strength on the stability.

    Keywords: emulsions, formation, stability, molecular properties,β-casein,β-lactoglobulin,α-lactalbumin, lysozyme, ovalbumin, peptides.

    Static and dynamic properties of proteins adsorbed at liquid interfaces
    Benjamins, J. - \ 2000
    Agricultural University. Promotor(en): J. Lyklema; E.H. Lucassen-Reynders. - S.l. : S.n. - ISBN 9789058083173 - 212
    colloïdale eigenschappen - eiwitten - adsorptie - schuim - emulsies - colloidal properties - proteins - adsorption - foams - emulsions

    The aim of the investigation described in this thesis was to increase the level of understanding of the role that proteins play in the preparation and subsequent stabilisation of foams and emulsions. One aspect of this role is facilitation of break-up, due to surface tension lowering. A second aspect is the formation of a viscoelastic interfacial layer, which affects both the short-term and long-term stability of the dispersion. Therefore, a systematic study of the changes in static and dynamic interfacial properties induced by proteins was carried out.

    For part of this study, dealing with the interfacial rheology, several experimental techniques were used. These techniques were either properly modified existing techniques (Chapter 3, modified longitudinal wave set-up) or newly developed (Chapter 4, Dynamic Drop Tensiometer; Chapter 5, Concentric Ring Surface Shear Rheometer) to meet the requirements for measuring the rheology of adsorbed protein layers at liquid/liquid interfaces. These requirements are (i) isotropic deformation, without leakage of the interfacial layer, for the dilational modulus measurements at air/water and oil/water interfaces and (ii) shear modulus measurements at small oscillatory deformation.

    The proteins chosen for this study wereβ-casein,β-lactoglobulin (BLG), bovine serum albumin (BSA), ovalbumin and lysozyme. This set of proteins was chosen, because they differ considerably in relevant aspects, such as molecular weight, molecular structure and iso-electric point.

    In Chapter 1 the scope and context of this study are given including a brief introduction into (i) the molecular properties of these proteins, that are relevant to the adsorption, (ii) protein adsorption and interfacial rheology, and (iii) the relation between interfacial properties and the properties of emulsions and foams.

    Chapter 2 deals with the adsorption of proteins at the air/water interface. The adsorption was determined by ellipsometry, a method by which not only the adsorbed amount but also the layer thickness and protein concentration in the adsorbed layer could be determined. The ellipsometric studies were combined with surface tension measurements at the same surface.

    All proteins examined show high affinity adsorption, i.e. strong adsorption at low concentration in solution. The initial rate of adsorption of all proteins is well described by a simple diffusion equation. For all proteins examined, the value of the surface pressure (Π) are protein-specific, but otherwise unique, time-independent functions of the adsorption (Γ). Time independence of theΠ(Γ) curve was concluded from the finding thatΠandΓpairs measured at different bulk concentrations and at different stages of adsorption, all collapse into one single curve. In other words, each protein has a unique surface equation of state indicated by its measuredΠ(Γ) curve. This curve reflects the relative rigidity of the protein molecule. For flexible molecules likeβ-casein and PVA ,Γ min (=ΓwhereΠstarts to deviate measurably from zero) is low and from this point onward the surface pressure increases gradually with increasingΓ. For rigid globular proteins (BSA, ovalbumin and lysozyme)Γ min is higher and with further increase of the surface concentration the surface pressure increases steeply. At high protein concentration and long adsorption times, for most proteins multilayer adsorption takes place.

    For ovalbumin, in the pH range 4-8 the effect of pH on theΠ-Γcurve is small, which indicates that electrostatic intermolecular forces do not contribute much to the surface pressure.

    In Chapter 3 a longitudinal wave technique, modified to ensure isotropic surface deformation, was used to determine the dilational modulus,ε, of adsorbed protein layers, at the air/water interface. This modification fully eliminated the complicating shear effects that became apparent in dilational modulus measurements with adsorbed layers of proteins in a conventional set-up.

    For all proteins examined at frequencies in the range from 0.01 to 1 rad/s, the initial part of theε(Π) plot is a straight line through the origin. The slope of this initial part ranges between +4 and +12 . No clear relationship between the slope and the rigidity of the protein molecule was found. However, the extent of this linear range is smaller for the flexible molecules (β-casein and PVA). From the fact that this slope significantly exceeds the ideal value of +1, it must be concluded that the behaviour of the adsorbed layer is far from ideal. In the linear range, the measured moduli coincide with the limiting moduli,ε 0 , calculated from theΠ(Γ) curve. This indicates that the surface pressure adjusts "instantaneously" to the changing adsorption during a compression-expansion cycle in time-scales ranging from 1 to 100 s. This also means that the modulus is purely elastic, i.e. the effect of relaxation processes is negligible. In this elastic range, differences between individual proteins are related to different degrees of non-ideality, reflected in the surface equation of state.

    At higher surface concentrations a relaxation mechanism becomes operative, which is most probably not caused by diffusional exchange between surface and solution. This conclusion is based on calculations of the diffusional transport rate and the theoretical frequency spectrum of the modulus. Relaxation due to conformational changes is plausible. In the visco-elastic regionε≥ε 0 for all proteins examined. This is an extra argument against diffusional exchange.

    The modulus increases in the order: PVA <β-casein

    Chapter 4 describes a new method, the Dynamic Drop Tensiometer, especially suitable for determining the dynamic properties of proteins adsorbed at oil/water interfaces. According to this method, a small drop is subjected to sinusoidal oscillations of its volume. The corresponding area changes produce interfacial tension changes, which are evaluated from measurements of the fluctuating shape of the drop, using the Young-Laplace equation. Compared to the conventional Langmuir trough set-up, this method is particularly suited for liquid/liquid interfaces, because (i) interfacial leakage is fully eliminated and (ii) uniform deformation is ensured even if one of the liquids is a viscous oil. An additional advantage of the method is its short response time. The dynamic properties of adsorbed protein layers at three interfaces (TAG (triacylglycerol)-oil/water, tetradecane/water and air/water) were compared. At the three interfaces, at low protein concentration, the conformation change upon adsorption is fairly fast, occurring within 1 min.. However, at high protein concentration (> 1g/l), during the first minutes after adsorption a situation exists that differs from the equilibriumΠ(Γ) curve. At low interfacial pressures, during a modulus measurement, the adaptation of the conformation is faster (< 1 s.). Non-ideality of the adsorbed layer increases in the sequence TAG-oil < tetradecane < air, which is probably related to a decrease of solution quality for the more hydrophobic amino acids, which decreases in the same sequence. At each of the different interfaces non-ideality increases with increasing rigidity of the protein molecule (β-casein<β-lactoglobulin

    The surface shear properties of adsorbed protein layers are described in Chapter 5. These properties were determined with a newly developed concentric ring surface shear rheometer. The technique allows measurements over a wide range of frequencies and deformations. As the magnitude of the shear deformation markedly affects the shear modulus,μ s , an extrapolation to zero deformation is required to asses the shear properties of the undisturbed surface. Because the surface dilational modulus and the surface shear modulus both increase in the sequence PVA< Na-caseinate s ≥3 indicates that the adsorbed protein layer can be modelled as a thin homogeneous gel layer. Such a model points to a significant ideal monolayer contribution toεat low to medium surface concentrations.

    In Chapter 6 models describing the surface equation of state of adsorbed macromolecules were applied to the experimentalΠ(Γ) curves. These models were also applied to understand the dynamic behaviour of these layers. Statistical models, in which it is assumed that the macromolecules adsorb with all segments in direct contact with the surface, e.g. Singer equation, only explain the very low pressure part of the experimental curves of PVA andβ-casein. To explain the higher pressure part, progressive loop formation and molecular interaction must be accounted for. For rigid globular proteins, simple statistical models are unable to fit any part of the experimental curves, because such molecules only slightly change their conformation upon adsorption and consequently, will adsorb with only a small fraction of the segments at the surface, even at very low pressures.

    A 2-D solution model, which accounts to first order for both entropy and enthalpy, is used to describe the non-ideal behaviour of adsorbed protein layers. This non-ideality was deduced from the highΓneeded to produce a measurableΠand the steep initial slopes of theε(Π) curves.

    All above models need modification to describe the S-shaped part of theΠ(Γ) curves at high surface concentrations. This part of the curve can be described by the Soft Particle concept, which is a modification of the surface equation of state of a 2-D hard sphere fluid. The S-shape is attributed to a decrease of the molecular cross-sectional area with increasing surface concentration. This effect appears to be more pronounced for flexible molecules like PVA andβ-casein than for globular rigid molecules like BSA, ovalbumin and lysozyme. Experimentalε(Π) curves are within the limits that are predicted by this concept. A promising option is combining a molecular compressibility as used in the Soft Particle concept with the 2-D solution model.

    In Chapter 7 it is shown that interfacial properties typical for proteins predict a larger drop size and a lower stability against recoalescence during production compared to low molecular weight (LMW) surfactants.

    In the presence of both types of surfactant, concentrations and conditions can be chosen such that the LMW surfactant determines the dispersion efficiency, while the protein determines the long-term stability. A comparison between the different proteins reveals that, in the production stage, a higher dilational modulus at short times correlates with a faster build-up of stability against recoalescence. For a good long term stability a high dilational modulus of adsorbed protein layers at longer times is more important. In foams, retardation of Ostwald ripening, i.e. the growth of large bubbles at the expense of small ones, is probably the major factor. This mechanism depends on the ratio of the modulus to the surface tension, which ratio is considerably higher for proteins than for LMW surfactants in relevant cases.

    For a measurable shear modulus a high surface concentration is required. Therefore, shear properties may only affect long term stability of emulsions and foams, but not break-up and stability against recoalescence during production.

    Dubbele emulsies controleren afgifte of scheiding voedingscomponenten
    Horst, B.M. van der; Langelaan, H.C. - \ 1999
    Voedingsmiddelentechnologie 32 (1999)24. - ISSN 0042-7934 - p. 10 - 16.
    ingrediënten - voedselindustrie - voedseltechnologie - emulsies - dispersie - emulgeermiddelen - scheiding - segregatie - zuiveren - filtratie - technologie - verwerking - vloeistoffen (liquids) - pigmenten - modellen - ingredients - food industry - food technology - emulsions - dispersion - emulsifiers - separation - segregation - purification - filtration - technology - processing - liquids - pigments - models
    Een nieuw concept voor het scheiden van voedingscomponenten, alsmede voor toepassing in de cosmetische en farmaceutische industrie. Het onderzoek op dit gebied door ATO-DLO is gericht op het encapsuleren van (vluchtige) componenten, nieuwe emulgeringsmethoden, bereidingsaspecten, lange termijnstabiliteit, reologische eigenschappen, typen emulgatoren en gecontroleerde afgifte. Tevens werd een modelsysteem ontwikkeld voor het verwijderen van ongewenste componenten uit voedingsmiddelen
    The adsorption of chymosin and lysozyme onto emulsion droplets and their association with casein
    Roos, A.L. de - \ 1999
    Agricultural University. Promotor(en): P. Walstra. - S.l. : De Roos - ISBN 9789058080332 - 115
    caseïne - chymosine - lysozym - emulsies - zuivelchemie - casein - chymosin - lysozyme - emulsions - dairy chemistry

    The proteolytic action of proteases present in cheese plays a major role in the ripening of cheese. These proteases originate from the rennet, the starter cultures and from the milk itself. The proteolysis in cheese results in the degradation of the casein proteins into smaller peptides and free amino acids, which act as flavour precursors. The ripening of cheese under conditioned storage is time consuming and costly. Addition of specific enzymes to the cheese milk is one of several options to accelerate ripening. A major problem then is that hardly any of these proteases end up in the cheese and most disappear with the whey stream. Entrapment of bacteria and milk fat globules into the casein matrix of the curd is due to their particle character. Immobilisation of proteases onto particles would thus result in retention of these proteins in the curd. Ideally, for reasons of acceptance, these particles should originate from the milk itself or at least be edible.

