Records 1 - 20 / 262
Horizontal and vertical diversity jointly shape food web stability against small and large perturbations
Zhao, Qinghua ; Brink, Paul J. Van den; Carpentier, Camille ; Wang, Yingying X.G. ; Rodríguez-Sánchez, Pablo ; Xu, Chi ; Vollbrecht, Silke ; Gillissen, Frits ; Vollebregt, Marlies ; Wang, Shaopeng ; Laender, Frederik De - \ 2019
Ecology Letters 22 (2019)7. - ISSN 1461-023X - p. 1152 - 1162.
Equilibrium - horizontal diversity - large perturbations - small perturbations - stability - vertical diversity
The biodiversity of food webs is composed of horizontal (i.e. within trophic levels) and vertical diversity (i.e. the number of trophic levels). Understanding their joint effect on stability is a key challenge. Theory mostly considers their individual effects and focuses on small perturbations near equilibrium in hypothetical food webs. Here, we study the joint effects of horizontal and vertical diversity on the stability of hypothetical (modelled) and empirical food webs. In modelled food webs, horizontal and vertical diversity increased and decreased stability, respectively, with a stronger positive effect of producer diversity on stability at higher consumer diversity. Experiments with an empirical plankton food web, where we manipulated horizontal and vertical diversity and measured stability from species interactions and from resilience against large perturbations, confirmed these predictions. Taken together, our findings highlight the need to conserve horizontal biodiversity at different trophic levels to ensure stability.
Stability of creatine monohydrate and guanidinoacetic acid during manufacture (retorting and extrusion) and storage of dog foods
Poel, Antonius F.B. van der; Braun, Ulrike ; Hendriks, Wouter H. ; Bosch, Guido - \ 2019
Journal of Animal Physiology and Animal Nutrition (2019). - ISSN 0931-2439
additive - creatinine - guanidinoacetic acid - heat sterilisation - stability - uniformity
The stability of creatine monohydrate (CrMH), crystallised guanidinoacetic acid (GAA-C) and granulated GAA (GAA-G) in a moist retorted and a dry extruded dog food formulation during production and storage was investigated. Commercial food mixtures were supplemented with CrMH, GAA-C or GAA-G. Uniformity after mixing and retorting or extrusion was determined based on replicate samples (moist n = 8, dry n = 10). Storage stability was evaluated at 25°C/60% relative humidity for 15 months and 40°C/75% for 6 months. Foods with CrMH were analysed for creatine (Cr) and creatinine (Crn), whereas GAA-C and GAA-G foods were analysed for GAA concentrations. Coefficients of variation (CV) for uniformity of the additives after mixing of moist and dry pet food formulations were below 15%, and the CV was lower in processed mixtures. Recoveries after retorting and extrusion were higher for GAA-G (79 and 99%) and GAA-C (89 and 86%) compared to CrMH (36 and 85%) foods. In moist CrMH food, Cr concentrations re-increased by 54% whilst Crn concentrations decreased by 39% after storage at 25°C for 15 months. With total molar Cr + Crn remaining stable throughout storage, Crn and Cr appeared to effectively interconvert. Storage of the extruded CrMH food at 25°C for 15 months resulted in a 63% decrease in Cr and a 39% increase in Crn concentration. The decrease in Cr concentration was larger at 6 months storage at 40°C compared to 15 months storage at 25°C. Both GAA-C and GAA-G moist and dry foods were stable during storage (<10% decrease). This study showed that GAA is highly stable during production and storage of moist and dry canine foods whilst CrMH is relatively unstable, particularly during storage. The latter makes it difficult to establish a guaranteed Cr content in finished moist retorted and dry extruded foods with CrMH.
Superior Stability of Au/SiO2 Compared to Au/TiO2 Catalysts for the Selective Hydrogenation of Butadiene
Masoud, Nazila ; Delannoy, Laurent ; Schaink, Herrick ; Eerden, Ad van der; Rijk, Jan Willem de; Silva, Tiago A.G. ; Banerjee, Dipanjan ; Meeldijk, Johannes D. ; Jong, Krijn P. de; Louis, Catherine ; Jongh, Petra E. de - \ 2017
ACS Catalysis 7 (2017)9. - ISSN 2155-5435 - p. 5594 - 5603.
butadiene - catalyst - gold - selective hydrogenation - stability - supported nanoparticles
Supported gold nanoparticles are highly selective catalysts for a range of both liquid-phase and gas-phase hydrogenation reactions. However, little is known about their stability during gas-phase catalysis and the influence of the support thereon. We report on the activity, selectivity, and stability of 2-4 nm Au nanoparticulate catalysts, supported on either TiO2 or SiO2, for the hydrogenation of 0.3% butadiene in the presence of 30% propene. Direct comparison of the stability of the Au catalysts was possible as they were prepared via the same method but on different supports. At full conversion of butadiene, only 0.1% of the propene was converted for both supported catalysts, demonstrating their high selectivity. The TiO2-supported catalysts showed a steady loss of activity, which was recovered by heating in air. We demonstrated that the deactivation was not caused by significant metal particle growth or strong metal-support interaction, but rather, it is related to the deposition of carbonaceous species under reaction conditions. In contrast, all the SiO2-supported catalysts were highly stable, with very limited formation of carbonaceous deposits. It shows that SiO2-supported catalysts, despite their 2-3 times lower initial activities, clearly outperform TiO2-supported catalysts within a day of run time. (Graph Presented).
