Elke Scholten

Self-assemblies of lecithin and α-tocopherol as gelators of lipid material

Amongst the different mechanisms that have been proposed and used to structure organogels, self-assembly of the gelators into supramolecular structures linked through non-covalent bonds is the most interesting. The gelator activity of LMGOs is often found most effective when micellar or lamellar phases are obtained, which is dependent on the gelator geometry and the specific packing parameter. Gelation can therefore be induced by altering the packing parameter of different gelators, but due to the law restrictions there are only a few edible gelators that can be used to structure edible lipids. Here, we show that a combination of α-tocopherol and phosphatidylcholine (PC) can be used to alter the packing geometry to provide supramolecular structures needed for the organogelation. We have observed that when the gelators were combined at 1 : 1 ratio in sunflower oil, edible organogels were obtained. The firmness of the solid-like material was enhanced when 1.0 wt% of water was added. The proposed mechanism for this assembly is that most likely cylindrical micellar structures are formed, due to combined assembly of the α-tocopherol and phosphatidylcholine, stabilized through physical interactions. Since these interactions, and the accompanied packing geometry, depends on temperature and application of external stresses, the formation of the organogels showed reversibility when the organogels were subjected to shear or when the temperature was increased to values above 35 °C. Polarized microscopy along with small angle X-ray scattering were used to provide a hypothesis for the mechanism behind the gelation.

Purified oleosins at air–water interfaces

Oleosins are low molecular mass proteins that are distinguished from other proteins for their extended central hydrophobic domain which covers almost half of its entity. For this work, they were extracted from isolated maize germ oil bodies. The purification steps included washing with diethylether and a chloroform–methanol–water mixture. Their amphipathic terminal domains positioned at the organelle surface, where they strongly interact with the surface polar phospholipids, made the application of a third washing step with acetone essential. Although oleosins are well known for their insolubility in water, we were able to prepare aqueous buffer solutions of 0.008 wt% at pH 8.0. The interfacial behaviour of oleosins was studied, in order to predict their ability to stabilize foams. Even at low concentrations they were capable of decreasing the interfacial tension of air–water interfaces to values similar to those obtained from milk protein or egg yolk apolipoproteins adsorption. Pendant drop profile analysis showed that the dilatational elastic modulus was frequency-dependent, but at the same time the elastic to viscous modulus ratio was frequency-independent and below 0.1. The results indicate that oleosins might have a high potential as foam stabilizers, a fast developing and challenging field.

Polymer organogelation with chitin and chitin nanocrystals

In this paper, we show that biodegradable and biocompatible organogels can be formed with chitin as the filler material and triglycerides as the continuous hydrophobic phase. When crude chitin was used, a large degree of aggregation was observed that prevented the formation of stable organogels. Two approaches were used to diminish this degree of aggregation and increase the stability. Either surfactants were used to increase the dispersability of the crude chitin, or the crude chitin was transformed into smaller rod-like nanocrystals by acid hydrolysis. Both approaches led to the formation of stable organogels with storage moduli up to 106 Pa for high chitin concentrations (20 wt%). Three different types of surfactants were used, namely phosphatidylcholine, enzymatically modified phosphatidylcholine and sorbitan monostearate (Span 60). The choice of surfactant has a large influence on the gel strength and the temperature sensitivity of the gels. With chitin nanocrystals, in the presence of surfactants, larger gel strengths were observed for lower concentrations (1–10 wt%), indicating more efficient packing of the particles. Gels were stable even after addition of considerable amounts of water up to 25 wt%. The increase in gel strength in the presence of water (storage modulus) was most likely an effect of the water absorption ability of chitin that increased the effective volume fraction of the fillers.

Protein Oleogels from Protein Hydrogels via a Stepwise Solvent Exchange Route.