    In this study, in a more general approach, soya oil emulsion droplets and casein micelles, being protein aggregates, were tested as the carrier system. Chymosin and lysozyme were taken as the enzymes to be immobilized, because of their relevance to the dairy industry and because they are scientifically well-known. Moreover, their biochemical divergence make them suitable models for study.

    The literature provides several studies on adsorption of proteins onto interfaces. Few of these proteins are enzymes. In cases where lysozyme was studied, it was included because of its extraordinary properties as a protein and not because of its enzymatic activity. Apart from (phospho)lipases there is scarcely any literature that describes activity of enzymes adsorbed onto the oil/water or air/water interfaces. Proteins tend strongly to accumulate in interfaces and for that reason are said to be very surface active. This adsorption is accompanied by a conformational change of the three-dimensional structure of the protein that results in some unfolding, ranging from almost full stretching of the peptide chain to a more conserved conformation. Hydrophobic residues or patches of the protein, mostly buried inside the molecule, will tend to position themselves next to or even protrude partly into, the hydrophobic phase of an oil/water interface.

    The extent of conformational change depends on the conformational stability of the protein, which, in turn, depends on pH, temperature, ionic strength etc. Furthermore, the extent of unfolding will be dependent on the surface area available and on the time scale, and hence, on protein concentration. In a static condition, adsorption at the interface will be diffusion driven, whereas during emulsification the time of adsorption will be determined by convection and will be very much shorter. Consequently, conformational changes of proteins during emulsification should be smaller because full surface coverage may be reached before unfolding can occur. In the case of enzymes being adsorbed the emulsification process would therefore offer an opportunity to retain activity.

    The relation between the surface pressureΠi.e. the extent of surface tension decrease and the amount of protein adsorbed per unit surface area availableΓ, provides a possibility to relate the size of the adsorbed protein molecules, and thereby the extent of unfolding, to the surface load. As mentioned earlier the extent of unfolding should be less at a higherΓvalue. It has been calculated that lysozyme, an enzyme of high conformational stability, hardly unfolds at the air/water interface, even at low surface coverage. For the oil/water interface, however, a considerable increase in the radius of the protein molecules was observed at low surface load. At surface coverage of > 1.5 mg.m -2, the radius remained more or less constant, indicating that substantial unfolding did not occur. Despite this rigidity, the enzyme had lost all of its enzymatic activity in situ and it even remained inactive after desorption.

    Apparently, conformational changes in the enzyme molecule do not necessarily become manifest in a larger size for the molecule. Chymosin, being an enzyme of smaller conformational stability, naturally lost all of its activity due to adsorption onto the oil/water interface. In experiments with the enzymes coadsorbed simultaneously with bovine serum albumin, or the one after the other, there was no retention of in situ activity. Chymosin also proved to be inactivated at the expanding air/water interface due to air incorporation, if this occurred e.g. during homogenization.

    During the cheese-making process enzymes like chymosin and lysozyme are retained in the curd. This retention must be due to association of the enzymes with the casein from milk. In order to adsorb the enzymes with retention of activity, the various casein fractions were used to make and stabilize a soya-oil emulsion, and the enzyme was subsequently allowed to associate with the casein. The extent of association of lysozyme with the casein fractions was in the orderα s -casein >β-casein >κ-casein. Only for theκ-casein stabilized emulsion, was lysozyme association dependent on pH within the range of pH 5.2 - 6.4 (greater for a lower pH). Furthermore, the association with the caseins was not dependent on temperature, indicating that hydrophobic interactions were not predominant. The same trends were found with the various caseins in solution, albeit that association withκ-casein hardly occurred. It should be kept in mind that casein adsorbed at an interface will expose other amino acid residues compared to its behaviour when free in solution. For that reason the association behaviour in the two systems may differ.

    Because the association varies between caseins the extent of association with lysozyme depended on the composition of the casein micelles (aggregates of many casein molecules and calcium phosphate, as occurring especially in milk). As expected, casein micelles containing a higher proportion ofκ-casein associated less with lysozyme. It was found that lysozyme did not lose activity due to association with casein adsorbed on soya oil droplets or free in solution. However, lysozyme activity was markedly reduced when the enzyme was associated with casein micelles. In this system lysozyme also associated with casein in the interior of the casein micelle. The apparent loss of activity was most probably due to internal diffusion limitation. The difference of association for the various systems was also reflected in the free equilibrium concentration at which the surface excess plateau value was reached. In the system of adsorbed caseins this value was reached at a free lysozyme concentration of about 3 _M, whereas for the micellar system this value was about 100 times higher.

    The association of chymosin with casein has been studied in the same three systems of casein adsorbed onto soya-oil emulsion droplets, caseins in solution and caseins aggregated in casein micelles. It appears that chymosin only associated with adsorbedκ-casein and not with adsorbedαs- orβ-casein. Preceding the association, the caseinomacropeptide part ofκ-casein is split off, followed immediately by the aggregation of the soya-oil emulsion droplets containing the remaining para-κ-casein. This coagulation behaviour is identical to the renneting of milk during the cheese-making process. The association characteristics for chymosin are also comparable. The association was strongly dependent on pH and ionic strength, and on chymosin and casein concentration. Theκ-casein stabilized emulsion has proven to be a good model system for studying chymosin retention in curd. The chymosin associated with para-κ-casein was shown to be still active on addedκ-casein or on a fluorescent small hexapeptide substrate. Consequently, the active centre of the enzyme is presumably not involved in the association with casein.

    The association of chymosin with caseins free in solution has also been studied. Only in a solution containingκ-casein will addition of chymosin result in protein flocculation and precipitation. This flocculation is due to splitting off the caseino-macropeptide part ofκ-casein and the consecutive aggregation of the fairly hydrophobic and almost electrically neutral para-κ-casein molecules. The precipitated protein fraction also contains associated chymosin, to an extent depending on conditions like pH, ionic strength and casein and chymosin concentrations. In this system time and temperature also affected the extent of chymosin association. The association decreased with increased contact time and was stronger at higher temperatures.

    The protein content in the supernatant after centrifugation increased not only due to dissociation of chymosin but also due to the presence of casein fragments. Apparently, the dissociation of chymosin was related to its proteolytic action. The dissociation rate increased with decreasing pH where chymosin becomes more active and less specific. The dissociation also increased with temperature for a given time of contact. However, when extrapolated to a contact time of t = 0 (i.e. when dissociation due to proteolysis has not occurred yet) the association was observed to be somewhat stronger for a higher temperature. The effect of temperature on the proteolysis-dependent dissociation, apparently was stronger than its effect on the increase of the association. Since chymosin association depends on mutual association of caseins (see below), it will also depend on the temperature dependence of the latter. Dissociation of chymosin was not found in the system of caseins adsorbed onto emulsion droplets.

    The addition of small amounts ofαs- orβ-casein strongly decreased the extent of association of chymosin with para-κ-casein. This effect was stronger forαs-casein than forβ-casein. It was also found that the extent of chymosin association (moles of chymosin per mole of para-κ-casein) was larger when the system was diluted or, in other words, when the casein concentration was reduced. Both phenomena can be explained by assuming that competitive association occurs between the caseins and chymosin for interaction with a para-κ-casein molecule. Chymosin is only able to associate with a para-κ-casein molecule when that is not associated with other casein molecules. Thermodynamically speaking, the extent of association of chymosin is determined by the association constants that exist between all caseins under conditions as in the system. These association constants vary with pH, ionic strength, casein concentration and temperature.

    The model of competitive association is further developed and applied to the association of chymosin with casein micelles of various composition. It follows that chymosin will associate less with casein micelles composed ofαs- andκ-casein than with micelles composed ofβ- andκ-caseins. Again, this behaviour can be explained by competitive association, since different association constants exist for the caseins and chymosin for association with para-κ-casein. The relations for association and dissociation found in this casein micelle system are comparable with those found with caseins in solution. The kinetic model for competitive association is only a crude approximation. It does not provide possibilities of calculating all the association constants occurring in milk from the relations found from retention of chymosin in curd.

    Physics of foam formation on a solid surface in carbonated liquids
    Zuidberg, A.F. - \ 1997
    Agricultural University. Promotor(en): A. Prins; H.J. Bos. - S.l. : Zuidberg - ISBN 9789054856979 - 236
    suspensies - emulsies - systemen - vloeistoffen (liquids) - gassen - fasen (chemie) - suspensions - emulsions - systems - liquids - gases - phases

    The amount and size of bubbles in a foam layer that have originated from a solid surface in a gas supersaturated solution is largely determined by the physical properties of that solid and liquid surface and the supersaturation level of the gas in the liquid. The presence of pre-existent nuclei - gas trapped in pores at the solid surface - as well as the wetting properties of the liquid on that surface contribute to the formation of the bubbles. The supersaturation level of gas in the liquid contributes to the rate at which this phenomenon occurs. The main objective of this study is to quantify their contributions in order to establish the relation between a) the physical properties of the supersaturated liquid and solid surface as well as the shapes of the liquid and the solid phases and b) the foam formation in a supersaturated solutions. Therefore: 1) the growth, detachment and rise of individual bubbles forming at an "active" site - a pre-existent nucleus trapped in a cavity in a solid surface - is studied, 2) based on this knowledge the requirements of an ideal "active" surface - a collection of "active" sites - is formulated, and 3) with a selected "active" surface, the hypothesis of the requirements are tested, by determining the foaming properties of the "active" surface in gas supersaturated solution.

    In the process of bubble formation at an "active" site in a gas supersaturated solution, the following steps can be distinguished:

    1) The growth of a bubble at an "active" site.
    By using a model "active" site in the experiments- the tip of a glass capillary which is closed at one end - the following variables have been studied: the mouth radius of the "active" site, the supersaturation of the surrounding solution, the wetting behaviour of the "active" site, the flow of the liquid along the bubble, and dynamic surface properties.

    We have observed the effect of the gas supersaturation on the individual bubble growth rate. With three different concentrations of carbon dioxide gas, the growth of individual bubbles on different sized glass capillaries were measured and compared to several growth models. The growth rates of the bubble could be best described by Bisperink's growth model, which takes into account the fact that the bubble is attached to a solid surface, for the movement of the bubble interface into the liquid as well as for the depletion of gas from the layer of liquid surrounding the bubble during growth (chapter 3).

    During the experiments, it was observed that the formation of extra, unwanted bubbles, also called "parasite bubbles", influenced the bubble growth rate as a result of the entrainment of liquid. Due to this phenomenon, it was difficult to make a theoretical assessment of bubble growth rates. The opposite effect was also observed, namely at the lowest concentration, where after the formation of a single bubble, the liquid surrounding
    the bubbles was so much depleted of gas, that the growth rate of the succeeding bubble was also affected (chapter 3). The dynamic surface properties were found not to play an important part in the process of bubble formation as the velocity of growth was slow enough to keep the system close to equilibrium (chapter 3).