Foam properties of proteins, low molecular weight surfactants and their complexes
Lech, F.J. - \ 2016
Wageningen University. Promotor(en): Harry Gruppen; Peter Wierenga; Marcel Meinders. - Wageningen : Wageningen University - ISBN 9789462576247 - 122
surfactants - proteins - bovine serum albumin - beta-lactoglobulin - lysozyme - foams - chemical properties - stability - mixtures - food chemistry - oppervlaktespanningsverlagende stoffen - eiwitten - runderserumalbumine - bèta-lactoglobuline - lysozym - schuim - chemische eigenschappen - stabiliteit - mengsels - voedselchemie
This thesis shows the effects that the addition of low molecular weight surfactants (LWMS) to proteins has on the foam stability of the mixture. For this, the bulk, interfacial, thin liquid films and foam properties are determined for different protein-LWMS mixtures at different molar ratios (MR). It was shown that the MR as well as the charge of the protein and LMWS determine the foam stability of the mixtures. For all mixtures it was found that the proteins have a select number of high affinity binding sites. So, the concentration of free LMWS in the solution is 0 until a critical MR (MRcr), at which all high affinity binding sites are saturated. Above this MRcr, part of the LMWS binds to low affinity binding sites of the proteins. The low affinity binding sites have a binding ratio < 1, which determines the concentration of bound and free LMWS. For similarly charged protein-LMWS mixtures (i.e. b-lactoglobulin (BLG) and sodium dodecyl sulphate (SDS) and bovine serum albumin (BSA) and SDS at pH 7) the foam stability typically decreases from the foam stability of the pure protein solution (MR 0) until MRcr is reached. At MR > MRcr the foam stability is dominated by the amount of free LMWS. For oppositely charged protein-LMWS mixtures, the binding of the LMWS to the proteins can be described in a similar way, although the number of high affinity sites and low affinity binding ratio are different. There is also a regime of MRs in which the protein-LMWS complexes form large aggregates. These aggregates were in some cases found to increase foam stability (lysozyme (LYS) and SDS and BLG-SDS at pH 3), while in another case (BLG and cetyltrimethylammonium bromide (CTAB)) they lead to decreased foam stability. Still, in all cases it was found that above MRD the aggregates dissociate and the foam stability becomes dominated by free surfactants, equivalent to what was observed for similarly charged protein-LMWS mixtures.
A multi-scale model was developed to describe the stability of thin liquid films in terms of rupture time and thickness. Initially, the model was used to predict the stability of surfactant free films of water and electrolyte solutions. Later, it was used to predict the foam stability in LYS-SDS mixtures. For that purpose, the model was combined with a foam drainage model to provide theoretical estimations of foam stability. This model is the basis to understand coalescence of bubbles in foam. Finally, the concept of the critical MRs and the free LMWS was introduced. Using this, the foam properties of protein-LMWS mixtures can partly be predicted by relative charge of the components and the binding to both high and low affinity binding sites.
Effect of Temperature and Pressure on the Stability of Protein Microbubbles
Rovers, Tijs A.M. ; Sala, Guido ; Linden, Erik Van Der; Meinders, Marcel B.J. - \ 2016
ACS Applied Materials and Interfaces 8 (2016)1. - ISSN 1944-8244 - p. 333 - 340.
buckling - heating - microbubble - pressure - stability - storage temperature
Protein microbubbles are air bubbles with a network of interacting proteins at the air-water interface. Protein microbubbles are commonly used in medical diagnostic and therapeutic research. They have also recently gained interest in the research area of food as they can be used as structural elements to control texture, allowing for the manufacture of healthier foods with increased consumer perception. For the application of microbubbles in the food industry, it is important to gain insights into their stability under food processing conditions. In this study, we tested the stability of protein microbubbles against heating and pressurization. Microbubbles could be heated to 50 °C for 2 min or pressurized to 100 kPa overpressure for 15 s without significantly affecting their stability. At higher pressures and temperatures, the microbubbles became unstable and buckled. Buckling was observed above a critical pressure and was influenced by the shell modulus. The addition of cross-linkers like glutaraldehyde and tannic acid resulted in microbubbles that were stable against all tested temperatures and overpressures, more specifically, up to 120 °C and 470 kPa, respectively. We found a relation between the storage temperatures of microbubble dispersions (4, 10, 15, and 21 °C) and a decrease in the number of microbubbles with the highest decrease at the highest storage temperature. The average rupture time of microbubbles stored at different storage temperatures followed an Arrhenius relation with an activation energy for rupture of the shell of approximately 27 kT. This strength ensures applicability of microbubbles in food processes only at moderate temperatures and storage for a moderate period of time. After the proteins in the shell are cross-linked, the microbubbles can withstand pressures and temperatures that are representative of food processes.
Plant Species Diversity Mediates Ecosystem Stability of Natural Dune Grasslands in Response to Drought
Rooijen, Nils M. van; Keersmaecker, Wanda de; Ozinga, Wim A. ; Coppin, Pol ; Hennekens, Stephan M. ; Schaminée, Joop H.J. ; Somers, Ben ; Honnay, Olivier - \ 2015
Ecosystems 18 (2015)8. - ISSN 1432-9840 - p. 1383 - 1394.
diversity - drought - dune grasslands - ecosystem functioning - NDVI - remote sensing - stability
How plant species diversity can mediate the temporal stability of ecosystem functioning during periods of environmental stress is still a pressing question in ecology, certainly in the context of predicted increasing frequencies and intensities of climate extremes, such as drought. The vast majority of empirical research in this context is based on relatively small-scaled experiments, where plant species composition is manipulated and ecosystem functions, such as biomass production, are monitored through time. Results of these studies have generally shown that ecosystem functioning is more stable in more species-diverse communities. Yet, there is very little evidence so far that these relations also hold in naturally assembled plant communities. In this study, we combined historical vegetation and climate data with time series of remotely sensed indicators of aboveground biomass production (MODIS NDVI), to quantify how plant species diversity and plant functional diversity correlate with the temporal stability of biomass production in naturally assembled Dutch dune grasslands under the influence of fluctuating drought. We found that the negative NDVI response to drought of grasslands with a higher plant species richness and diversity was significantly lower than the response of less species rich and species-diverse grasslands, indicating a stabilizing role of plant species richness and diversity on biomass production through time. We found no relation between plant functional diversity and NDVI response to drought. This is the first study to generalize experimentally established relations between species diversity and stability of ecosystem functioning to naturally assembled grasslands across a large spatial and temporal scale.
