Ton van Vliet

Interfacial rheological properties of adsorbed protein layers and surfactants: a review

Proteins and low molecular weight (LMW) surfactants are widely used for the physical stabilisation of many emulsions and foam based food products. The formation and stabilisation of these emulsions and foams depend strongly on the interfacial properties of the proteins and the LMW surfactants. Therefore these properties have been studied extensively. In this review an overview is given of interfacial properties of proteins at a mesoscopic scale and the effect of LMW surfactants (emulsifiers) on them. Properties addressed are the adsorbed amount, surface tension (reduction), network formation at interfaces and possible conformational changes during and after adsorption. Special attention is given to interfacial rheological behaviour of proteins at expanding and compressing interfaces, which simulate the behaviour in real emulsions and foams. It will be illustrated that information on interfacial rheological properties, protein conformation and interactions between adsorbed molecules can be obtained by changing experimental conditions. The relation between interfacial rheology and emulsion and foam stabilisation is subsequently addressed. It is concluded that there is a need for new measuring devices that monitor several interfacial properties on a mesoscopic and microscopic scale at the same time, and for physical models describing the various processes of importance for proteins. Real progress will only be possible if both are combined in an innovative way.

Theoretical and experimental study of the fractal nature of the structure of casein gels

Casein gels consist of particulate disordered structures, formed by aggregation of casein micelles. The aggregation leads to flocs of a fractal nature, which form a gel as soon as the occupy the total volume. A model was developed to describe gel formation and structure usnig the concept of fractal geometry. Parameters derived from this model were compared with experimental results concerning the structure and related properties.The model was in good agreement with the dependence of the permeability or rheological properties, such as the storage modulus of the gels, on the volume fraction of casein, and with the dependence of the turbidity of the gel on wavelength.It is shown that acid casein gels can be described very well as a collection of fractal clusters with a fractal dimensionality, D, of 2.3. Rennet-induced casein gels appeared to lose their fractal behaviour due to microsyneresis. Consequently, the model was not fully valid for the latter gels.

On the relation between texture perception and fundamental mechanical parameters for liquids and time dependent solids

Aspects of the relationship between texture perception in the mouth and fundamental mechanical parameters for liquids and time dependent solids are discussed. The emphasis is on the physical side of the relation. The importance is stressed of the incorporation of a thorough knowledge of rheology and fracture mechanics for the establishment of physical sound relations. Various examples are discussed where basic rheology and fracture mechanics (can) contribute to a better understanding of the relation between texture perception and fundamental mechanical parameters. Further progress in the establishment of casual relations is still hampered by weak definition of texture terms, insufficient knowledge of the deformations involved in the mouth, the inhomogeneous character of many products and insufficient development and understanding of relevant theoretical concepts in the field of rheology and fracture mechanics.

Interaction of water unextractable solids with gluten protein: effect on dough properties and gluten quality

In a previous study, we have shown that water unextractable solids (WUS) interfere with gluten formation and affect the quality of the resulting gluten. In this study we aim to explain how WUS can affect the process of gluten formation. To this end, WUS were modified with NaOH, xylanase, horseradish peroxidase (HRP) and hydrogen peroxide (H2O2). Effects of modified WUS on gluten yield, dough properties, and gluten and glutenin macropolymer (GMP) composition and properties were studied. The results showed that addition of WUS to wheat flour led to a lower gluten yield and gluten starch yield, a higher Rmax and a lower E at Rmax of gluten and a more concentrated and elastic GMP gel. Pretreatment of WUS by NaOH, xylanase, HRP and H2O2 cannot correct its negative effect on gluten yield, but addition of xylanase or free ferulic acid (FA) during gluten separation can remove or prevent the negative effect of WUS on gluten yield. Compared to addition of only WUS, addition of WUS and FA together to wheat flour resulted in a higher gluten yield, a higher E at Rmax of gluten, and a less concentrated and elastic GMP gel. Similar to water extractable pentosans (WEP), FA bound WUS plays a key role in the effect of WUS on gluten yield and properties. It appears that there is a common mechanism regarding the effect of WUS and WEP that the oxidative cross-linking during gluten formation could be prevented by FA addition. The difference between both is that WUS have a higher water binding capacity, which is reflected in a higher Rmax of dough and gluten in the presence of WUS.

Large deformation and fracture behaviour of gels.