    2) The detachment of a bubble from an "active" site.
    By using the same glass capillary as model "active" site the following variables have been studied: the mouth radius of the "active" site, the angle of inclination of the "active" site, the surface tension and the surface rheological properties of the bubble surface, the hinterland effect, the bubble growth rate and the liquid flow around the bubble.

    The volume of the detaching bubble has a linear relationship with the internal radius of the "active site", for a well- wetted surface and as long as the shape of the bubble resembles a sphere (capillary radius < 0.5 mm). For a larger capillary size, and thus a large bubble size, the bubble resembles a pear shape (as shown on the cover of this thesis) and detaches with a smaller volume than predicted theoretically, as a result of mechanical instabilities (chapter 3).

    When the capillary or model "active site" is tilted, the bubble size at detachment is decreased as a result of a lower effective adhesive force to the surface. The bubble size as a function of the tilting angle can be predicted theoretically, but experimental assessment of the theory shows some deviation (chapter 4).

    To simulate the effect of liquid flow on bubbles attached to a solid surface, a horizontal oscillating movement was imparted to the capillary on which the bubbles grow. Increasing both amplitude and frequency of the oscillation decreases the size of the bubble at detachment. A theoretical examination of this phenomenon is given in chapter 4, and one of the conclusions is that the decrease in bubble size cannot be explained alone with the shear and inertial forces, We have suggested that a "surface skin" is formed, and that the bubble becomes unstable and detaches as soon as the surface skin is mechanically ruptured by shear forces parallel to the surface.

    The internal volume of the cavity, also called the "hinterland volume" was found to have less effect on the bubble growth rate than was expected, as the dry internal walls of the cavity behaved more hydrophobically than expected. However, due to a sometimes considerable retreat of the air-liquid interface into the glass capillary after bubble detachment, it was concluded that a large hinterland volume could, in extreme cases, lead to an almost complete stop of bubble formation (chapter 3).

    Surface rheological properties were found to be of influence on the detachment of bubbles under the influence of liquid flow. It was found that the bubbles detached at an earlier stage than could be predicted from forces alone. We therefore speculate that due to periodic surface expansion and compression, a "skin" is formed which, when ruptured, unbalances the bubble and induces it to detach (chapter 4).

    3) The rise and growth of the detached bubble on its way to the foam layer. In a model experiment, single bubbles released from a tilted capillary tip were studied during their rise through the liquid. Variables were: the bubble size, the surface rheological properties, the liquid viscosity, the travelled distance, the degree of supersaturation and the distance between two consecutive bubbles. After the bubbles detach, they rise through the supersaturated liquid to the foam layer. In a gas supersaturated solution relatively free of surfactant, the bubbles are found to grow considerably during the travelled distance, but bubbles rising in a gas supersaturated solution like beer hardly grow during rise. It was therefore speculated that in beer, a surface "skin" could be formed at the bubble interface which is either mechanically too rigid to grow, or is partly insoluble to gas molecules which reduces the mass transfer of gas through the interface to the bubble (chapter 5).

    Based on the knowledge obtained in the first part, the second part of this thesis was dedicated to formulating the requirements of an ideal "active" surface, selecting such an "active" surface, and relating the foamability to the material properties of that surface (chapters 6 and 7). From a fimdamental point of view, an ideal "active" surface for the production of foam out of a supersaturated solution has to fulfil the following requirements:
    1) The "active" sites are situated at such a lateral distance from each other that coalescence between neighbouring bubbles is prevented.
    2) The number of "active" sites is big enough to produce a foam layer of pre-set volume in the required time.
    3) The "active" surface is situated in a vertical position allowing that bubbles are formed on both sides.
    4) The outside part of the "active" surface is hydrophilic to ensure a controlled small bubble size.
    5) The inside surface of the "active" sites are hydrophobic to ensure a long shelf life of such "active" sites.

    In an empirical set-up, a variety of surfaces was tested on their ability to produce a foam in a gas supersaturated solution. A suitable and very "active" surface, Tyvek, was selected for further study. The structure of this paper like material is very heterogeneous and porous. It is comprised of polyethylene fibres which are pressed together to form a hydrophobic porous matrix. The outside of this paper is made hydrophilic for printing purposes. This kind of structure could produce a great amount of bubbles simultaneously, and was stable in foamability. The liquid is prevented from entering the pores due to the hydrophobic nature of the material and due to the curvature of the liquid inside the pores, which makes the gas inside the Tyvek material stable over long periods of time, when submerged in the liquid (chapter 6).
    The degree of internal wetting of the "active" surface is found to affect foam formation considerably. Almost complete wetting results in almost no bubble formation. When the surface is not previously wetted this may lead to extremely turbulent bubble formation. This proves our hypothesis that pre-existent nuclei are needed to help in the formation of bubbles at the levels of gas supersaturation used in this work. The "active" surface is doubly effective when it is placed in an upright position due to its two sides. A horizontal positioning of the surface leads to the formation of very large bubbles trapped underneath the surface, which help to destabilise the foam as soon as the become a part of that foam (chapter 6).

    The surface area of the Tyvek used, and therefore the amount of "active" sites, has a direct effect on the initial rate of foam formation. For example, an area of 16 cm 2forms a foam layer at a much faster rate than an area of 4 cm 2. However, this latter surface may stay active for a longer period of time, as the dissolved gas in the solution is not exhausted as rapidly. Eventually, the bubble formation stops when the gas supersaturation in the liquid is exhausted. The surface, however, can be inserted into a fresh supersaturated liquid and the process starts anew (chapter 7).

    With this work, we have shown that, in order to enable an "active" surface to foam, it should contain pre-existent nuclei trapped within pores or cavities and these should be stable over a long period of time. This can be accomplished with a material that contains pores or cavities with a hydrophobic internal surface. If this is not the case, the liquid will penetrate into the solid surface and dissolve the gas nuclei within. Without the gas nuclei, no foaming will occur at the gas supersaturation levels observed in this work.

    Instant foam physics : formation and stability of aerosol whipped cream
    Wijnen, M.E. - \ 1997
    Agricultural University. Promotor(en): A. Prins; H.J. Bos. - S.l. : Wijnen - ISBN 9789054856931 - 157
    suspensies - emulsies - room - suspensions - emulsions - cream

    The formation and stability of aerosol whipped cream, as an example of an instant foam, were studied from a physical point of view. Instant foam production out of an aerosol can is based on the principle that a soluble gas (laughing gas) is dissolved under elevated pressure (5-10 bar) in the product (cream). By opening the nozzle of the can the product is allowed to leave the can. The resulting decrease in pressure causes the gas to come out of the cream and a foam is formed.

    Formation of the foam occurs in the smallest opening of the nozzle. Here, the velocity is limited to the speed of sound in the foam, which indicates that the flow through the nozzle is controlled by choking conditions. These conditions determine apart from the velocity also the density and pressure of the aerated cream in the nozzle. The whole process of instant foam formation is therefore regulated by the physics of choking.

    Aerosol whipped cream is characterised by a high overrun (400-600%) which provides firmness to the close-packed foam. The overrun is determined by the amount of laughing gas (nitrous oxide) dissolved in the cream. Since the pressure in an aerosol can knows practical limits, a high solubility of the gas in the cream is required to ensure that a sufficient amount of gas is dissolved. However, this high solubility enhances the process of disproportionation. Disproportionation involves gas diffusion from smaller to larger bubbles and out of the foam, which negatively affects the foam stability. Cream showed not to have the surface rheological properties that are required to stop this destabilisation process. This explains the fast deterioration of the product. Obviously, optimising the foam properties of aerosol whipped cream involves several compromises.

    Peptiden als emulgator en schuimvormer.
    Caessens, P.W.J.R. ; Kalsbeek, H.K.A.I. van; Smulders, P.E.A. - \ 1997
    Voedingsmiddelentechnologie 30 (1997)25. - ISSN 0042-7934 - p. 11 - 14.
    eiwitten - emulsies - schuim - functionele reacties - peptiden - proteins - emulsions - foams - functional responses - peptides
    Physical stability of caseinate - stabilized emulsions during heating
    Cruijsen, J.M.M. - \ 1996
    Agricultural University. Promotor(en): P. Walstra; M.A.J.S. van Boekel. - Delft : Eburon - ISBN 9789056510305 - 126
    caseïne - dispersie - vloeistoffen (liquids) - emulsies - melk - verwarming - casein - dispersion - liquids - emulsions - milk - heating

    The physical stability of caseinate stabilized emulsions was studied during heating (80- 120°C). Coagulation, coalescence and phase separation of the caseinate emulsions was studied using objective heat stability tests. The physical changes were characterized by light microscopy, particle size measurements and by determination of the solubility of coagula in various reagents. Additional information about physico-chemical changes in caseinate dispersions was obtained by determination of association of salts with caseinates, association of caseins with emulsion droplets, 1H-NMR, 31P-NMR and SDS- Page.

    The susceptibility towards heat coagulation of caseinate emulsions was mainly determined by volume fraction of oil, decrease of pH during heating and initial Ca 2+- activity. The heat coagulation time could be related to association of calcium and magnesium and caseinate and to association of caseinate and emulsion droplets. Association of calcium and magnesium and caseinate in these conditions would diminish electrostatic repulsion between caseinate molecules, which would lead to aggregation of the caseinate. It was shown that the formation of chemical cross-links is not rate determining in heat coagulation.

    During heating of caseinate emulsions containing malto-dextrins or a lactose/sucrose mixture, phase droplets were formed. The appearance of these phase droplets was accompanied by multi-layer formation of caseinate on the oil-droplets and a decreased stability to heating, often resulting in a highly viscous mass. Phase separation could be prevented in several ways; by using phosphates as stabilizing salt, by using maltodextrins or glucose syrups with relatively high DE values or by using soya lecithin. The effect of lecithin was only found when lecithin was added prior to homogenization. It appeared that the association of caseinate in solution or with the emulsion droplet was modified and thereby prevented phase separation.

    Although soya lecithin proved to be a very effective stabilizer, coalescence of the emulsion droplets could be observed when the concentration exceeded a certain value or when the phosphatidylcholine fraction of soya lecithin was used. The lecithins affected the coalescence behaviour, presumably by lowering the interfacial tension and lowering the caseinate surface load, and possibly by changing the structure of the caseinate layer on the emulsion droplets.