Shape and stability in liquid threads and jets : a link to droplet formation
Heugten, W.G.N. van - \ 2015
Wageningen University. Promotor(en): Cees van Rijn. - Wageningen : Wageningen University - ISBN 9789462575707 - 172
droplets - controlled droplet application - threads - viscosity - stability - fluid mechanics - druppels - controlled droplet application - draden - viscositeit - stabiliteit - vloeistofmechanica
This thesis explores relevant fluid dynamic processes for the formation of uniformly sized droplets in microfluidic systems. Growing droplets made from a bulk source have often liquid threads or jets in between to supply liquid to the droplet. Liquid threads and jets are however known to be instable and finding parameters determining their instability/stability will possibly promote a more controlled formation of uniformly sized droplets. Different droplet formation processes in microfluidic devices are explained, such as cross-flow, co-flow and flow focussing.
Dimensionless numbers (introduced in chapter 1) represent the ratio of relevant forces or pressures acting on the fluids and/or their interfaces. These forces and pressures originate from their related fluid dynamic parameters, such as viscosity, interfacial tension, mass density and velocity of the fluid within a specific fluidic confinement with a certain length scale. We show that the dimensionless Reynolds, Weber and Capillary numbers can be associated with the stability of liquid threads and/or jets and provide insight in droplet formation processes.
The phenomenon of spontaneous droplet formation at low flow rates of an inner fluid confined in a microfluidic channel is studied in chapter 2. A short overview of known processes of spontaneous droplet formation with micro-engineered microfluidic devices is presented. We have studied the process of auto breakup with rectangular and round glass capillaries, the latter provided with micro-corrugations and uniform sized droplets were obtained, but only if the outer fluid is able to enter the capillary during droplet formation.
The process of auto breakup is described by a new analytical model described in chapter 3. The model states that the instability of a liquid thread is induced by the decrease of a local liquid thread pressure inside the capillary near the growing droplet. Predicted droplet sizes have been experimentally verified accurately, and also the predicted breakup length inside a micro-corrugated capillary has been verified. The model states that viscous flow stabilises the liquid thread and that auto breakup happens as long as the capillary number is below a critical capillary number of 0.0625. Above 0.0625 droplets grow infinitely large. Auto breakup is however already hampered at Capillary numbers above 0.03, because between 0.03 and 0.0625 no well controlled droplet sizes could be obtained by auto breakup. This is explained by the observed formation of a partially collapsed inner liquid thread that remains open and supplies the growing droplet with inner fluid.
In chapter 4 the formation and stability of a liquid thread in free surface flow feeding a large growing droplet is demonstrated and discussed. The shape of the liquid thread is positively tapering (towards the droplet) and can be described accurately by a Navier-Stokes based ordinary differential equation (ODE) assuming steady state, axisymmetry and an averaged fluid velocity over the cross section of the liquid thread. The axial shape of a viscous liquid thread is concave and its radial dimension has initially a cubic dependence with respect to the axial dimension. A driving force to stabilise the liquid thread was identified, which is a pressure gradient Q = Q0/L – Q1. Q0 is the pressure drop over thread length L, and Q1 is interfacial based dissipation of energy of the outer fluid. The maximum length of the liquid thread is predicted to be reached when Q goes towards 0 as the ratio Q0/Q1.
Shape and stability of emanating liquid jets, which appear after impact of falling droplets from a deep liquid, is presented in chapter 5. During rise and fall of the jet due to gravity, the jet is additionally decelerated towards the liquid surface by a tensile retraction force from the surface tension force exerted on the jet surface by the liquid bath. The retracting force generates an inertial deceleration pressure inside the jet that is balanced by the local Laplace pressure, herewith defining its local curvature and therefore also the shape of the complete jet. A deceleration based Young-Laplace equation is introduced and the predicted shape is experimentally verified for different fluids. Furthermore, the size of droplets forming on the tip of the jet can also be explained by the found pressure balance between the local Laplace pressure and the inertial deceleration of the jet (including the forming droplet).
In general we found that the stability of a liquid thread or jet seems correlated with an applied pressure difference that is distributed between the begin and end of the thread or jet. Studying auto breakup (chapters 2 and 3) of a confined liquid thread it was found that only when the applied pressure is high enough the liquid thread is stable and infinitely large droplets are formed. For the free surface flow liquid thread (chapter 4) it was found that breakup happens when the applied pressure gradient over the length of the thread goes to zero. For the emanating jet (chapter 5) an inertial pressure difference between the base and tip of the jet comes into existence that opposes the squeezing Laplace pressure that wants to break up the liquid jet. Furthermore we found that the last stages of droplet breakup from a liquid thread or jet appeared to follow universal pinch-off, and also that micro-thread formation is observed between droplet and liquid thread or jet.