We investigated the use of whey protein isolate (WPI) as oleogelator in liquid oil. First, heat-set WPI hydrogels were prepared varying in microstructure and network density. Then, by applying a stepwise solvent exchange procedure via an intermediate solvent, full replacement of the internal aqueous phase within the protein matrix by sunflower oil was achieved. The solvent exchange procedure was performed by using either acetone or tetrahydrofuran (THF) as intermediate solvent. The oil inside the protein matrix was homogeneously distributed without any noticeable damage to the structure. Analyzing the weight change of the protein gel as a result of the solvent exchange shows that the oil holding capacity depends on the microstructure, the polarity of the intermediate solvent, and the kinetics of the solvent exchange. Depending on the gel microstructure and protein concentration of the preceding hydrogel, the oil content in the oleogels was found to be as high as 91 wt %. Oil holding capacity correlated well with the water holding capacity of the preceding hydrogel, and its Youngs modulus (stiffness). It was found that the oleogels, compared to the hydrogels, were much stiffer, as the Youngs modulus increased by 2 orders of magnitude and showed a lower strain at fracture. Our novel route of structuring oil by immobilizing liquid oil inside a biodegradable protein gel matrix with tunable mechanical properties could be relevant for developing novel materials, e.g., in pharmaceutical, nutraceutical, and food applications.

Descriptive sensory profiling of double emulsions with gelled and non-gelled inner water phase

The use of double emulsions (w1/o/w2) has been acknowledged as a promising strategy to reduce oil content in several food applications. Despite the potential of double emulsions for oil reduction, their sensory properties have not been investigated. In this study, we investigated sensory perception of double emulsions by descriptive sensory profiling using a trained panel (n=11). Two sets of emulsions with either 30 or 50% dispersed phase fraction were studied. Each set differed in composition (gelled and non-gelled inner w1 phase, gelatin as gelling agent) and fat reduction level (30 to 50%), but was similar in oil droplet size and viscosity. Fat reduction level depended on the amount of water droplets entrapped inside the oil droplets. Emulsions were evaluated on nine attributes describing taste (T), mouth-feel (MF) and after-feel (AF) perception, including thickness (MF), creaminess (MF, AF), fattiness (MF, AF), and cohesiveness (MF). The replacement of oil by small water droplets w1 did not decrease the intensity of fat-related attributes. When inner w1 droplets were gelled, 47wt.% of oil could be replaced while increasing the intensity of fat-related attributes. This indicates that the sensory perception of single and double emulsions with gelled and non-gelled w1 phase is mainly determined by the total oil droplet surface area. The composition of the inner water phase (gelled or not) also influences the sensory perception of double emulsions. We conclude that fat reduction up to 47wt.% can be achieved in double emulsions while maintaining or enhancing fat-related sensory perception.

Organogel formation via supramolecular assembly of oleic acid and sodium oleate

To create materials with novel functionalities, the formation of gels within hydrophobic media has become popular. This is often accomplished through the assembly of low molecular weight organogelators into a variety of complex phases through intermolecular interactions. In the case of edible materials, the assembly of saturated fatty acids to form fat crystal networks is often used for structuring. Here, the first example of structuring with unsaturated fatty acids is reported, namely mixtures of oleic acid and sodium oleate, to structure edible lipid phases. Small-angle scattering demonstrates that the resultant structures, which vary with oleic acid and sodium oleate molar ratio, comprise either inverse micellar or lamellar phases, combined with the formation of crystalline space-filling networks. Network formation was found for filler concentrations above 10 wt%. Rheological measurements show that gel strength depends on the ratio of oleic acid to sodium oleate, and is greater when only oleic acid is used. The addition of up to 1.5 wt% of water enhanced the strength of the organogels, probably through supplementary hydrogen bonding but, for concentrations greater than 2.0 wt%, the assembly was inhibited leading to collapse of the gel.

The effect of calcium on the composition and physical properties of whey protein particles prepared using emulsification.