    Partial coalescence in oil-in-water emulsions
    Boode, K. - \ 1992
    Agricultural University. Promotor(en): P. Walstra. - S.l. : Boode - 159
    emulsies - oliën - water - colloïden - coagulatie - uitvlokking - emulsions - oils - water - colloids - coagulation - flocculation
    The influence of crystals on the stability against partial coalescence at rest and during Couette flow was examined in emulsions of saturated triglycerides in SDS- or caseinate solutions and in natural cream. Partial coalescence was characterized by determining changes in globule size distribution and fat content. In the absence of crystals emulsions were stable at rest and in Couette flow. At rest partially crystallized emulsions remained stable unless numerous large fat crystals were present or a temperature cycle was applied (= rebodying process). A theory was developed to explain this temperature controled phenomenon. In Couette flow considerable partial coalescence was observed if the fat network inside the globules was continuous. Due to a lack of liquid oil crystals were sticking out of the globule further, thereby increasing aggregation. Aggregation could be nullified within a few hours after clumping by changing the wetting properties, so that the fat crystals became preferentially wetted by the aqueous phase. Deaggregation could occur also in a flow field if the solid fat fraction had exceeded the optimum, which depended mainly on the properties of the fat and on the velocity gradient applied. A theoretical model was developed that accurately describes the course of the partial coalescence process up to the point where most of the fat creamed out of the emulsion, when warming it. The model is based on Smoluchowski's frequency equation and distinguishes between singlets and clumps with and without crystals. From the model it was deduced that the kind of fat, the solid fat content and the number of globules that contains crystals are the main factors that determine the instability of the emulsion globules.
    Toepassing van optische sensoren (I). Meting van bellengrootteverdeling in luchtige levensmiddelen.
    Bisperink, C.G.J. ; Akkerman, J.C. ; Prins, A. - \ 1991
    Voedingsmiddelentechnologie 24 (1991)11. - ISSN 0042-7934 - p. 20 - 22.
    voedselindustrie - voedseltechnologie - optische instrumenten - meting - dispersie - gassen - schuim - vloeistoffen (liquids) - emulsies - glasvezel - minerale kunstvezels - componenten - sensors - transductoren - automatische regeling - instrumentatie - systemen - straling - licht - lichtdoorlating - reflectie - refractie - absorptie - emissie - optica - food industry - food technology - optical instruments - measurement - dispersion - gases - foams - liquids - emulsions - glassfibre - manmade mineral fibres - components - sensors - transducers - automatic control - instrumentation - systems - radiation - light - light transmission - reflection - refraction - absorption - emission - optics
    Beschrijving van een methode, ontwikkeld op het laboratorium voor Zuivel en Levensmiddelennatuurkunde van de LU Wageningen in nauwe samenwerking met de TU Delft, waarmee goed en snel de verdeling van de gasfase in de vloeibare fase in de tijd en op verschillende plaatsen in een monster kan worden bepaald. Deze methode is gebaseerd op glasvezeltechniek
    Electrostratic stabilization of suspensions in non-aqueous media
    Hoeven, P.C. van der - \ 1991
    Agricultural University. Promotor(en): J. Lyklema. - S.l. : Van der Hoeven - 179
    suspensies - emulsies - colloïden - suspensions - emulsions - colloids
    Concentrated suspensions of detergent powder solids in a liquid nonionic surfactant are considered for practical application as liquid detergent products. If no precautions are taken, upon storage the viscosity of such suspensions increases and the pourability drops because the suspensions are colloidally unstable. It has been found that after the addition of a small amount of dodecylbenzene sulphonic acid (DoBS-acid or HDoBS) good pourability is maintained on storage. All the phenomena observed with such suspensions suggest that the addition of DoBS-acid reduces coagulation and improves colloidal stability. It was hypothesized that the colloidal stability obtained is of an electrostatic nature. In a liquid non-aqueous medium this is unexpected. A study of the mechanism of stabilization is described in this thesis.

    After a general introduction to the topic in Chapter 1, in Chapter 2 we discuss the character of the interactions which play a role in nonionic suspensions. The used nonionics are condensates of long chain alcohols and 3 to 9 alkylene oxide units. The dispersed solids are sodium salts as are usually present in current detergent powders, or oxides. They are aggregates or agglomerates of smaller crystalline primary particles and consist of irregular spheroids. The solids, the liquid nonionics and the anionic acid have been characterized with respect to a number of properties, including the molecular and crystalline structure, specific density, specific surface area, porosity, axial ratio and water content. The refractive indices and dielectric constants of the liquids and solids are also measured. Elemental analysis of the supernatants of our suspensions is carried out by Atomic Absorption, by Plasma Emission and by X-ray Fluorescence Spectroscopy. Since analysis of the supernatants indicated only very limited dissolution of the solids, it is concluded that the suspensions are lyophobic. It is demonstrated that, when DoBS- acid is added to a suspension of sodium salts in nonionic, it is converted quantitatively into anionic NaDoBS.

    Sedimentation rates, sediment volumes and viscosities are important physical characteristics of concentrated nonionic suspensions; they reflect the interactions between the suspended particles. The interactions follow the DLVO-theory, meaning that they are governed by the balance between attractive and repulsive or 'stabilizing' forces.

    The literature on van der Waals attraction (energy and forces) between particles in suspension is discussed in Chapter 3. It shows that for particles in the micron-size range, geometrical parameters (differences in particle size, interparticle distance), retardation and surface roughness are of more importance than in colloidal systems, having smaller particles. This means that the van der Waals bonding energy obtained on approach is larger, but, as a function of increasing interparticle distances, it decays more rapidly.

    In the van der Waals attraction, material properties are reflected in the Hamaker constant. Hamaker constants for the inorganic crystalline solids considered in this study are not available in the literature. Therefore it was necessary to evaluate them theoretically. Two approaches have been applied, a macroscopic theory and a microscopic theory. In a comparison they gave identical results within a few tens of percent. For the crystalline detergent solids the constants have been evaluated from their dielectric constants and refractive indices. The results showed the Hamaker constants for the detergent solids (except activated Zeolite 4A) to exceed those of the nonionics, but to be lower than those of the metal oxides. The differences between the constants of crystalline detergent solids and those of nonionics are relatively small, implying that suspensions of detergent solid particles in nonionics can be made to relatively high volume fractions and can be stabilized easily.

    In Chapter 4 the electrostatic theory for interactions of particle pairs in suspension is evaluated for its applicability in non-aqueous media, using models of plates and spheres. For both models the conclusion is that, for the calculation of the repulsive energies and forces, approximated equations can be used. They result in repulsive energies, pressures and forces, which are in good agreement with those of exact computations at distances>10 nm, but underestimate the repulsions at shorter distances.

    DLVO energy and force curves have been constructed and demonstrate the dependence of the repulsion on five parameters that govern the behaviour, viz. the dielectric constant, the ionic strength, the electric surface potential, the Hamaker constant and the particle size. For our suspensions with surface potentials ≥20 mV, significant repulsions already develop at distances between 2 and 40 nm. The theoretical repulsions are much higher than the van der Waals attractions and cause much larger repulsive barriers than those usually reported for non- polar, nonaqueous media. They are expected to play a role in the colloidal stabilization of nonionic suspensions and to influence the resistance against coagulation under pressures at the bottom of sediments. Secondary minima are only a few kT at most and coagulation is only expected at the protrusion points of contact and at relatively high ionic strengths.

    Ionic strengths in HDoBS-stabilized suspensions in the nonionics Plurafac LFRA30 and Imbentin C91/35 are evaluated from the conductivity in the supernatants and from their respective limiting molar conductivities. The methodology is described in Chapter 5. It was found that in both nonionics the limiting molar conductivity was lower than predicted from the values in water assuming Walden's rule applies. The results indicate that solvation interactions of Na +and DoBS -ions in nonionics are stronger than in water and stronger in Imbentin than in Plurafac.

    In Chapter 6 the results of the electric and dielectric measurements have been given. It is shown that the dielectric constant of nonionic is increased by HDoBS. Taking this increase into account, the ionic strengths found can be satisfactorily explained from theory. Only at high HDoBS concentration and relatively high dielectric constants are the ion concentrations lower than theoretically predicted, a feature that could be due to the formation of 'molecular associates'.

    From the limiting conductivities, at HDoBS concentrations between 10 and 150 mM, the ionic strengths have been found to range from 0.05 to 4 mM in Plurafac and from 0.08 to 30 mM in Imbentin. These results demonstrate a weak dissociation of the NaDoBS electrolyte. However, the ionic strengths obtained are considerably larger than those in supernatants of unstable suspensions and are higher than ever reported in the 'non-polar' hydrocarbon media, commonly considered in non-aqueous studies. Liquid nonionic media have a dielectric constant between 5 and 12 and are denoted 'low-polar'. At these ionic strengths, and considering the enhancement of the dielectric constant by HDoBS, in the HDoBS concentration regime between 0.5 and 150 mM, Debye lengths range from 33 to I nm in Plurafac and from 13 to 1 nm in Imbentin, i.e. in the same range as in aqueous media.

    Electrokinetic (ζ-)potentials of particles of detergent solids suspended in nonionics, given in Chapter 6, are found to be a function of the HDoBS concentration. The surface potentials tend to level off at HDoBS concentrations as low as 0.5 % w/w (15 mM dm -3), to a maximum value ranging from +25 to +70 mV, depending on the nature of the solid and the nonionic liquid. Addition of water or of a crown-complexant (15-Crown-5), reduces the ζ-potential. The formation of positive surface charges can be explained from the dissociation of adsorbed HDoBS.

    Mechanical properties of concentrated non-aqueous suspensions are discussed in Chapter 7, including their relation to the electrostatic repulsion. Rheology is used to monitor the properties under dynamic conditions. The consistency, which quantifies the particle interactions and shear thinning index was derived from the Sisko model.

    Addition of HDoBS was found to have little or no influence on the high shear rate viscosity of nonionic suspensions. This viscosity is governed by hydrodynamic interactions, which are, in turn, determined by the viscosity of the nonionic phase, the volume fraction and the temperature. The nature of the solid also has an influence on the 'infinite shear' viscosity, probably due to variations in protrusion size, causing their effective volumes to be larger than the actual volume. Measurements of the intrinsic viscosity of sodium tripolyphosphate (STP) indicated that the particles of this substance are almost spherical.

    Low shear rate viscosities monitor effects of interparticle interactions. The consistency was found to be inversely proportional to the particle volume. Addition of HDoBS reduces the consistency. As with the ~-potentials, the main effect is already obtained from 0.5 % w/w HDoBS. In this respect the behaviour of the viscosity is correlated with that of the ~-potential of the particles. It is further found that the drop in the 'normalized' consistency has a direct relation to the electrostatic force. These results support the conclusion that the nature of the obtained stabilization is electrostatic. The correlation of the viscosity with the Péclet number further supports this conclusion. It shows that under shear HDoBS-stabilized systems can be considered as hard-sphere suspensions.

    Creep compliance measurements of suspensions of STP in Plurafac at high volume fractions demonstrated that at low shear stresses the interactions are completely elastic. Under those conditions, relaxation of the stress leads to almost complete recovery. The shear moduli derived from creep compliance, drop less steeply as a function of the volume fraction than predicted from the electrostatic repulsive barrier. It is possible that this difference is a result of secondary minimum coagulation by the particle protrusions.

    In static sediments the volume fractions can be measured as a function of height by γ-ray absorption. Measurements of γ-ray absorption shows that the particle concentration from top to bottom in a stable sediment shows a concentration gradient. For HDoBS-stabilized suspensions this gradient is more continuous, whereas in unstable suspensions, due to coagulation, it is very irregular. From these results the relations between the static pressure or the network modulus and the volume fraction are derived. Pressures show an exponential relation with the interparticle distances. With low levels of DoBS-acid the interparticle distances are larger than for high concentrations of HDoBS. These results are in agreement with the dependency predicted by electrostatic repulsion, although the experimental pressure drop as a function of distance is much smoother than that theoretically predicted. The experimental network moduli derived from the pressure-volume fraction relation also drop much more slowly than theoretically predicted. This may again be a result of secondary minimum coagulation occurring by the protrusions.

    The overall conclusion is that the suspensions under consideration are electrostatically stabilized with DoBS-acid as the charge-determining electrolyte.