Microbubble stability and applications in food
Rovers, T.A.M. - \ 2015
Wageningen University. Promotor(en): Erik van der Linden, co-promotor(en): Marcel Meinders; Guido Sala. - Wageningen : Wageningen University - ISBN 9789462574755 - 138
microbubbles - eiwit - stabiliteit - karakterisering - voedsel - voedseladditieven - oppervlaktespanningsverlagende stoffen - zuurbehandeling - reologische eigenschappen - sensorische evaluatie - tribologie - druk - verwarming - koelen - microbubbles - protein - stability - characterization - food - food additives - surfactants - acid treatment - rheological properties - sensory evaluation - tribology - pressure - heating - cooling
Aeration of food is considered to be a good method to create a texture and mouthfeel of food products that is liked by the consumer. However, traditional foams are not stable for a prolonged time. Microbubbles are air bubbles covered with a shell that slows down disproportionation significantly and arrests coalescence. Protein stabilized microbubbles are seen as a promising new food ingredient for encapsulation, to replace fat, to create new textures, and to improve sensorial properties of foods. In order to explore the possible functionalities of microbubbles in food systems, a good understanding is required regarding the formation of protein stabilized microbubbles as well as their stability in environments and at conditions encountered in food products. The aim of this research was to investigate the key parameters for applications of microbubbles in food systems. In Chapter 1 an introduction to this topic is given.
In Chapter 2, the effect of the microbubble preparation parameters on the microbubble characteristics, like the microbubble yield, size and stability, was investigated. The protein Bovine Serum Albumin (BSA) and the method sonication was used to manufacture the microbubbles. The manufactured number and stability of microbubbles was highest when they were prepared at a pH around 5 to 6, just above the isoelectric point, and at an ionic strength of 1.0 M. This can be related to the protein coverage at the air/water interface of air bubbles formed during sonication. At a pH close to the isoelectric point the BSA molecules is in its native configuration. Also the repulsion between the proteins is minimized at these pH values and ionic strength. Both the native configuration and the limited repulsion between the proteins result in an optimal protein coverage during the first part of sonication. Also a high protein concentration contributes to a higher surface coverage. The surface coverage is proportional to the protein concentration up to a concentration of 7.5% after which an increase in protein concentration did not lead to a substantial increase in the number of microbubble . In the second part of sonication the protein layer around the air bubble becomes thicker and stronger by heat induced protein-protein interactions. We found that and at a preheating temperature of 55-60°C, about 5 °C below the BSA denaturation temperature, and a final solution temperature of 60-65°C most microbubbles were obtained, while at higher temperatures mainly protein aggregates and (almost) no microbubbles are formed. This suggests that at temperature of around 60°C to 65°C protein aggregated mostly at the air-water interface creating a multi-layered shell, while at higher temperature, they also aggregated in bulk. These aggregates cannot form microbubbles. We found that optimal preparation parameters strongly depend on the protein batch. We hypothesize that the differences in microbubble formation between the protein batches is due to (small) differences in the protein molecular and denaturation properties that determine the temperature at which the molecules start to interact at the air-water interface. Microbubbles made with different protein concentration and preheating temperatures shrunk in time to a radius between 300 nm and 350 nm, after which the size remained constant during further storage. We argue that the driving force for the shrinkage was the Laplace pressure, resulting in an air flux from the bubbles to the solution. We argue that the constant final size can be explained by a thickening of the microbubble shell as a result of the microbubble shrinkage, thereby withstanding the Laplace pressure.
In Chapter 3 and Chapter 4, microbubble stability at environments and conditions representative for food products were studies. In Chapter 3 we investigated the stability upon addition of surfactants and acid, When surfactants or acid were added, the microbubbles disappeared in three subsequent steps. The release of air from the microbubble can be well described with the two-parameter Weibull process. This suggests two processes are responsible for the release of air: 1) a shell-weakening process and 2) a random fracture of the weakened shell. After the air has been released from the microbubble the third process is identified in the microbubble disintegration: 3) the shell disintegrated completely into nanometer-sized particles. The probability of fracture was exponentially proportional to the concentration of acid and surfactant, meaning that a lower average breaking time and a higher decay rate were observed at higher surfactant or acid concentrations. For different surfactants, different decay rates were found. The disintegration of the shell into monomeric proteins upon addition of acid or surfactants shows that the interactions in the shell are non-covalent and most probably hydrophobic. After surfactant addition, there was a significant time gap between complete microbubble decay (release of air) and complete shell disintegration, while after acid addition the time at which the complete disintegration of the shell was observed coincided with the time of complete microbubble decay.
In Chapter 4 the stability of the microbubbles upon pressure treatment, upon fast cooling after heating and at different storage temperatures was studied. The microbubble stability significantly decreased when microbubbles were pressurized above 1 bar overpressure for 15 seconds or heated above 50°C for 2 minutes. Above those pressures the microbubbles became unstable by buckling. Buckling occurred above a critical pressure. This critical pressure is determined by the shell elastic modulus, the thickness of the shell, and the size of the microbubble. Addition of crosslinkers like glutaraldehyde and tannic acid increased the shell elastic modulus. It was shown that microbubbles were stable against all tested temperatures (up to 120°C) and overpressures (4.7 bar) after they were reinforced by crosslinkers. From the average breaking time at different storage temperatures, we deduced that the activation energy to rupture molecular bonds in the microbubbles shell is 27 kT.
