E. van der Linden

Rheology and structure of ovalbumin gels at low pH and low ionic strength

The objective of this study was to relate the rheological behavior of ovalbumin gels at low pH and low ionic strength to their mesoscopic structure, using rheological measurements and Transmission Electron Microscopy (TEM). Varying pH, ionic strength and protein concentration, we obtained transparent solutions, transparent gels, opaque gels, or turbid gels, upon heating and subsequent cooling of the ovalbumin solutions. At equal pH and increasing salt concentration we found an increase in turbidity, suggesting that the structure of the aggregates changed from linear or branched to more clustered aggregates. The gel strength increased with increasing salt concentration. A similar trend was observed at equal ionic strength and increasing pH. TEM micrographs of ovalbumin aggregates at pH 2 and 15 mM NaCl show that the ovalbumin aggregates are linear. Increasing the pH from 2 to 3.5 results in structures that are still linear, but with a higher degree of clustering. No random aggregates were observed. TEM micrographs of the gel phase at pH 3.5 and 30 mM NaCl (turbid) show that the structure consists of linear aggregates organized in large clusters of approximately 230–350 nm. At pH 2 and 30 mM NaCl (transparent) the gel consists of single strands with a diameter of about 3.3–3.9 nm, i.e. one or at most two monomers thick.

Polymer Microcapsules with a Fiber-Reinforced Nanocomposite Shell

Polymer microcapsules can be used as controlled release systems in drugs or in foods. Using layer-by-layer adsorption of common food proteins and polysaccharides, we produced a new type of microcapsule with tunable strength and permeability. The shell consists of alternating layers of pectin and whey protein fibrils, yielding a fiber-reinforced nanocomposite shell. The strength can be tightly controlled by varying the number of layers or the density and length of the fibrils in the protein layers. The mechanical stability of these microcapsules appears to be superior to that of currently available multilayer capsules. The method involves only standard unit operations and has the potential for scaling up to industrial production volumes.

Organogel-Emulsions with Mixtures of β-Sitosterol and γ-Oryzanol: Influence of Water Activity and Type of Oil Phase on Gelling Capability

In this study, water-in-oil emulsions were prepared from water containing different salt concentrations dispersed in an oil phase containing a mixture of β-sitosterol and γ-oryzanol. In pure oil, the β-sitosterol and γ-oryzanol molecules self-assemble into tubular microstructures to produce a firm organogel. However, in the emulsion, the water molecules bind to the β-sitosterol molecules, forming monohydrate crystals that hinder the formation of the tubules and resulting in a weaker emulsion-gel. Addition of salt to the water phase decreases the water activity, thereby suppressing the formation of sitosterol monohydrate crystals even after prolonged storage times (∼1 year). When the emulsions were prepared with less polar oils, the tubular microstructure was promoted, which significantly increased the firmness of the emulsion-gel. The main conclusion of this study is that the formation of oryzanol and sitosterol tubular microstructure in the emulsion can be promoted by reducing the water activity and/or by using oils of low polarity.

Influence of protein hydrolysis on the growth kinetics of β-lg fibrils.

Recently it was found that protein hydrolysis is an important step in the formation of β-lactoglobulin fibrils at pH 2 and elevated temperatures. The objective of the present study was to further investigate the influence of hydrolysis on the kinetics of fibril formation. Both the hydrolysis of β-lactoglobulin and the growth of the fibrils were followed as a function of time and temperature, using SDS polyacrylamide gel electrophoresis and a Thioflavin T fluorescence assay. As an essential extension to existing models, the quantification of the effect of the hydrolysis on the fibrillar growth was established by a simple polymerization model including a hydrolysis step.

Suppression of depletion flocculation in oil-in-water emulsions: a kinetic effect of β-lactoglobulin

Abstract This paper reports on creaming and flocculation in 10% (w/w) oil-in-water emulsions, stabilised by β-lactoglobulin (β-lg) and flocculated by dextran. Dextran and an additional amount of β-lg were added at various concentrations after emulsion formation. A substantial effect of the β-lg concentration was observed. At higher β-lg concentrations, a larger dextran concentration was required to induce network formation. This effect was explained by a retardation of the flocculation process at larger β-lg concentrations, shown by diffusing wave spectroscopy (DWS). This retardation was caused by the unexpectedly high apparent viscosity at low shear-rates of mixed solutions of β-lg and dextran.

Effects of flow on hen egg white lysozyme (HEWL) fibril formation: length distribution, flexibility, and kinetics.