    Beer foam physics
    Ronteltap, A.D. - \ 1989
    Agricultural University. Promotor(en): A. Prins. - S.l. : Ronteltap - 133
    bieren - bierbereiding - suspensies - emulsies - fysica - mechanica - beers - brewing - suspensions - emulsions - physics - mechanics

    The physical aspects of beer foam behavior were studied in terms of the four physical processes, mainly involved in the formation and breakdown of foam. These processes are, bubble formation, drainage, disproportionation and coalescence. In detail, the processes disproportionation and coalescence were studied. The mechanism of coalescence was determined using, amongst others, a falling film apparatus. The spreading of surface active material on the film surface proved to initiate coalescence. Disproportionation in a foam is mainly influenced by partial gas pressure differences. Surface rheological aspects dominate the rate of disproportionation when the gas composition throughout a foam is uniform. The effect of the four physical processes on various foam phenomena can be explained. The disappearance of beer foam is a result of the combined action of drainage and gas diffusion from the foam to the surrounding atmosphere. When spreading substances are added to beer foam from an external source, coalescence is initiated and foam collapse occurs. The four physical processes have a different effect on foam behavior. Therefore, a distinction between these processes was made using an optical glass-fibre probe technique. With this technique the bubble-size distribution, the gas fraction in the foam, the height of the foam and the level of the foam-liquid interface can be measured as a function of time.

    The stability of recombined milk fat globules
    Melsen, J.P. - \ 1987
    Agricultural University. Promotor(en): P. Walstra. - S.l. : Melsen - 146
    dierlijke producten - colloïden - zuivelindustrie - dispersie - emulsies - vet - melkproducten - oliën - malsheid - textuur - water - vetgehalte - animal products - colloids - dairy industry - dispersion - emulsions - fat - milk products - oils - tenderness - texture - water - fat content

    The stability of the fat globules in recombined milk products against creaming, flocculation, clustering, partial coalescence and real coalescence, with the emphasis on partial coalescence, was studied. (partial) Coalescence was characterized by determining changes in globule size distribution and fat content. Without crystals the emulsions were mostly stable at rest and during flow. If crystals were present, natural cream and emulsions of milk fat-in-whey were unstable in a flow, while emulsions of milk fat and skimmilk or milk fat and buttermilk remained fairly stable and only gave partial coalescence if high shear rates were applied to emulsions with a high fat content and a large average diameter.

    In some cases partial coalescence resulted in the formation of a few large clumps that coalesced into floating fat upon heating the emulsion to above the melting point of milk fat thus causing a reduction of the fat content of the underlying emulsion, in other cases partial coalescence resulted in the formation of many small clumps that coalesced into larger fat globules upon heating, thus causing an increase in average globule size. Coalescence course and rate appeared to depend on emulsion type and applied treatment. Partial coalescence of milk fat-in-whey emulsions nearly always resulted in a decrease of the fat content. With a model starting from a small fraction of reactive globules gradually growing into clumps during the treatment, the coalescence process of these emulsions was fairly good quantitatively described. The ideas whereupon this model is based were used to qualitatively explain the different partial coalescence processes observed with the other emulsions.

    The effect of electrolytes on emulsions stabilized by nonionic surfactants
    Boomgaard, A. van den - \ 1985
    Landbouwhogeschool Wageningen. Promotor(en): J. Lyklema, co-promotor(en): T.F. Tadros. - Wageningen : Van den Boomgaard - 198
    dispersie - dissociatie - elektrolyten - emulsies - ionen - vloeistoffen (liquids) - dispersion - dissociation - electrolytes - emulsions - ions - liquids
    The objective of this study was to investigate the effect of high electrolyte concentrations on the stability of oil-in-water- emulsions stabilized by nonionic surfactants.

    In chapter 1 several stability mechanisms are briefly outlined and the distinction between coalescence and flocculation of an emulsion is explained.

    The structural characterization of the nonionic surfactants used (Synperonic NPE series) is described in chapter 2. These surfactants consist of a hydrophobic nonylphenol part, a less hydrophobic propylene oxide part (PO) and a hydrophilic ethylene oxide moiety (EO). Various samples differing with respect to the length of the EO part have been studied. Consistent results for the molecular masses have been obtained by pmr- and uv- spectroscopy. It has been demonstrated that the hydrophobic nonyl group of the surfactant is branched in different ways but there is no significant preference for any of the aliphatic nonyl isomers in the ethoxylation process.

    In chapter 3 attention is paid to the properties of surfactant solutions by studying the surface- and interfacial tension, density, heat capacity, light scattering and viscosity. It was found that Synperonic NPE surfactant solutions have properties similar to other ethoxylated surfactant solutions and that the solvent quality of the EO-moiety plays the main role in the solution behavour of NPE surfactants. With respect to the effect of concentrated electrolytes it was established that there are two opposing trends which lead to a maximum in the micelle interaction coefficient as a function of salt concentration.

    Chapter 4 is devoted to the adsorption behaviour of Synperonic surfactants and to the flocculation of dispersions stabilized with these surfactants. The adsorption was found to depend on the nature of the adsorbent, which had consequences for the stability of the dispersions. Polystyrene latices and emulsions could be stabilized with Synperonic NPE surfactants in electrolyte solutions, whereas
    pyrogenic silica could not. The PS latex flocculated under theta conditions for poly ethylene oxide indicating that the ethylene oxide moieties of the adsorbed surfactants protrude into the aqueous solution.

    Free liquid film thickness measurements are treated in chapter 5. For comparison purposes also a number of experiments has been carried out with Synperonic NP, a surfactant similar to NPE but without the propylene fraction. The measured film thicknesses were in good agreement with layer thicknesses obtained from the reduction in the streaming potential in a narrow capillary. This agreement supports the correctness of the two different techniques. The film thickness depended strongly on the ethoxy chain length and did not change with increase of surfactant concentration in the range studied. From thickness measurements of films stabilized by binary mixtures of surfactants it was demonstrated that fractionation did not occur in the free liquid films. The film thickness could be analyzed using a simple model in which the ethoxy chain behaved as a terminally adsorbed chain with dimensions similar to a coil trapped in a tube. The effect of electrolytes obeyed laws similar to those observed in chapter 3 and was entirely attributable to the interaction of the EO parts of the molecule.

    Chapter 6 deals with the stability against coalescence of emulsions stabilized by Synperonic NPE 1800 in the presence of electrolytes. All emulsions studied were very stable according to criteria given in the literature, only in the course of several weeks some coalescence could be observed. The trend as a function of electrolyte concentration and temperature could be described with the film rupture theory of Vrij and Overbeek, but the observed effect of the oil volume fraction upon coalescence could not be explained with this theory.

    Finally in chapter 7 the conclusion is obtained that with the present system the influence of electrolytes on the flocculation and coalescence are entirely different processes. Flocculation is due to interaction between the ethylene oxide moieties; with respect to this phenomenon the effect of electrolytes is similar to that observed in solutions of the surfactants and in the thin films.

    The preparation and stability of homodisperse colloidal haematite (alpha-Fe2-O3)
    Penners, N.H.G. - \ 1985
    Landbouwhogeschool Wageningen. Promotor(en): J. Lyklema, co-promotor(en): L.K. Koopal. - Wageningen : Penners - 97
    colloïden - dispersie - emulsies - hematiet - ijzerhydroxiden - ijzeroxiden - isolatie - verwerking - zuiveren - suspensies - colloids - dispersion - emulsions - haematite - iron hydroxides - iron oxides - isolation - processing - purification - suspensions

    Since the foundation of colloid chemistry as a branch of science, much attention has been paid to the subject of colloid stability, i.e. the stability of colloid systems against aggregation. Gradually, our knowledge of the mechanisms involved has improved and models were developed, comprised in the DUO theory, which form the basis of a quantitative description of the stability of a colloidal system. There is plenty of experimental evidence which substantiate the correctness of the principles of the DUO theory, and hence, this theory is regarded as one of the fundaments of colloid chemistry. However, in one respect the theory is not confirmed by experiments: calculations predict pronounced size effects, but in practice stability seems to be little affected by particle size.

    It was the purpose of this study to gain insight in this contradictory matter. Chapter 1 offers a more extended introduction to the problem as well as the outline of this study.

    Chapter 2 focusses attention on a model system which meets the outlined requirements regarding surface charge, homodispersity, sphericity and particle size range: a method is described for the synthesis of homodisperse haematite (α-Fe 2 O 3 ) sols containing particles whose sizes vary from 35 nm up to 700 nm. This method is principally based on the gradual growth of haematite seeds in supersaturated FeCl 3 solutions (heterogeneous nucleation) up to the desired paticle size. As slight deviations in the composition of the growth medium have drastic effects on the shape of the final colloid, emphasize is given to the description of optimal synthesis conditions for spherically shaped particles. Kinetic experiments, performed to unravel the principles of particle growth revealed that the precipitation process is governed by diffusion.

    The coagulation experiments described in chapter 5 are monitored by turbidity measurements. Therefore the characterisation of the optical properties of the sols is a prerequisite for further studies. The sols under investigation are homodiperse and contain spherically shaped particles. This makes them particularly suited for such an evaluation, as their scattering behaviour can be interpreted in terms of the Mie theory. Such a comparsion, leading to values for the refractive index (n) and the absorption coefficient (K) in the wavelength range from 400 to 800 nm is made in chapter 3 . The agreement between calculations and experiments is good for any wave length in the visible range, and for any particle size studied, although the particles are monocrystalline and no perfect spheres.

    Any study dealing with electrostatic stabilisation demands some knowledge of the electrochemical behaviour of the system under study. Chapter 4 pays attention to the electrochemical characterisation of the haematite surface by comparing haematite samples from different origines. In these studies, potentiometric titrations, streaming potential measurements on haematite-coated capillaries and micro electrophoresis were used as the experimental tools to get access to the surface properties. Instead of providing unambiguous data, being valid for all iron oxides occuring in the (α-Fe 2 O 3 ) modification, the reported experiments emphasize that the crystal structure of the bulk phase is not the exclusive parameter in determining the electrochemical behaviour of an oxide. The purification procedure, or a heat treatment of the sol (aqueous or dried) plays an important part as well. There is some evidence that the crystal habit of the haematite surface is pH-dependent and that in some cases precipitated amorphous oxide may share in determining the surface properties. Though not going too much into details, the study gives evidence of the fact that the charging mechanisms of haematite are more complicated than expected on the grounds of purely crystallographic considerations.

    Inevitably this finding has its impact on the question of colloid stability, which is the dominating item of chapter 5 . Considering the outcome of the electrochemical study, is it still justified to assume that the haematite particles meet the demands of the stability theory regarding the sharpness of the boundary between bulk material and surrounding liquid? How could a diffuse surface layer be accounted for in existing stability models? Within the restrictions set by such questions, chapter 5 deals with the item of colloid stability with special reference to the influence of particle size on stability: though the value of the critical coagulation concentration depends on particle size and shows a minimum (!), the slopes of the log W - log C plots are virtually size independent. Such trends can be accounted for by the concepts of the DUO theory, if coagulation reversibility and shear effects are incorporated in the analysis. Deviations from sphericity, which are definitely observed for the systems under investigation. might explain some of the observed effects as is shown by some simple double layer calculations dealing with orientational effects in the interaction of a cubic particle with a half space.Finally, chapter 6 reflects on the preceding Items and pays attention to its limitations. Furthermore, it points to subjects which deserve further elaboration and mentions the means to make them experimentally accessible.