In Chapter 5, we investigated the effect of microbubbles on the rheological, tribological sensorial properties of model food systems and we compared this effect to the effect on food systems with emulsion droplets and without an added colloid. We investigated the effect in three model food systems, namely fluids with and without added thickener and a mixed gelatine-agar gel. In a sensory test panellists were asked whether they could discriminate between samples containing microbubbles, emulsion droplets or no added colloid. Emulsions could be sensorially well distinguished from the other two samples, while the microbubble dispersion could not be discriminated from the protein solution. Thus, we concluded that at a volume fraction of 5% of these BSA covered microbubbles were not comparable to oil-in-water emulsions. The good discrimination of emulsion might be ascribed to the fact that emulsion had a lower friction force (measured at shear rates form 10 mm/s to 80 mm/s) than that microbubbles dispersions and protein solutions. Upon mixing emulsions and microbubble dispersions the friction value approximated that of emulsions. This effect was already noticed at only 1.25% (v/v) oil, indicating that microbubbles had not a significant contributions to the friction of these samples. Also microbubble dispersions with and without protein aggregates were compared. The microbubble dispersions with and without thickener containing protein aggregates had a higher viscosity than the those samples without protein aggregates. Protein aggregates in the gelled microbubble sample yielded a higher Young’s modulus and fracture stress. The differences between the gelled samples could be well perceived by the panellists. We attribute this mainly to the fracture properties of the gel. In general we concluded that microbubbles, given their size of ~ 1 mm and volume fraction of 5%, did not contribute to a specific mouthfeel.
Finally in Chapter 6, the results presented in the previous chapters are discussed and put in perspective of the general knowledge on microbubbles production, stability, and applications in food. We described the main mechanisms leading to microbubble formation and stability. We showed that the production parameters significantly influence the interactions in the microbubble shell, and the those interactions highly determine the stability of the microbubbles under several conditions. We reported about limitations of sonication as a method to produce microbubbles suitable for food applications and we provided some ways to overcome these limitations. The use of microbubbles in food systems has been explored and we clearly see possible applications for microbubbles in food. We reported about directions for possible further research.
In this work we made significant progress in understanding the interactions in the microbubble shell and their relation to microbubble stability. We also advanced in comprehension towards possible applications of microbubbles in food.
Lipid bilayer stability in relation to oxide nanoparticles
Pera, H. - \ 2015
Wageningen University. Promotor(en): Frans Leermakers, co-promotor(en): Mieke Kleijn. - Wageningen : Wageningen University - ISBN 9789462574670 - 144
lipids - membranes - stability - nanotechnology - particles - analytical methods - models - modeling - lipiden - membranen - stabiliteit - nanotechnologie - deeltjes - analytische methoden - modellen - modelleren
Lipid bilayer stability in relation to oxide nanoparticles
All living organisms are composed of cells that are filled with a thick molecular soup. These molecules constitute a complex machinery that brings these cells to life. To contain these molecules, and to protect them from the hostile outer environment, a phospholipid bilayer envelopes the cell. It is essential that this lipid bilayer, also known as the cell membrane, should remain intact and form a perfect barrier at all times. Industrially manufactured nanoparticles are suspect to be able to penetrate this barrier, and thus endanger living organisms in the environment. This thesis deals with some aspects of the structural integrity of lipid bilayers, and especially how this integrity is affected by the interaction with nanoparticles.
Experiments were performed with silica and titanium dioxide nanoparticles, interacting with lipid bilayers, using a variety of experimental techniques. In addition, a theoretical model was applied that is based on the Scheutjens-Fleer Self Consistent Field (SCF) theory. This model delivered detailed structural and thermodynamic information about the lipid bilayer. The modelling work helped us to improve our understanding of lipid bilayer stability, and showed the effect of the interaction with the nanoparticles on the phospholipid bilayer. These latter results could be related directly to our experiments.
Let us first experimentally regard the interaction of lipid bilayers with synthetic oxide nanoparticles. We developed a protocol for high-throughput screening of the nanoparticle-bilayer interaction using a fluorescence technique. Results from this method were combined with reflectometry measurements and atomic force microscopy (AFM). The combination of these methods allowed us to relate lipid bilayer integrity to its interaction with nanoparticles and their adsorption onto the bilayer. In addition, the AFM results yielded detailed structural information at the nano-scale. We found that the interaction strongly depends on both lipid bilayer and nanoparticle charge. However, the specific interaction that depends on the nanoparticle type, starts to play a role when the charges are low. When the total interaction strength is regarded, a regime was found at which interaction is strong enough for the nanoparticles to adsorb onto the bilayer, but too weak to disrupt the bilayer. If, however, the bilayer is disrupted by the nanoparticles, the particle may steal away some lipid molecules from the bilayer, and leave again to disrupt the bilayer elsewhere.
Let us now go into more detail on the SCF modelling. Bilayers are composed of phospholipids, which consist of a hydrophilic head group, and a hydrophobic tail. These bilayers were modelled using a single lipid molecule type, of which the head group structure and lipid tail length could be varied. We thus obtained bilayers that varied in their thickness, and the space that a single lipid takes within the bilayer. Changes in bilayer composition affect the bilayer mechanical properties, such as those constants that describe bilayer stretching or bending. This thesis shows how vesicles, which are bilayers in a globular shape, may become unstable if the bilayer lipid composition is changed. Under certain conditions, a vesicle would prefer to fall apart into many smaller vesicles, which is when highly charged head groups start to repel each other. Or the bilayer may form continuous cubic phases, which might occur if lipids with uncharged head groups but with very long tails are used to form the bilayer. Under very specific and finely tuned conditions, a lipid bilayer may become unstable to form stable pores in the membrane, or to fall apart into tiny lipid discs.
