T. van Vliet

Gel formation by β-conglycinin and glycinin and their mixtures

Gel formation and gel properties of β-conglycinin, glycinin and their mixtures were studied as a function of pH using small and large deformation rheology and differential scanning calorimetry. We conclude that heat denaturation is a prerequisite for gel formation. Gelation temperatures of β-conglycinin were lower than those of glycinin and more dependent on protein concentration. At pH 7.6, protein solutions gelled at a higher temperature than at pH 3.8. Glycinin gels were stiffer than β-conglycinin gels at the same pH and protein concentration, and fractured at a higher strain and stress. At pH 7.6, G′ is lower than at pH 3.8 for both proteins and the gels could be deformed to a larger extent. Based on the appearance of the gels (turbid at pH 7.6, white at pH 3.8) and the fracture properties, we conclude that different network structures are formed as a function of pH. The reason why glycinin gives a better gel than β-conglycinin is believed to be due to a difference in network structure as well as in strength of interaction between the protein molecules. Mixing of both soy proteins resulted in improved gelling properties at pH 3.8. The elastic modulus of the mixture was larger than the weighed sum of the separate contributions. Furthermore, mixing reduced the protein dispersability at pH 7.6. This strongly indicates the presence of an interaction between the proteins. Gels of the 1:1 mixture (pH 3.8) had a fracture stress and strain in between those of the gels of the separate proteins.

Effects of structural rearrangements on the rheology of rennet-induced casein particle gels.

During ageing of casein or skim milk gels, structural changes take place that affect gel parameters, such as pore size and storage modulus. These changes can be explained in terms of rearrangements of the gel network at various length scales. In this paper, rheological experiments on rennet-induced casein gels and a general model on rearrangements are presented. The results of experiments (e.g. microscopy, permeametry) and computer simulations, the model, and recent literature on casein gels and other types of particle gels are compared to each other. Experiments presented include measurements of storage and loss moduli and maximum linear strain of the casein gels. Parameters varied were pH (5.3 and 6.65) and temperature (25 and 30 degrees C). In addition, the casein volume fraction (5-9 vol.%) was varied, which enables application of fractal scaling models. For rennet-induced casein gels, it is demonstrated that at the lower pH, all types of rearrangements proceed significantly faster. The rearrangements include: an increase in the size of compact building blocks; partial disappearance of fractal structure; and the formation of straightened strands, some of which eventually break. All of these rearrangements seem to be a consequence of particle fusion. There are indications of universality of the relation between particle fusion and gel syneresis for gels composed of viscoelastic particles.

Gelation and retrogradation of concentrated starch systems. 1. Gelation.

Abstract Small deformation properties of potato and wheat starch suspensions were studied during heating and cooling at rest by a small amplitude dynamic rheological test method. Starch concentrations used were 10 to 30% w/w. The temperature to which the suspensions were heated varied from 65 to 90°C. During heating the moduli of the starch sytems at first increase and subsequently decrease. On prolonged heating the moduli depend greatly on heating temperature: the higher the temperature, the lower the moduli. Properties of concentrated starch systems during heating are related to swelling of granules, melting of crystallites, separation of amy lose from amylopectin and disentanglement of non-covalent bonds. The differences between the rheological properties of potato and wheat starch suspensions are discussed, and compared with those of chemically cross-linked potato starches. Our hypothesis for the explanation of the observed phenomena is that physical entanglements or chemical cross-links, naturally or artificially present in starch granules, reduce the swelling capacity of the granules and increase the stiffness of the swollen granules; as a consequence, they affect the rheological properties of concentrated starch suspensions.

Colloidal aspects of texture perception.

Recently, considerable attention has been given to the understanding of texture attributes that cannot directly be related to physical properties of food, such as creamy, crumbly and watery. The perception of these attributes is strongly related to the way the food is processed during food intake, mastication, swallowing of it and during the cleaning of the mouth after swallowing. Moreover, their perception is modulated by the interaction with other basic attributes, such as taste and aroma attributes (e.g. sourness and vanilla). To be able to link the composition and structure of food products to more complicated texture attributes, their initial physical/colloid chemical properties and the oral processing of these products must be well understood. Understanding of the processes in the mouth at colloidal length scales turned out to be essential to grasp the interplay between perception, oral physiology and food properties. In view of the huge differences in physical chemical properties between food products, it is practical to make a distinction between solid, semi-solid, and liquid food products. The latter ones are often liquid dispersions of emulsion droplets or particles in general. For liquid food products for instance flow behaviour and colloidal stability of dispersed particles play a main role in determining their textural properties. For most solid products stiffness and fracture behaviour in relation to water content are essential while for semi-solids a much larger range of mechanical properties will play a role. Examples of colloidal aspects of texture perception will be discussed for these three categories of products based on selected sensory attributes and/or relevant colloidal processes. For solid products some main factors determining crispness will be discussed. For crispiness of dry cellular solid products these are water content and the architecture of the product at mesoscopic length scales (20-1000 microm). In addition the distribution of water at mesoscopic length scales was found to be important. For semi-solid foods, sensory characteristics as spreadability, watery and crumbliness are primarily determined by food properties at mesoscopic length scales. Crumbliness is directly related to the formation of free running cracks that occur during eating of the product. Exudation of the continuous liquid phase of gels during compression gives rise to watery/juicy sensory attributes. For liquid food products, colloidal interactions of emulsion droplets, particles, proteins, and polysaccharides with saliva and oral surfaces were found to affect texture characteristics as creaminess, fattiness, roughness and astringency.