    Enzymatic reactions in reversed micelles
    Hilhorst, M.H. - \ 1984
    Landbouwhogeschool Wageningen. Promotor(en): C. Veeger, co-promotor(en): N.C.M. Laane. - Wageningen : Hilhorst - 95
    emulsies - oliën - oxidoreductasen - water - emulsions - oils - oxidoreductases - water
    It has been recognised that enzymes in reversed micelles have potential for application in chemical synthesis. Before these expectations will be realised many problems must be overcome. This thesis deals with some of them.
    In Chapter 1 the present knowledge about reversed micelles and micellar enzymology is reviewed. Encapsulation of enzymes in reversed micelles enables the enzymatic conversion of apolar compounds. In the literature only a few cases have been reported of conversions of apolar compounds, and only initial enzyme activities were measured. In Chapters 3 and 4 of this thesis, the conversion of the apolar steroids progesterone and prednisone by 20β-hydroxysteroid dehydrogenase is described. In Chapter 3 it is shown that the reaction proceeds for at least nine hours, indicating that the steroid dehydrogenase used here is fairly stable in a reversed micellar environment under operational conditions. The stability of hydrogenase in reversed micelles was even higher than in an aqueous solution (Chapter 2).
    In order to function for longer periods of time, redox enzymes require a continuous and sufficient supply of reducing equivalents. Several systems have been described that provide reducing equivalents in aqueous solutions, but no such a system was known for reversed micellar media. In this thesis three methods for the generation of reducing equivalents in reversed micellar media have been applied. Chapter 2 describes a photochemical system for the vectorial transport of electrons from a donor in the continuous phase to an acceptor in the water pool of the reversed micelle. The spatial arrangement of the components is an important factor in determining the efficiency of such systems. The reducing equivalents thus generated can be converted to hydrogen by hydrogenase located in the water pool. In Chapter 3 a combined enzyme system consisting of hydrogenase and lipoamide dehydrogenase converts hydrogen gas into reducing equivalents in the form of NADH that are consumed by 20β-hydroxysteroid dehydrogenase during the conversion of apolar steroids. Based on the results of these two chapters, patent applications were filed in several countries, e.g. in Europe. A copy of this patent application is added as an Appendix. There, a third method is mentioned, i.e. the electrochemical regeneration of NADH and NAD(+).
    In Chapter 4 it is investigated how the composition of a reversed micellar medium affects the rate of conversion of apolar steroids by 20β-hydroxysteroid dehydrogenase. Evidence was obtained that the steroid concentration in the interphase dictates the rate of conversion. This concentration depends on the hydrophobicity of the substrate as compared to the hydrophobicity of the interphase and the hydrophobicity of the continuous phase. These observations are generalized to guidelines indicating that the difference between the hydrophobicity of the substrate and the interphase must be minimal to ensure a high substrate concentration in the interphase and that the difference between the hydrophobicity of the substrate and the hydrophobicity of the continuous phase must be maximal to keep the substrate concentration in the continuous phase as low as possible. These guidelines will prove useful to predict the optimal composition of a medium for enzymatic conversion of apolar compounds.
    Not only the composition of a reversed micellar medium with respect to organic solvent and cosurfactant is important, but also the charge of the surfactant used. In Chapter 5, evidence is presented that the charge of the surfactant head groups influences the kinetic parameters K m and k cat of enzymes in reversed micelles, either resulting in an increase or a decrease of activity. Furthermore, an expression is derived for the initial reaction rate of the enzymatic conversion of an apolar substrate in a pseudo two-phase system where the partition equilibrium of the substrate over the two phases can be shifted due to enzyme catalysis.
    In another section of the Discussion the advantages and disadvantages of reversed micellar media for the enzymatic conversion of apolar compounds are compared with those of other systems that have been proposed for that purpose.
    In conclusion, in a reversed micellar medium:
    - a highly organised photochemical system can be created (Chapter 2)
    - enzymes can be immobilized while retaining their activity (Chapters 1- 5)
    - enclosure of enzymes can lead to enhanced stability (Chapter 2)
    - enzymes experience an essentially aqueous micro-environment (Chapter 5)
    - enzymatic activity can be higher than in aqueous solution (Chapter 4 and 5)
    - apolar compounds can be converted enzymatically (Chapters 3, 4 and Appendix)
    - multi-enzymatic reactions can be performed for longer periods (Chapter 3)
    - cofactors can be regenerated with hydrogen and electricity (Chapter 3 and Appendix)
    - enzyme activity can be regulated by changing the composition of the interphase and continuous phase (Chapter 4)
    - the optimal composition for enzymatic conversion of any given apolar compound can be predicted (Chapter 4)
    - the thermodynamic equilibrium can be shifted in the desired direction (Chapter 5)
    - the product can be isolated while the other components can be recycled (Chapter 3).

    Bepaling van chlooramfenicol in melk met reduktieve LCEC
    Hooijerink, H. ; Ruig, W.G. de; Buizer, F.G. - \ 1983
    Wageningen : RIKILT (Verslag / RIKILT 83.69) - 5
    chlooramfenicol - vloeistofchromatografie - antibiotica - emulsies - elektroanalytische analyse - melk - analytische scheikunde - chloramphenicol - liquid chromatography - antibiotics - emulsions - electroanalytical analysis - milk - analytical chemistry
    Doel van dit onderzoek is: Ontwikkelen van een kwantitatieve bepaling van chlooramfenicol in melk op het ug/1 niveau met behulp van reduktieve LCEC. Het is bekend dat bij het extraheren van lichaamsvloeistoffen, zoals urine en melk, zeer snel emulsies optreden. Door gebruik te maken van Extrelut kolommen is het emulsieprobleem opgelost. Om het eventueel aanwezige vet te verwijderen volgt nog een vloeistof/vloeistof extraktie. Uiteindelijk vindt reduktieve elektrochemische detektie plaats na een vloeistofchromatografische scheiding.
    Ontwikkeling methode voor de kwantitatieve bepaling van benzoëzuur en sorbinezuur in (vlees)salades
    Oostrom, J.J. van; Struijs, T.B.D. van der; Ruig, W.G. de - \ 1982
    Wageningen : RIKILT (Verslag / RIKILT 82.27) - 3
    emulsies - conserveermiddelen - voedselconserveermiddelen - analytische methoden - benzoëzuur - sorbinezuur - vloeistofchromatografie met omgekeerde fase - emulsions - preservatives - food preservatives - analytical methods - benzoic acid - sorbic acid - reverse phase liquid chromatography
    Ontwikkeling van een analysemethode voor de kwantitatieve bepaling van conserveermiddelen in sterk emulsievormende produkten. Er wordt een methode beschreven voor de bepaling van benzoëzuur en sorbinezuur in sterk emulsievormende produkten, bestaande uit een voorbewerking m.b.v. Extrelut kolommen van de Fa. Merck, gevolgd door hogedrukvloeistofchromatografische scheiding m.b.v. een reversed phase systeem.
    Influence of fat crystals in the oil phase on stability of oil-in-water emulsions
    Boekel, M.A.J.S. van - \ 1980
    Landbouwhogeschool Wageningen. Promotor(en): P. Walstra. - Wageningen : Pudoc - ISBN 9789022007396 - 94
    dichtheid - emulsies - vetten - vloeistofmechanica - vloeistoffen (fluids) - vloeistoffen (liquids) - vermenging - oliën - water - density - emulsions - fats - fluid mechanics - fluids - liquids - mixing - oils - water
    Coalescence at rest and during flow was studied in emulsions of paraffin oil in water with several surfactants and with crystals of solid paraffin or tristearate in the oil phase. Solid fat in the oil phase was estimated by pulsed nuclear magnetic resonance. Without crystals, oil-in-water emulsions were mostly stable and flow hardly influenced coalescence, even of unstable emulsions. Emulsions with crystals in the dispersed oil phase were less stable if crystals appeared at the interface. The contact angle indicated that crystals could be oriented in the interface; if so, instability was promoted by creaming, Couette flow, turbulence or flow with Taylor vortices. Coalescence in such systems could be caused by crystals sticking through the interface and piercing the film between the globule and a second approaching globule. The effect of variables such as type of surfactant, type of crystal, amount of crystalline fat, globule size, volume fraction of fat and ionic strength fitted this view. Natural cream with part of the globular fat crystallised behaved to some degree like the model systems but there were deviations.
    Interactions between adsorbed macromolecules : measurements on emulsions and liquid films
    Vliet, T. van - \ 1977
    Landbouwhogeschool Wageningen. Promotor(en): J. Lyklema. - Wageningen : Veenman - 131
    kunststoffen - industrie - chemie - colloïden - adsorptie - oppervlakten - suspensies - emulsies - macromoleculaire stoffen - oppervlaktechemie - moleculen - intermoleculaire krachten - plastics - industry - chemistry - colloids - adsorption - surfaces - suspensions - emulsions - macromolecular materials - surface chemistry - molecules - intermolecular forces