Hydrodynamic model for drying emulsions
Feng Huanhuan, Huanhuan ; Sprakel, J.H.B. ; Gucht, J. van der - \ 2015
Physical Review. E, Statistical nonlinear, and soft matter physics 92 (2015). - ISSN 2470-0045 - 8 p.
latex film formation - atomic-force microscopy - foam drainage - phase inversion - coalescence - dynamics - surfactant - stability - fracture
We present a hydrodynamic model for film formation in a dense oil-in-water emulsion under a unidirectional drying stress. Water flow through the plateau borders towards the drying end leads to the buildup of a pressure gradient. When the local pressure exceeds the critical disjoining pressure, the water films between droplets break and the droplets coalesce. We show that, depending on the critical pressure and the evaporation rate, the coalescence can occur in two distinct modes. At low critical pressures and low evaporation rates, coalescence occurs throughout the sample, whereas at high critical pressures and high evaporation rate, coalescence occurs only at the front. In the latter case, an oil layer develops on top of the film, which acts as a diffusive barrier and slows down film formation. Our findings, which are summarized in a state diagram for film formation, are in agreement with recent experimental findings.
Preliminary UHPLC–PDA–ESI-MS screening of light-accelerated autoxidation products of the tetrapyrrole biliverdin
Weesepoel, Y.J.A. ; Gruppen, H. ; Vincken, J.P. - \ 2015
Food Chemistry 173 (2015). - ISSN 0308-8146 - p. 624 - 628.
spirulina-platensis - mass-spectrometry - c-phycocyanin - stability - preservatives - antioxidants - pigments
The application of phycobiliproteins, e.g. blue C-phycocyanin, as natural water-soluble food colourants is emerging. The chromophore of these proteins comprises a number of tetrapyrroles (or phycocyanobilins), which have an extensive conjugated system, vulnerable to autoxidation. To assess the autoxidation products, a simplified model system was used in which the free tetrapyrrole biliverdin, instead of phycobiliprotein, was subjected to light-accelerated autoxidation. Degradation products of biliverdin were subsequently annotated by reversed-phase ultra-high performance liquid chromatography with a photo diode array and positive mode in-line electrospray ionization mass spectrometry. To facilitate the analysis of degradation products, autoxidation of the three methine bridges in biliverdin was mimicked in silico. It was found that both the peripheral and central methine bridges of biliverdin were susceptible to light-accelerated autoxidation. Scission products tentatively annotated with MS2 and MS3 were propionic acid-containing pyrroles. From this, it can be speculated that tetrapyrroles attached to phycobiliproteins are susceptible to autoxidative degradation.
Effect of species and harvest maturity on the fatty acids profile of tropical forages
Khan, N.A. ; Farooq, M.W. ; Ali, M. ; Suleman, M. ; Ahmad, N. ; Sulaiman, S.M. ; Cone, J.W. ; Hendriks, W.H. - \ 2015
The JAPS 25 (2015)3. - ISSN 1018-7081 - p. 739 - 746.
neutral detergent fiber - dairy-cows - perennial ryegrass - maize silages - degradation - stability - grass
The aim of this study was to quantify the fatty acid (FA) content and composition of forages commonly fed to dairy animals in the tropics. Twelve forage species, namely, Trifolium alexandrinum, Cichorium intybus, Hordeum vulgare L., Medicago sativa, Avena sativa, Pennisetum purpureum Setaria anceps, Sorghum almum, Panicum maximum, Rumex nepalensis, Panicum coloratum and Panicum antidotale were evaluated. Each forage species was grown in four replicate plots under standard agronomic conditions, and sampled at early, normal and late stages of maturity. The result of the present study showed that the chemical composition, dry matter digestibility and FA contents varied (P <0.001) among forage species and harvest maturity. Linolenic acid (C18:3n-3), palmitic acid (C16:0) and linoleic acid (C18:2n-6) were the predominant FAs with an average content of 8.65, 3.61 and 2.38 g/kg dry matter (DM), contributing on average to 53%, 22% and 14% of the total measured FAs, respectively. Among the individual FAs, C18:3n-3 had the largest variation ranging from 4.26 to 17.43 g/kg DM at first harvest. The content of C16:0, C18:2n-6 and C18:3n-3 decreased (P <0.001) with maturity, with the largest decrease being observed in C18:3n-3. This study highlights that harvest management is an important tool to manipulate the FA contents and composition within a forage species
Interactions among drainage flows, gravity waves and turbulence: a BLLAST case study
Román Cascón, C. ; Yagüe, C. ; Mahrt, L. ; Sastre, M. ; Steeneveld, G.J. ; Pardyjak, E. ; Boer, A. van de; Hartogensis, O.K. - \ 2015
Atmospheric Chemistry and Physics 15 (2015). - ISSN 1680-7316 - p. 9031 - 9047.
stable boundary-layer - intermittent turbulence - density-current - flux - field - wind - disturbances - simulations - stability - dynamics
The interactions among several stable-boundary-layer (SBL) processes occurring just after the evening transition of 2 July 2011 have been analysed using data from instruments deployed over the area of Lannemezan (France) during the Boundary Layer Late Afternoon and Sunset Turbulence (BLLAST) field campaign. The near-calm situation of the afternoon was followed by the formation of local shallow drainage flows (SDFs) of less than 10 m depth at different locations. The SDF stage ended with the arrival of a stronger wind over a deeper layer more associated with the mountain-plain circulation, which caused mixing and destruction of the SDFs. Several gravity-wave-related oscillations were also observed on different time series. Wavelet analyses and wave parameters were calculated from high resolution and accurate surface pressure data of an array of microbarometers. These waves propagated relatively long distances within the SBL. The effects of these phenomena on turbulent parameters (friction velocity and kinematic heat flux) have been studied through multi-resolution flux decomposition methods performed on high frequency data from sonic anemometers deployed at different heights and locations. With this method, we were able to detect the different time-scales involved in each turbulent parameter and separate them from wave contributions, which becomes very important when choosing averaging-windows for surface flux computations using eddy covariance methods. The extensive instrumentation allowed us to highlight in detail the peculiarities of the surface turbulent parameters in the SBL, where several of the noted processes were interacting and producing important variations in turbulence with height and between sites along the sloping terrain.