Rheological properties of soybean protein isolate gels containing emulsion droplets

Rheological properties of soybean protein gels containing various volume fractions oil droplets have been studied at small and large deformations. Dynamic viscoelastic properties of soybean protein isolate gels were determined as a function of the volume fraction of oil droplets stabilised by the same protein, both in absence and presence of 0.2 M sodium chloride (NaCl). The storage and loss moduli were higher if NaCl was added. For both conditions, they increased with increasing oil volume fraction during the heating as well as the cooling stage. Furthermore, gel formation started at a lower temperature with increasing oil volume fraction. The increase in the moduli with increasing volume fraction of oil droplets was stronger than predicted by Van der Poels theory for a simple filled gel containing single interacting stiff emulsion droplets. This effect was attributed to aggregation of the emulsion droplets. Fracture properties of gels with different oil volume fraction and oil droplet size were determined at pH 7.0, in the presence of 0.2 M NaCl, by a uniaxial compression test. Compressive stresses of the gels containing oil droplets of 1.17 μm were higher than those containing droplets of 2.70 μm, especially at higher volume fractions of oil droplets. The fracture strain did not depend on the volume fraction of oil droplets.

Categorization of rheological scaling models for particle gels applied to casein gels

Rennet-induced casein gels made from skim milk were studied rheologically. A scaling model or framework for describing the rheological behavior of gels is discussed and used for classification of the structure of casein gels. There are two main parameters in the model that describe the number of deformable links in a strand and the bendability of the links. In the model at least five types of gel structure can be distinguished. Application of the model to experimental data on rennet-induced casein gels at pH of 6.0–6.6 and 25 °C shows that they contain straight strands with a large number of deformable links. Analysis of the experimental data of the storage modulus, maximum linear strain and yield stress as a function of the volume fraction results in the same information about the gel structure.

Water in Casein Gels; How to Get it Out or Keep it In

Abstract An important problem related to food preparation starting from milk gels is to remove just the required amount of water in the case of cheese production or to prevent syneresis of liquid in most other cases. About 90% of the water present in milk gels is mechanically enclosed between the casein strands forming the network and most of the other water is mechanically enclosed in the casein particles (strands) forming the network. In this respect the presence or absence of so-called bound water is of no importance. It is the structure of the casein aggregates which determines the ease of removal of water from the gel. The effects of pH and temperature on the structure of the casein aggregates as analysed by rheometry, permeametry and NMR and the consequences for the ease of removal of most of the moisture from the gel are discussed.

Interfacial rheological properties and conformational aspects of soy glycinin at the air/water interface

Interfacial (rheological) properties of soy glycinin were studied at different pH. At acidic and high alkaline pH glycinin (11S form, Mw ∼ 350 kDa) dissociates into smaller subunits, the so called 3S form (Mw ∼ 44 kDa) and 7S form (Mw ∼ 175 kDa). This dissociation behaviour is expected to affect the interfacial rheological properties of glycinin. Adsorption kinetics at the air/water interface were followed with an automated drop tensiometer (ADT) and ellipsometer. The changes in surface concentration, surface pressure, dilational modulus and layer thickness were determined. At acidic pH where glycinin is in the 3S/7S form, it adsorbs much faster at the air/water interface giving a higher surface concentration and a higher dilational modulus than at pH 6.7 (where glycinin is in the 11S form). Surface shear viscosity measurements showed that after short ageing times glycinin gives a protein network that is much more resistant to deformation in shear at pH 3 than at pH 6.7. After ageing for 24 h, the surface shear viscosity is about the same at both pH. Foaming experiments failed to give a glycinin stabilised foam at pH 6.7 while at pH 3 glycinin behaves as a good foaming agent. All results indicate that changing the pH influences the conformation of glycinin and that this has a great impact on the interfacial rheological properties and foaming properties. Based on the results a model is postulated for the modes of glycinin adsorption at pH 3 and pH 6.7, respectively.

Relating colloidal particle interactions to gel structure using Brownian dynamics simulations and the Fuchs stability ratio

Brownian dynamics simulations of aggregation of hard-sphere dispersions at intermediate volume fractions (∼3–10 vol%) have been performed. A long-range activation energy for aggregation was incorporated. The bonds formed were irreversible and flexible. Cluster growth rate and fractal properties of the gel matrix could be related to particle interactions by using a Fuchs stability ratio WF. Although this approach is expected to apply only to the very early stages of gelation, WF was shown to be a useful parameter, especially for predicting gel matrix parameters like the fractal dimensionality Df (which is a measure of the compactness of the clusters in the intermediate or fractal length scale regime) and the correlation length ξ (which is a measure of the average gel pore size). The number of aggregates, Nagg, was found to be a convenient measure of the stage of aggregation for the range of volume fractions and interactions studied. For high values of WF, the value of Df was more generic (i.e., less depend...

Gelation and retrogradation of concentrated starch systems: 3. Effect of concentration and heating temperature

In a previous paper we discussed the relation between the structure of concentrated starch gels and their mechanical properties at large deformations, including effects of storage time and type of starch. The effects of concentration and heating temperature on the mechanical properties of concentrated starch gels are presented in this paper. In general, the fracture stress and the modulus increased, and the fracture strain decreased with increasing starch concentration. For starch gels with tightly-packed swollen granules, a linear relation between starch concentration and the logarithm of the modulus was observed. The heating temperature substantially affected the mechanical properties of concentrated starch gels. An explanation for these observations is proposed.