    The aim of this study was to gain more insight into the factors, determining the inter- and intramolecular interactions between adsorbed macromolecules. To that end several experimental and theoretical approaches were followed, using well-defined systems. It was shown that these interactions could conveniently be studied by measurements on emulsions and thin free liquid films. Two different macromolecules were used: a nonionic one: polyvinyl alcohol (PVA) and an ionic one: a copolymer of methacrylic acid and the methyl ester of methacrylic acid (PMA-pe) in the molar ratio 2:1.
    The characterization of the used materials has been described in chapter 2. The conformational transition, occurring in dissolved polymethacrylates was briefly discussed. At low pH, the molecules occur in a compact form, the hypercoiled form, or a-conformation. At pH above ~ 6 the molecules occur in the common. more extended b-conformation. From viscometry on PVA solutions conformational parameters, such as the root mean square end-to-end distance, the length of a statistical chain element and the linear expansion factor were determined. These conformational parameters were determined in a I M aqueous glycerol solution because in the film experiments 1 M glycerol was present in the PVA solutions in order to lower the water vapour pressure.
    In chapter 3 the experimental methods have been described. In the first part attention was paid to the preparation of the emulsions and to the determination of basic properties, such as specific area and adsorbed amount. A variety of rheological measurements were described in the second part. A more detailed description was given of the apparatus for the dynamic measurements (the rheometer) and of that for creep measurements.
    The end of chapter 3 deals with the thickness measurements of polymerstabilized free liquid films. First, a description of the apparatus and the experimental procedure was given. Subsequently, a discussion followed of the calculation of thicknesses from the intensity of the reflected light. It was shown that, for the calculation of the correction to be applied to the equivalent aqueous solution thickness, the smeared out adsorbed polymer segment layers may be formally replaced by a block distribution.
    The inter- and intramolecular interactions between the PMA-pe segments and the effect of these interactions on the conformation of the polyelectrolyte molecule and on the rheological properties of emulsions stabilized by this polyelectrolyte, have been discussed in chapter 4. As possible attractive forces responsible for the compact conformation at low pH, VAN DER WAALS attraction and hydrophobic bonding between the methyl groups in the main chain were considered. In addition, the strength of the COULOMBIC interaction between the carboxyl groups also plays a role in the conformational transition.
    The conformational transition from the a- to the b-conformation in free and adsorbed PMA-pe, was studied by potentiometric titration. Data for adsorbed PMA-pe were obtained by titrating polyelectrolyte-covered emulsion droplets. It was found that the conformational transition also occurs in adsorbed PMA-pe. This conformational transition is reflected in the rheological properties of paraffin in water emulsions, stabilized by PMA-pe. It could be concluded both from viscosity and dynamic data, that strong attraction between the emulsion droplets occurs only at a low degree of neutralization α, that is, if a substantial part of the adsorbed PMA-pe is in the a-conformation. Then both the dynamic moduli and the viscosities are very high. On the contrary at high a the emulsions were very fluid with little or no indication of attraction between the adsorbed polyelectrolyte sheets.
    The main conclusions from the potentiometric titration data and the rheological measurements are:
    a. the attraction between the polyelectrolyte segments, observed at low αin solution occurs also between loops and/or tails, adsorbed on one emulsion droplet;
    b. the high values of the dynamic moduli and of the viscosities at low αare due to attraction between extending loops and/or tails, adsorbed on different droplets;
    c. the two types of interaction are very similar.
    This conclusion was confirmed by the influence of methanol on Na-PMA-pe stabilized emulsions and the effect of temperature. Moreover, from these experiments it could also be concluded, that probably the hyper-coiled conformation at low α, is to a large extent due to hydrophobic bonding.
    The influence of Ca ++ ions on the properties of the polyelectrolyte was also investigated. Potentiometric titration showed that, in the presence of Ca ++ ions, the conformational transition is moved to higher a. Again the transition is reflected in the rheological properties of emulsions, stabilized by Ca-PMA-pe. The balance between the inter- and intramolecular interaction forces and the interactions themselves are more complicated than in the case of Na-PMA-pe. This complex character is reflected in the more complex rheological functionalities (η(α), G' (α) curves) of emulsions stabilized by Ca-PMA-pe.
    The interactions between adsorbed macromolecules were further investigated by studying the properties of polymer stabilized thin free liquid films. Measurements on films, stabilized by PVA or PMA-pe, were reported in chapter 5.
    The interaction forces which must be taken into account in a PVA film are VAN DER WAALS attraction, hydrostatic pressure and steric interaction. The VAN DER WAALS attraction over a film can be calculated. The equilibrium film thicknesses of the films were determined at varying hydrostatic pressure. Then the steric repulsion force Fs between the two adsorbed PVA layers was obtained by equalizing - Fs with the hydrostatic pressure and the VAN DER WAALS attraction. So the steric repulsion force could be calculated for different equilibrium thicknesses. Next the free energy of steric interaction was found by graphic integration of the force-distance curve. These values can be compared with theoretical predictions.
    In order to calculate the free energy of steric interaction theoretically, a model of the segment density distribution had to be developed. The proposed semiquantitative model was based on the consideration that the molecular weight distribution of the used PVA samples is wide and the presumption that a large fraction of the segments is adsorbed as tails. Indications for this presumption were found by comparing the extrapolated ( Fs - 0) film thickness with the ellipsometric thickness of an adsorbed layer. This model leads to the conclusion, that the properties of the outer part of the adsorbed layers are dominated by a few extending tails. The free energy of steric repulsion, thus calculated with the HESSELINK et al. (1971b) theory of steric repulsion, between two adsorbed PVA ( M v = 27,000 or 86,000) layers, agrees well with the experimentally determined values for reasonable lengths of the tails.
    In chapter 5 also the drainage behaviour and the equilibrium thicknesses of PMA-pe films, made at different values of the degree of neutralization a, were discussed. The measured equilibrium thicknesses correlated well with ellipsometric measurements of an adsorbed layer. The drainage pattern changes if or is varied. At low αthe films are rigid, whereas at high αthey are mobile. Also the dilatational modulus decreased from α= 0.1 to α= 1.0. Probably the interaction forces between the polyelectrolyte segments which are responsible for these phenomena, are the same as those which induce the conformational transition in the molecule or which are responsible for the drastic changes in the rheological properties of emulsions stabilized by PMA-pe if αis varied.
    A more elaborate discussion of the rheological properties of PMA-pe stabilized emulsions is given in chapter 6. Both dynamic and creep measurements were reported.
    In the dynamic experiments the storage modulus G' and the loss modulus G'' were measured as a function of the frequency ω. The degree of neutralization, the polyelectrolyte supply and the salt concentration were variables. By comparing the gel point concentration of free PMA-pe with the polyelectrolyte concentration in the layer between two emulsion droplets, it was concluded that there also a gel could be formed if attractive forces between the polyelectrolyte segments dominate. This conclusion is supported by analyses of the G' (ω) and G'' (ω) curves. In cases where such a kind of gel is formed, it is possible to relate G' to the number of polyelectrolyte cross-links between two droplets. Equations were given for the case of an ideal network model and for an aggregate model of the emulsion structure. For both models equations were also derived relating G' to the VAN DER WAALS attraction between the droplets. It was found that the ideal network model was good enough to interpret semiquantitatively the results obtained for the viscoelastic emulsions. The VAN DER WAALS attraction between the emulsion droplets proved to be much less important than the interactions between the polyelectrolyte sheets. It was calculated that at αPMA-pe = 0.1, about 400-1000 polyelectrolyte bonds were formed between two emulsion droplets at interparticle distances of 30 to 50 nm. It implies that about 10-20% of the polyelectrolyte molecules, present in the contact region between two emulsion droplets, are directly involved in the formation of these bonds.
    A short discussion was given of the unusually high values of the loss factor tg δ. The suggestion was put forward that these high values follow from the fact that liquid must move in and out of the micro gels between adjacent emulsion droplets or from the relaxation of the polyelectrolyte cross-links, during a deformation cycle.
    The creep curves were analysed by assuming the existence of both strong and weak bonds between the emulsion droplets. If measured under the proper conditions the weaker (secondary) bonds are broken, but the stronger (primary) bonds are not. Then it is possible to calculate from a non-linearity in the deformation as a function of the shear stress, if any, the contribution of the secondary bonds to the shear stress. The secondary bonds were identified as VAN DER WAALS attraction between the emulsion droplets and the primary bonds as interactions between polyelectrolyte molecules adsorbed on different droplets. Again it was found that the VAN DER WAALS attraction is relatively unimportant. From the found contribution to the shear modulus of an emulsion due to VAN DER WAALS attraction and steric repulsion between the droplets the interparticle distance was calculated to be 25-30 nm. This value was of the same order of magnitude as the results of the film thickness measurements. A semiquantitative assessment of the activation energy necessary to break a polyelectrolyte-polyelectrolyte bond showed that the interactions between the methyl groups must have a cooperative character.
    It was concluded that the results of creep and dynamic measurements support each other.

    In conclusion, this study shows that both rheological measurements of sterically stabilized dispersions and the investigation of polymer stabilized thin liquid films are excellent tools for investigating the interactions between adsorbed macromolecules. Intramolecular interactions and interactions between macromolecules adsorbed on different interfaces are very similar. The latter interactions are dominated by the outer part of the adsorbed macromolecule layers.

    Adsorption of polyelectrolytes at liquid-liquid interfaces and its effect on emulsification
    Böhm, J.T.C. - \ 1974
    Landbouwhogeschool Wageningen. Promotor(en): J. Lyklema. - Wageningen : Veenman - 110
    oleïnezuur - onverzadigde vetzuren - carbonzuren - acrylzuur - kunststoffen - industrie - emulsies - oliën - water - adsorptie - sorptie - chemie - colloïden - oppervlakten - macromoleculaire stoffen - oppervlaktechemie - oleic acid - unsaturated fatty acids - carboxylic acids - acrylic acid - plastics - industry - emulsions - oils - water - adsorption - sorption - chemistry - colloids - surfaces - macromolecular materials - surface chemistry

    In this study we have investigated the adsorption behaviour of a number of synthetic polyelectrolytes at the paraffin oil-water interface and the properties of paraffin oil-in-water emulsions stabilized by these polyelectrolytes.

    Polyacrylic acid (PAA), polymethacrylic acid (PMA) and the copolymers of the monomeric acids with their methyl esters (resp. PAA-pe and PMA-pe) are used. Most of the experiments have been performed with PMA-pe, whereas the other polyelectrolytes are mainly used for the sake of comparison. In chapter 2 the synthesis and some relevant bulk properties are described. These properties are determined by potentiometric titrations and by viscosimetry. In agreement with the literature it is found that PMA and PMA-pe undergo a reversible conformational transition as a function of pH. At low charge density these polyelectrolytes are characterized by a compact hypercoiled conformation (aconformation) and by increasing the charge density they unfold, resulting finally in an expanded conformation (b-conformation). This conformational transition is not observed for PAA and PAA-pe.

    Several techniques are used to obtain information about the mode of adsorption of these polyelectrolytes at the paraffin oil-water interface and about the properties of emulsions stabilized by them. At undisturbed interfaces the interfacial tension as a function of time γ( t ) has been determined from interfacial tension measurements with the Wilhelmy-plate technique. Adsorbed (section 3.2.) as well as spread (section 3.3.) monolayers are investigated. It is found that the attainment of the steady state of the interfacial tension γ(∞) is faster the more compact the molecules are (i.e. lower charge density, higher ionic strength, Ca 2+counterions) and the higher the polyelectrolyte concentration. These findings agree with the fact that the rate of reduction of the interfacial tension dγ/d t is determined by three processes, viz. diffusion of the polymer molecule to the interface with adsorption in its bulk conformation, reconformation and spreading of the arrived molecules. As all three processes usually occur simultaneously, the adsorption behaviour of polymers is very complex. For compact molecules diffusion leads to a rapid accumulation of segments at the interface and reconformation and spreading hardly take place. For more expanded molecules diffusion is slow and the rate of reduction of γtakes place mainly through reconformation. As at higher c p the supply by diffusion is faster, it is understandable that the steady state is attained faster according as c p is higher and reconformation and spreading are reduced.

    If spreading can take place to a high degree, the fraction of segments per molecule adsorbed in the first layer p , becomes relatively high. From a semiquantitative interpretation of the interfacial tension measurements it is indeed found that p increases the less compact the molecules are (section 3.5.). To be able to do these calculations also adsorption experiments at an undisturbed liquid-solid (L/S) interface have been performed (section 3.4.). The amount of adsorbed PMA-pe at the polystyrene latex of low surface charge has been determined and it is assumed that Γ at the oil-water interface and the latex surface does not differ.

    Additional information about the mode of adsorption has been obtained from experiments in which the conditions in the water phase were changed after adsorption (chapter 4). Especially changes in pH produced interesting results (so-called γ-pH cycles). It was found that the mode of adsorption determines the degree of (ir)reversibility of the adsorbed layer by increasing pH. The longer the average train length of the adsorbed molecule, the higher the degree of irreversibility. These findings agree with the mode of adsorption as described already in chapter 3. If reconformation and spreading can take place to a high degree (i.e. at high pH) the average train length is large and the molecules are irreversibly adsorbed. If diffusion is the main factor (i.e. at low pH) the average train length is small and desorption is possible.

    The properties of emulsions stabilized by these polyelectrolytes have been investigated by the specific interfacial area S (expressed in m 2per ml paraffin oil), the amount of adsorbed polyelectrolyte at the emulsified interface Γ e (expressed in mg per m 2) and the viscosity of the emulsions. Γ e has been obtained by a depletion method (section 5.3.) and S by turbidity measurements (section 5.4.). As these properties did not change with time after finishing emulsification, they enable us to characterize the adsorbed layer at an emulsified interface and they reflect the interesting but complex dynamic processes occurring during emulsification with polyelectrolytes as stabilizers. The results of these investigations are summarized in chapter 5 ( S , Γ e and the mechanism of emulsification) and in chapter 6 (rheology of emulsions and characterization of the adsorbed layer at an emulsified interface).