The effect of steady shear and turbulent flow on the formation of amyloid fibrils from hen egg white lysozyme (HEWL) was studied. The conversion and size distribution of fibrils obtained by heating HEWL solutions at pH 2 were determined. The formation of fibrils was quantified using flow-induced birefringence. The size distribution was fitted using decay of birefringence measurements and transmission electron microscopy (TEM). The morphology of HEWL fibrils and the kinetics of their formation varied considerably depending on the flow applied. With increasing shear or stirring rate, more rod-like and shorter fibrils were obtained, and the conversion into fibrils was increased. The size distribution and final fibril concentration were significantly different from those obtained in the same heat treatment at rest. The width of the length distribution of fibrils was influenced by the homogeneity of the flow.

Origin of Water Loss from Soy Protein Gels

Water holding (WH) of soy protein gels was investigated to identify which length scales are most contributing to WH when centrifugal forces are applied. More specifically, it was attempted to differentiate between the contributions of submicron and supramicron length scales. MgSO4 and MgCl2 salt specificities on soy protein aggregation (submicron contribution) were used to create different gel morphologies (supramicron contribution). Obtained results showed that the micrometer length scale is the most important contribution to WH of gels under the applied deformation forces. WH of soy protein gels correlated negatively with Youngs modulus and positively with recoverable energy. The occurrence of rupture events had only a limited impact on WH. The ease by which water may be removed from the gel, but not the total amount, seemed to be related to the initial building block size. These insights could be exploited in product development to predict and tune oral perception properties of (new) products.

Relation between Gelation Conditions and the Physical Properties of Whey Protein Particles

Whey protein particles have several applications in modulating food structure and for encapsulation, but there is a lack of methods to prepare particles with a very high internal protein content. In this study whey protein particles with high internal protein content were prepared through emulsification and heat gelation of 25% (w/w) whey protein isolate solution at different pH (6.8 or 5.5) and NaCl concentrations (50, 200, or 400 mM). Particles formed at pH 6.8 were spherical, whereas those formed at pH 5.5 were irregular and had a cauliflower-like appearance. Both particles had an average size of few micrometers, and the particles formed at pH 5.5 had higher protein content (∼39% w/v) than the particles formed at pH 6.8 (∼18% w/v). Similarly, particle morphology and protein density were also affected by initial NaCl concentration: particles formed at 50 mM NaCl (pH 6.8) were spherical, whereas particles formed at either 200 mM NaCl (pH 6.7) or 400 mM NaCl (pH 6.6) were irregular and protein density of the particles increased with increasing initial NaCl concentration. Whey protein particles formed at pH 5.5 showed an excellent heat stability: viscosity of the suspensions containing approximately 30% of protein particles formed at pH 5.5 did not show any change after heating at 90 °C for 30 min while the viscosity of suspensions containing protein particles prepared at other conditions increased after heating. In summary, whey protein particles with varying microstructure, shape, internal protein density, and heat stability can be formed by using heat-induced gelation of whey protein isolate at different gelling conditions.

Gelation: Principles, Models and Applications to Proteins

Publisher Summary Gelation is an important process for a wide range of applications in food, pharmaceutical, and material sciences. The investigation of food protein gels is complex in that it encompasses everything from the condensed matter physics of forming the gel to the physiological and psychological processes involved in food evaluation. This chapter focuses on the formation of food protein gels and the mechanical and sensory evaluation of protein gels as model foods. It describes and defines the gel state based on key historical developments and presents various examples associated with protein gels to better illustrate key concepts. A common feature to all protein gelation reactions is that they require some initial structural transition that can be considered transformation from an unreactive to a reactive structure that increases the probability of intermolecular interactions. Various models are analyzed that describe the denaturation and aggregation processes that occur prior to formation of a gel network. The protein gel is discussed in terms of its relation to sensory response, in particular its sensory response in terms of structural and mechanical properties of the gel. Secondly, the mechanical response of protein gels is examined in terms of the various structures that exist on a meso-scale. Thirdly, the phenomena that describe how to obtain the various meso-structures are reviewed. One of the future challenges is to identify the characteristic and distinct interaction–concentration regimes for the existing regimes in the phase diagram of proteins in solution.

Modulation of the Gelation Efficiency of Fibrillar and Spherical Aggregates by Means of Thiolation

Fibrillar and spherical aggregates were prepared from whey protein isolate (WPI). These aggregates were thiolated to a substantial degree to observe any impact on functionality. Sulfur-containing groups were introduced on these aggregates which could be converted to thiol groups by deblocking. Changes on a molecular and microstructural level were studied using tryptophan fluorescence, transmission electron microscopy, and particle size analysis. The average size (nm) of spherical aggregates increased from 38 to 68 nm (blocked variant) and 106 nm (deblocked variant) after thiolation, whereas the structure of fibrillar aggregates was not affected. Subsequently, gels containing these different aggregates were prepared. Rheological measurements showed that thiolation decreased the gelation concentration and increased gel strength for both WPI fibrillar and spherical aggregates. This effect was more pronounced upon thiolation of preformed fibrillar aggregates. The findings suggest that thiolation at a protein aggregate level is a promising strategy to increase gelation efficiency.