Protein microparticles were formed through emulsification of 25% (w/w) whey protein isolate (WPI) solutions containing various concentrations of calcium (0.0-400.0mM) in an oil phase stabilized by polyglycerol polyricinoleate (PGPR). The emulsions were heated (at 80°C) and the microparticles subsequently re-dispersed in an aqueous phase. Light microscopy and scanning electron microscopy (SEM) images revealed that control particles and those prepared with 7.4mM calcium were spherical and smooth. Particles prepared with 15.0mM calcium gained an irregular, cauliflower-like structure, and at concentrations larger than 30.0mM, shells formed and the particles were no longer spherical. These results describe, for the first time, the potential of modulating the properties of dense whey protein particles by using calcium, and may be used as structuring agents for the design of functional food matrices with increased protein and calcium content.

Natural amphiphilic proteins as tri-block Janus particles: Self-sorting into thermo-responsive gels

Design of particles for controlled assembly into complex 3D structure has been extensively investigated. However, their use in the preparation of such 3D structures has received less attention due to low availability. Here, we use a Janus-type particle in the form of a tri-block of natural origin, the protein zein, to create such 3D structures in the form of a thermo-responsive gel. Assembly behaviour of the tri-blocks can be initiated onto spherical hydrophobic surfaces, which leads to stacking of the tri-blocks into a space-spanning network. Rheological behaviour is dependent on the interplay between the hydrophobic interactions between the tri-blocks and the solvent quality, and can be used to alter the temperature at which the network collapses and the gel strength decreases by two orders of magnitude, representing a more fluid-like behaviour.

The effect of oil type on network formation by protein aggregates into oleogels

The aim of this study was to assess the effect of oil type on the network formation of heat-set protein aggregates in liquid oil. The gelling properties of such aggregates to structure oil into so-called ‘oleogels’ are related to both the particle–particle and particle–solvent interactions. To change these interactions, four different oils (medium chain triglyceride oil, sunflower oil, extra virgin olive oil and castor oil) differing in polarity were used. The rheological properties of the protein aggregate oleogels were determined and compared to gels prepared with hydrophilic and hydrophobic colloidal silica particles at the same concentration. The results show that gel strength of the network formed by protein aggregates is affected by the polarity of the oil, resulting in weaker gels in more polar oils as a result of larger particle–solvent interactions. Similar results were obtained for hydrophilic silica particles. In the case of castor oil, the increased particle–solvent interactions though hydrogen bonds limited gel formation for all particle types. Large deformation rheology shows that protein oleogels exhibit a yielding behaviour under large deformation, but regenerate its elasticity quickly after deformation is reduced. The rapid recovery of the protein network and the fact that the interactions between the protein aggregates can be tuned by changing the characteristics of the oil, may be interesting features for various applications in foods.

Effect of outer water phase composition on oil droplet size and yield of (w 1 /o/w 2 ) double emulsions

In this study the effect of various emulsifiers (whey protein isolate (WPI), Na-caseinate, and Tween 20) and thickeners (xanthan and pectin) present in the outer water phase, w2, on oil droplet size and yield of the inner water phase, w1, of (w1/o/w2) double emulsions was investigated. Double emulsions stabilized by Tween 20 had smaller oil droplet sizes and higher yields in comparison to emulsions stabilized by WPI and Na-caseinate. Gelation of the inner water droplets w1 increased yield by 20% for all emulsifiers. Upon the addition of thickeners, the increasing viscosity of the outer water phase, w2, facilitated oil droplet breakup. This resulted in smaller oil droplets and lower yields. When pectin was used as a thickener, in comparison to xanthan, an additional decrease in yield was observed. The yield decreased to values close to zero indicating that all inner water droplets w1 were lost during emulsification. We conclude that type of hydrophilic emulsifier, properties of inner water droplets, viscosity ratio of continuous and dispersed phase, as well as type of thickener influence oil droplet size and yield of w1 phase of double emulsions. This work provides a better understanding of how composition influences the properties of double emulsions and how this can be used to design double emulsions as fat replacers in more complex food systems.