    The interfacial tension at undisturbed interfaces and the emulsion properties have been investigated as a function of a number of parameters. These parameters are polyelectrolyte concentration c p or polyelectrolyte supply c p ', degree of neutralization α(charge density) or pH, ionic strength, nature of the counterion (Na +, Ca 2+) and chemical constitution of the polyelectrolytes. Moreover, it was investigated whether the a- and b-conformation in bulk was reflected in the adsorbed layer, the emulsion properties and the behaviour during emulsification. We will here summarize the effects of these parameters on the interfacial tension (undisturbed interface) and on the emulsion properties (disturbed interface). The comparison between both conditions of the interface is not meant to predict emulsion properties from the adsorption behaviour at an undisturbed interface. We intend to find out which of the investigated parameters play a major role in the interfacial activity at undisturbed interfaces, the emulsification process and the properties of the emulsions once they are formed. To predict the emulsifying behaviour of a polyelectrolyte it is necessary to investigate its interfacial behaviour under dynamic conditions. Such measurements are available (section 3.6.). However, it appears that the theory for the interpretation of these results is not yet applicable to polymers.

    In the course of this study the parameters mentioned before have been investigated seperately for each emulsion property or for each technique to determine the interfacial properties at undisturbed interfaces. Therefore it seems worth while to summarize the results here systematically for each parameter.

    Polyelectrolyte concentration c P or polyelectrolyte supply c P '

    The steady-state value of the interfacial pressure for adsorbed layers always increases with c p , d Π(∞)/d c p >0 (see figs. 3.4. and 3.5.). For spread monolayers it is always found that d Π(∞)/d Γ sp >0 (see figs. 3.11.-3.14.). From the literature it is known that at L/S interfaces d Γ/d c p ≥0. Hence it is acceptable to assume dΓ ad /d c p ≥0 for the adsorption of polyelectrolytes onto undisturbed L/L interfaces. However, for adsorbed polymer layers an unambiguous relation between c p , Πand Γ ad cannot be given. In this study it is argued that Πis primarily related to the occupation of the first layer θ ad (see section 3.5.).

    At emulsified interfaces it is always found that dΓ e /d c P ' ≥0 (see section 5.5.2.). S as a function of c P ' is not univocal and depends on both c P ' and the flexibility of the polyelectrolyte (see below: degree of neutralization). In the region of low c P ' it is always found that S increases with c P ' and that the flexibility only affects the value of S . Above a given c P ' the relation S ( c P ') depends on the flexibility of the polyelectrolyte molecule (see section 5.6.). As the flexibility depends strongly on the degree of neutralization the relation between S and c P ' is a function of α.

    The viscosity of emulsions - restricted to the highly viscous emulsions with PMA-pe at α ≤0.30 as the stabilizer - increases with c P '. It is found that the viscosity is a function of the amount of PMA-pe adsorbed in loops in the a-conformation (see chapter 6).

    Degree of neutralization α(or pH)

    As shown in chapter 2 an increase of α gives an expansion of the coil in bulk. At an oil-water interface a higher charge density will reduce the interfacial activity and if adsorption takes place it will result in a thin layer. Moreover, a higher charge density of the adsorbed molecules will give and increased stability of the emulsion droplets by electrostatic repulsion.

    To summarize our findings for this parameter it is necessary to distinguish between esterified (PAA-pe and PMA-pe) and non-esterified (PAA and PMA) polyelectrolytes.

    In general the steady-state interfacial pressure is reduced at higher pH or α(see figs. 3.6.-3.9.). PAA and PMA are even not interfacially active at pH>6. From the Π(∞) values of PMA-pe it is deduced that the adsorption mechanism at high pH differs from that at low pH. The γ-pH cycles underline this conclusion (chapter 4). It appears that adsorption of PMA-pe at pH = 4 enables desorption upon pH increase. However, adsorption at pH = 9, followed by a reduction of the pH to 4, gives an adsorbed layer that is not desorbable upon pH increase. Adsorption at pH 4 takes place with short sequences of segments in trains, whereas at pH 9 these sequences are longer. Desorption of the long sequences is unlikely.

    Spread monolayers of PMA-pe do not confirm the important differences in the interfacial properties as a function of pH. The Π- A isotherms of PMA-pe are hardly dependent on the pH (figs. 3.12-3.14.).

    The differences between esterified and non-esterified polyelectrolytes are also reflected in the emulsifying and stabilizing capacity of these polyelectrolytes as a function of α. At α< 0.5 emulsions stabilized by any of the polyelectrolytes are stable to coalescence, whereas at α>0.5 emulsions prepared with PMA and PAA coalesce directly but emulsions prepared with PMA-pe and PAA-pe remain stable to coalescence (see fig. 5.3.). The obtained area of emulsions stabilized by PMA-pe and PAA-pe as a function of αreflects again the difference in adsorption mechanism between low and high pH.

    As the emulsions have been prepared under standard conditions (time of agitation, intensity of agitation and volume fraction of dispersed phase constant) the differences in S as a function of αand c P ' reflect the mechanism of emulsification. The main factor is the way in which the gradients of γalong the interfaces of newly created droplets are reduced. The supply by diffusion to the interface and the rate of reconformation and spreading reduce these gradients and it is the ratio of their contributions that determine the mechanism of emulsification. From earlier experiments with PVA as emulsifying agent it was found that for polymers with a relatively low flexibility S passes through a maximum as a function of c P '. For polymers with a relatively high flexibility S levels off with increasing c P '. Analogous results are now found for PMA-pe as a function of α. Especially at α ≥0.70 the flexibility is low and a maximum in S is observed. However, the higher αthe less pronounced the maximum is, because of additional factors such as the increased electrostatic repulsion and the reduced diffusion (see section 5.6.). At 0.30 ≤ α ≤0.50 S levels off with increasing c P '. At α ≤0.10 an enhanced coalescence without desorption results in an increasing S with c P ' and the leveling-off or the maximum is not reached in the region of c P ' investigated by us.

    Γ e decreases with increasing αfor all types of polyelectrolytes (see fig. 5.5.). From a comparison with the adsorption at the polystyrene latex surface it is concluded that emulsions stabilized by Na +PMA-pe at α< 0.30 and emulsions stabilized by Ca 2+PMA-pe over the whole region of αare characterized by a coalescence without desorption during emulsification (see section 5.5.4.).

    The viscosity of emulsions stabilized by PMA-pe decreases considerably with higher α. For the explanation see below the section on the conformational transition.

    Ionic strength (NaCI)

    A high ionic strength reduces the electrostatic repulsion on the polyelectrolyte chain and hence increases its flexibility. Moreover, the diffusion to the interface increases with increasing ionic strength. These effects will be more pronounced the higher the degree of neutralization. It is anticipated that Γand Π(∞) will increase with increasing ionic strength. The effect on the emulsion interface is not easy to predict. It depends on the sum effect of the increased diffusion and flexibility and the reduced electrostatic repulsion.

    The effect of ionic strength is only investigated for PMA-pe. In general the interfacial pressure increases with higher ionic strength (see fig. 3.4). It is notable that the ionic strength has no effect on the interfacial pressure at pH ~ 9, whereas it is still measurable at pH = 4. This again indicates the differences in adsorption mechanism as a function of pH.

    At the polystyrene latex surface it is indeed found that Γ increases with increasing ionic strength.

    S decreases and Γ e increases slightly with increasing ionic strength. The viscosity of PMA-pe stabilized emulsions at α< 0.30 does not depend on ionic strength.

    In general the effects of ionic strength are small compared with the more drastic effects for αand the nature of the counterion.

    Nature of the counterion (Na+, Ca 2+)

    It is anticipated that Ca 2+counterions considerably affect all investigated properties. The bivalent bonding of Ca 2+ions with dissociated carboxylic groups reduces the flexibility of the polyelectrolyte molecule.

    The interfacial pressure is increased and becomes much less a function of αand c p (see figs. 3.5. and 3.8).

    S is decreased and Γ e considerably increased in the presence of Ca 2+ ions. Moreover, S and Γ e are also hardly dependent on α. It is concluded that bivalent bonding is indeed responsible for this behaviour. From a comparison with the adsorption on polystyrene latex it is deduced that emulsions stabilized by PMA-pe in the presence of Ca 2+ ions undergo coalescence without desorption during emulsification (see section 5.5.4). The effect of Ca 2+ions on the viscosity of emulsions is not investigated.

    Chemical constitution of the polyelectrolytes

    A number of the results are already mentioned in the summary of the effect of the degree of neutralization. On account of bulk properties a distinction between PMA(-pe) and PAA(-pe) is obvious (see chapter 2). However, it appears that in the interfacial pressure, S and Γ e the difference between esterified and non-esterified polyelectrolytes is more pronounced. The interfacial activity of the esterified polyelectrolytes is higher than that of the non-esterified ones (see fig. 3.9). In general S is higher for the esterified polyelectrolytes, although at α< 0.30 the presence of the ester groups is responsible for an additional coalescence during emulsification, resulting in a lower S for emulsions stabilized by esterified polyelectrolytes (see fig. 5.3). Γ e is higher for the esterified polyelectrolytes than for the non-esterified ones. This effect is very obvious at α< 0.30.

    It is found that the viscosity of emulsions is strongly affected by the presence of the methyl groups in the main chain but not by the presence of esterified groups (see fig. 6.1.). It appears that the high viscosity of emulsions stabilized by PMA-pe at α< 0.30 is related to the occurrence of a hypercoiled conformation in the molecules.

    Conformational transition

    The occurrence of two different conformations in bulk is partly reflected in the interfacial and emulsion properties. S and Γ e are not affected by the presence of the hypercoiled conformation, but only by the presence of the ester groups. Moreover, the interfacial pressure as such does not depend on the conformation. However, the γ-pH cycles (chapter 4) allow us to locate a transition region in the conformation of the adsorbed layer, which is related to the occurrence of the a- and b-conformation in bulk. Adsorption of the PMA-pe molecules onto the interface from the solution in which the a-conformation exists, leads to reversibility of the adsorbed molecules upon increasing pH after adsorption. However, the molecules are irreversibly adsorbed if the b-conformation exists in solution. This difference in the conformation is only detectable for the esterified polyelectrolytes, since for PMA and PAA desorption always occurs upon increasing pH.

    The reversible adsorption of PMA-pe at low pH as compared to the irreversibility at high pH is interpreted by proposing an adsorption model in which the length of the trains plays a dominant role. Adsorption of PMA-pe at pH = 4 gives relatively short trains which are desorbable by increasing pH. However, adsorption from solutions in which the b-conformation exists gives relatively long trains and desorption is less probable by increasing pH.

    The occurrence of two different conformations in the adsorbed layer is indicated in the viscosity of emulsions stabilized by PMA-pe and/or PAA-pe. The viscosity is only very high for emulsions stabilized by PMA-pe at α ≤0.30. It appears to be caused by intermolecular foces of attraction between molecules adsorbed at different droplets and not by a bridging of the droplets by adsorption of one molecule at two or more droplets. These forces are of the same origin as those responsible for the stabilization of the hypercoiled a-conformation. From the viscosity of emulsions stabilized by PMA-pe at α= 0.10 in a mixture of water and methanol as the continuous phase it is concluded that the main stabilizing factor for the hypercoiled structure is hydrophobic bonding. This conclusion disagrees with some literature references in which v.d. Waals forces of attraction are held responsible.

    It appears that the prediction of emulsion properties from the interfacial tension measurements at undisturbed interfaces and from the bulk properties of the polyelectrolyte is a difficult problem, because of the very complex dynamic processes occurring during emulsification. However, this study demonstrates that a number of parameters exert an influence on one or more of the investigated emulsion properties and on the interfacial properties at undisturbed interfaces. It provides more insight into the factors which influence the behaviour of polyelectrolytes at disturbed and undistrubed liquid-liquid interfaces. It contributes to a better conception of the behaviour of proteins at interfaces in more practical systems.

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