When gels are used in practice, their large-deformation and fracture characteristics are mostly far more relevant than small-deformation characteristics. In this paper fracture behaviour is discussed of various types of gels, viz. polymer and particle gels, the latter with fairly low and very high volume fraction of particles. First, a general introduction is given on theoretical aspects of fracture mechanics of gels, which involves an essential extension of classical fracture theories. The relationship between large-deformation and fracture behaviour of a gel and its structure proves to be far more complicated than for small-deformation properties. The main reasons for this difference are: (i) the much more important effect of relatively large inhomogeneities on fracture properties and (ii) some very different causes for the strain rate dependence. Not only are the average distance between cross-links and the average stiffness of the strands connecting them of importance, but also the distribution of these parameters. Moreover, inhomogeneities, be it defects (of µm to mm scale) or weak regions (e.g. in composite gels) may have an overriding effect on the fracture properties. To understand the strain rate dependence, one should consider the energies involved as a function of the deformation rate and distinguish between the amount elastically stored during deformation, the amount dissipated due to viscous flow or due to friction processes and the net fracture energy. Moreover crack initiation and fast ‘spontaneous’ crack growth (crack propagation) have to be distinguished. The factors mentioned cause large deformation and fracture properties to be much more strongly dependent on the physical structure of a gel than are the small deformation properties.

Gelation and interfacial behaviour of vegetable proteins

Recent studies on gelation and interfacial properties of vegetable protiens are reviewed. Attention is focused on legume proteins, mainly soy proteins, and on wheat proteins. The rheological properteis of vegetable protein gels as a function of heating time or temperature is discussed as well as the interfacial gelation upon adsorption of soy and wheat proteins at the air/water interface. It is shown that modification of proteins improves functionality and application. ©2002 Elsevier Science Ltd. All rights reserved. Chemicals/CAS: glycinin, 9007-93-6; soybean protein, 9010-10-0; water, 7732-18-5

Gelation of soy glycinin; influence of pH and ionic strength on network structure in relation to protein conformation

Abstract Formation and structure of glycinin gels were studied in relation to protein conformation for two pH values and three ionic strengths. While at I =0.03 the gels were found to be fine stranded, gel coarseness increased when the ionic strength was higher. At I =0.03 finer gel network structures were formed at pH 3.8 than at pH 7.6, whereas for I =0.2 and 0.5 the reverse was found. The observed differences in gel stiffness (rheological dynamical measurements) did not correspond to coarseness of the gels. It was found that the nature of the primary network particles was different at pH 7.6 compared to pH 3.8, since at pH 7.6 only 51–69% of total protein was incorporated in the gel network (predominantly basic polypeptides), while at pH 3.8 all protein was present in the network. The higher water holding capacities observed at pH 7.6 compared to pH 3.8 support the idea that at pH 7.6 the non-network protein resides in the pores. At all conditions studied denaturation coincides with the induction of β-sheet at a secondary level (IR measurements), and with gel formation (except for I =0.03). The largest increase in gel stiffness did not take place directly after denaturation but during the cooling part of the temperature cycle used. This increase in gel stiffness could not be related to changes in secondary structure.

Foams and surface rheological properties of β-casein, gliadin and glycinin

Interfacial rheological properties and their suitability for foam production and stability of two vegetable proteins were studied and compared to β-casein. Proteins used ranged from flexible to rigid/globular in the order of β-casein, gliadin and soy glycinin. Experiments were performed at pH 6.7. Network forming properties were characterised by the surface dilational modulus (determined with the ring trough) and the critical falling film length (Lstill) at which a stagnant protein film will break. Gliadin had the highest dilational modulus, followed by glycinin and β-casein, whereas glycinin formed the strongest film against fracture in the overflowing cylinder. The rate of decrease in the surface tension was studied at the air-water (Wilhelmy plate method) and the oil-water interface (bursting membrane) and the dynamic surface tension during compression and expansion in the caterpillar. Gliadin had the lowest equilibrium interfacial tensions and β-casein the lowest dynamic surface tension during expansion. Hardly any foam could be formed at a concentration of 0.1 g/l by shaking. At a concentration of 1.4 g/l most foam was formed by β-casein, followed by gliadin and glycinin. It seems that in the first place the rate of adsorption is important for foam formation. For the vegetable proteins, adsorption was slow. This resulted in lower foamability, especially for glycinin.

Large deformation and fracture behaviour of gels

Most recent experimental and theoretical work on large deformation and fracture behaviour of gels has focused on fracture stress and strain. Other parameters characterizing fracture behaviour, such as fracture energy and inherent defect length, were much less studied. Progress has been made in the development of theories relating fracture stress and strain to gel matrix structure.

A New Device for Studying Deep-Frying Behavior of Batters and Resulting Crust Properties

ABSTRACT The formation and properties of a crust during and after deep frying are difficult to study. Batter pickup (the amount of batter adhering to a product) and core properties affect crust formation and properties of the crust in such way that it is difficult to compare batters of different viscosity or cores with different properties. Moreover, it is often difficult and laborious to separate the crust/batter from the core. Another problem is the poor reproducibility of many fried products. A deep-fried model (DFM) was designed, making it possible to study crust formation and crust properties without the difficulties stated above. Two different batter types and three cores have been used to test the system. Crusts obtained from the DFM were evaluated on several physiochemical properties and compared with crusts found around commercial deep-fried products. Results show that crusts obtained with the DFM system are comparable to crusts of commercial products. The good reproducibility of the DFM crusts r...