Foam preparation at high-throughput using a novel packed bed system
Nazir, A. ; Maan, A.A. ; Sahin, S. ; Boom, R.M. ; Schroën, C.G.P.H. - \ 2015
Food and Bioproducts Processing 94 (2015). - ISSN 0960-3085 - p. 561 - 564.
stability - size
We investigated the formation of food foams using a novel packed bed system at different operating pressures.The foaming process was based on simultaneous injection of continuous (whey protein solution) and dispersed(nitrogen gas) phases into a column containing a packed bed of glass beads. Bubbles were produced by entrapmentof nitrogen by thin films of continuous phase inside the porous medium. Initial results show a proof of principle thatthe proposed system can be an effective method for the controlled production of foams at overruns of up to 600%.The entire window of operation regarding all process and formulation possibilities is expected to be much wider butneeds to be established in future research.
Linkages between aggregate formation, porosity and soil chemical properties
Regelink, I.C. ; Stoof, C.R. ; Rousseva, S. ; Weng, L. ; Lair, G.J. ; Kram, P. ; Nikolaidis, N.P. ; Kercheva, M. ; Banwart, S. ; Comans, R.N.J. - \ 2015
Geoderma 247-248 (2015). - ISSN 0016-7061 - p. 24 - 37.
water-retention capacity - field-flow fractionation - natural organic-matter - iron-oxides - phosphate adsorption - mineral surfaces - tropical soils - size classes - humic-acid - stability
Linkages between soil structure and physical–chemical soil properties are still poorly understood due to the wide size-range at which aggregation occurs and the variety of aggregation factors involved. To improve understanding of these processes, we collected data on aggregate fractions, soil porosity, texture and chemical soil properties of 127 soil samples from three European Critical Zone Observatories. First, we assessed mechanistic linkages between porosity and aggregates. There was no correlation between the fractions of dry-sieved aggregates (N1 mm, DSA) and water-stable aggregates (N0.25 µm, WSA). Soil microporosity and micro + mesoporosity increased with increasing abundance of aggregates, though this correlation was only significant for the WSA fraction. The fraction of DSA did not affect the overall porosity of the soil, but affected the ratio between micro- and mesopores (¿30 kPa/¿0.25 kPa), suggesting that micropores are dominantly located within DSA whereas mesopores are located in betweenDSA and loose particles. Second,we studied the relations between the physical and chemical soil properties and soil structure. Soil texture had only a minor effect on the fractions ofWSA and DSAwhereas Fe-(hydr)oxide content was correlated positively with bothWSA fraction and porosity. This may be attributed to Fe-(hydr)oxides providing adsorption sites for organic substances on larger minerals, thereby enabling poorly reactive mineral particles to be taken up in the network of organic substances. The fraction ofWSA increased with an increase in the soil organic carbon (SOC) and Fe-(hydr)oxides content and with a decrease in pH. This pH-effect can be explained by the enhanced coagulation of organically-coated particles at a lower pH. Overall, this study indicates that mechanistic linkages exist between soil chemical properties, aggregate formation and soil porosity.
Disintegration of protein microbubbles in presence of acid and surfactants: a multi-step process
Rovers, T.A.M. ; Sala, G. ; Linden, E. van der; Meinders, M.B.J. - \ 2015
Soft Matter 11 (2015)32. - ISSN 1744-683X - p. 6403 - 6411.
bovine serum-albumin - microspheres - nanoparticles - fabrication - stability
The stability of protein microbubbles against addition of acid or surfactants was investigated. When these compounds were added, the microbubbles first released the encapsulated air. Subsequently, the protein shell completely disintegrated into nanometer-sized particles. The decrease in the number of intact microbubbles could be well described with the Weibull distribution. This distribution is based on two parameters, which suggests that two phenomena are responsible for the fracture of the microbubble shell. The microbubble shell is first weakened. Subsequently, the weakened protein shell fractures randomly. The probability of fracture turned out to be exponentially proportional to the concentration of acid and surfactant. A higher decay rate and a lower average breaking time were observed at higher acid or surfactant concentrations. For different surfactants, different decay rates were observed. The fact that the microbubble shell was ultimately disintegrated into nanometer-sized particles upon addition of acid or surfactants indicates that the interactions in the shell are non-covalent and most probably hydrophobic. After acid addition, the time at which the complete disintegration of the shell was observed coincided with the time of complete microbubble decay (release of air), while in the case of surfactant addition, there was a significant time gap between complete microbubble decay and complete shell disintegration.
