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).
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).
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 m3h-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.