Epigenetic Basis of Morphological Variation and Phenotypic Plasticity in Arabidopsis thaliana
Kooke, R. ; Johannes, F. ; Wardenaar, R. ; Becker, F.F.M. ; Etcheverry, M. ; Colot, V. ; Vreugdenhil, D. ; Keurentjes, J.J.B. - \ 2015
The Plant Cell 27 (2015)2. - ISSN 1040-4651 - p. 337 - 348.
quantitative trait loci - dna methylation - transcription factor - qtl analysis - population - plant - inheritance - stability - evolution - performance
Epigenetics is receiving growing attention in the plant science community. Epigenetic modifications are thought to play a particularly important role in fluctuating environments. It is hypothesized that epigenetics contributes to plant phenotypic plasticity because epigenetic modifications, in contrast to DNA sequence variation, are more likely to be reversible. The population of decrease in DNA methylation 1-2 (ddm1-2)-derived epigenetic recombinant inbred lines (epiRILs) in Arabidopsis thaliana is well suited for studying this hypothesis, as DNA methylation differences are maximized and DNA sequence variation is minimized. Here, we report on the extensive heritable epigenetic variation in plant growth and morphology in neutral and saline conditions detected among the epiRILs. Plant performance, in terms of branching and leaf area, was both reduced and enhanced by different quantitative trait loci (QTLs) in the ddm1-2 inherited epigenotypes. The variation in plasticity associated significantly with certain genomic regions in which the ddm1-2 inherited epigenotypes caused an increased sensitivity to environmental changes, probably due to impaired genetic regulation in the epiRILs. Many of the QTLs for morphology and plasticity overlapped, suggesting major pleiotropic effects. These findings indicate that epigenetics contributes substantially to variation in plant growth, morphology, and plasticity, especially under stress conditions
The natural occurrence of chloramphenicol in crops
Berendsen, B.J.A. ; Zuidema, T. ; Jong, J. de - \ 2015
Wageningen : RIKILT Wageningen UR (RIKILT report 2015.009)
chlooramfenicol - antibiotica - stro - halfwaardetijd - stabiliteit - bodemgiftigheid - streptomyces venezuelae - opname van contaminanten - voedselveiligheid - chloramphenicol - antibiotics - straw - half life - stability - soil toxicity - streptomyces venezuelae - contaminant uptake - food safety
Unexpected findings of the banned antibiotic chloramphenicol in products of animal origin, feed and straw prompted urgent investigation. Therefore a monitoring study in straw was carried out. The monitoring showed that in 37 sample chloramphenicol was detected. In 7 samples a concentration above 0.3 μg kg-1 was found with the highest result at 6.8 μg kg-1. Next the hypothesis was studied that the chloramphenicol is naturally present in soil, through production by soil bacteria, and subsequently can be transferred to crops. First, the stability of chloramphenicol in soil was studied. The fate of chloramphenicol highly depends on soil type and showed a half-life of approximately one day in non-sterile topsoil. It was found to be more stable in sub-soil and sterile soils. Second, the production of chloramphenicol in soil was studied and it was confirmed that Streptomyces venezuelae can produce chloramphenicol at appreciable amounts in non-sterile soil. Third, a transfer study was carried out using wheat and corn grown on three different soils, that were weekly exposed to aqueous chloramphenicol solutions at different levels. Chloramphenicol was taken up by crops as determined by chiral liquid chromatography coupled to tandem mass spectrometric analysis and the levels in crop were found to be bioavailability related. It was concluded that chloramphenicol residues can occur naturally in crops as a result of the production of chloramphenicol by soil bacteria in their natural environment and subsequent uptake by crops.
Loss of Plant Species Diversity Reduces Soil Erosion Resistance
Berendse, F. ; Ruijven, J. van; Jongejans, E. ; Keesstra, S.D. - \ 2015
Ecosystems 18 (2015)5. - ISSN 1432-9840 - p. 881 - 888.
biodiversity loss - productivity - ecology - stability - future - impact
In many estuarine areas around the world, the safety of human societies depends on the functioning of embankments (dikes) that provide protection against river floods and storm tides. Vegetation on land-side slopes protects these embankments from erosion by heavy rains or overtopping waves. We carried out a field experiment to investigate the effect of plant species diversity on soil loss through erosion on a simulated dike. The experiment included four diversity treatments (1, 2, 4, and 8 species). In the third year of the experiment, we measured net annual soil loss by measuring erosion losses every 2 weeks. We show that loss of plant species diversity reduces erosion resistance on these slopes: net annual soil loss increased twofold when diversity declines fourfold. The different plant species had strongly diverging effects on soil erosion, both in the single-species and in the multi-species plots. Analysis of the dynamics of the individual species revealed that the main mechanism explaining the strong effects of plant species diversity on soil erosion is the compensation or insurance effect, that is, the capacity of diverse communities to supply species to take over the functions of species that went extinct as a consequence of fluctuating environmental conditions. We conclude that the protection and restoration of diverse plant communities on embankments and other vegetated slopes are essential to minimize soil erosion, and can contribute to greater safety in the most densely populated areas of the world. Key words: plant species diversity; soil erosion; plant competition; insurance effect; compensation effect.
Composition properties in the river claims problem
Ansink, E.J.H. ; Weikard, H.P. - \ 2015
Social Choice and Welfare 44 (2015)4. - ISSN 0176-1714 - p. 807 - 831.
international water law - climate-change - game-theory - rights - rules - allocation - resources - networks - stability
In a river claims problem, agents are ordered linearly, and they hve both an initial water endowment as well as a claim to the total water resou8rce. We provide characterizations of two solutions to this problem, using Composition properties which have particularly relevant interpretations for the river claims problem. Specifically, these properties relate to situations where river flow is uncertain or highly variable, possibly due to climate change impacts. The only solution that satisfies all says that agents are free to use any water available on their teerritory, without concern for downstream impacts. The other solution that we assess is the "No-harm rule", an extreme interpretation of the "no-harm" principle from international water law, which implies that water is allocated with priority to downstream needs. In addition to characerizing both solutions, we show their relation to priority rules and to sequential sharing rules, and we extend our analysis to general river systems.