T. M. Bikbov

Thermotropic gelation of food proteins

Abstract This review paper is an attempt to summarize the results of investigations over several years in the field of thermotropic gelation of proteins which have been carried out in the Laboratory of Novel Food Forms of the A.N.Nesmeyanov Institute of Organo-Element Compounds in Moscow. The gelling behavior of food proteins [soybean globulin fraction, 11S globulin from broad beans (legumin) and ovalbumin] is studied. Heat-setting mechanisms and general physical approaches for the phenomenological description of gel formation from globular proteins are discussed. Behavior of gelling systems with protein concentrations below and above the gelation threshold (the gel-point) is analysed in detail. The classical and modern theories of gelation, and their use in the description of concentration dependences of the average size of finite clusters in the sol-fraction, and gel equilibrium elasticity modulus are compared. The modern theory of gelation based on the 3d percolation model has been shown to reflect more correctly the main features of the thermotropic gelation of proteins. Detailed consideration is given to viscoelasticity of protein thermotropic gels under small deformations. It is shown that, due to microheterogeneity of the structure, protein gels display some characteristic features of the rheological behavior of both amorphous and crystalline polymer solids.

A study on gelation of soybean globulin solutions

SummaryA study has been undertaken on the linear viscoelastic properties (stress relaxation) of thermotropic gels of soybean globulins (SBG) over a wide range of concentration and temperature. Experimental data on isothermal stress relaxation for SBG gels of various concentrations were generalized in the form of a concentration-invariant relaxation curve using the reduction by modulus, but without the reduction by time. The concentration dependence of the modulus reduction parameter was shown to be similar to that of the equilibrium gel modulus with an accuracy of up to a constant factor. The temperature-invariant relaxation curve of 17.5% gel was obtained in the traditional way in the range of reduced time equal to about seven decimal orders. Its form is characteristic of ordinary cross-linked high elastic polymers at the end of the transition zone and the beginning of plateau zone. The activation energy for relaxation of SBG gels amounts to 136 kJ/mol irrespective of their concentration. Based on assaying of sol-fraction content and its composition, it has been found that 7 and 11 S SBG fractions play a predominant part in gelation. SBG gel are soluble in 8 M urea, but insoluble in 0.01 M mercaptoethanol. Under the action of urea the gels and native SBG are decomposed into practically identical molecular fragments (subunits). These facts attest that the hydrophobic interaction of subunits play a leading role in SBG gelation. Gelation is not accompanied by changes in the integral intensity of a comparatively well resolved leucine band in PMR (proton magnetic resonance) spectrum of SBG at 100 MHz. This fact serves as a basis for assumption that the packing densities of polypeptide chains in the structural elements of gels and native molecules of SBG are comparable. The relaxation properties of gels at the end of the transition zone and the beginning of the plateau zone are determined predominantly by the “internal viscosity” of their elastic structural elements. It should be noted in this connection that the role of the “local viscosity” as an effective criterium for hydrodynamic interaction of these elements is apparently small due to their limited flexibility. The elastic elements of gels are long-term fluctuations of protein concentration. In the event of gel with 17.5% concentration one elastic element consists of approximately 20 subunits. In their thermo-rheological behaviour SBG gels can be classed with the systems having entropic elasticity.

Studies on gelation of soybean globulin solutions

Thermal denaturation of soybean globulin fraction (SBGF) in diluted solution (protein concentration 0.15–0.63%) has been studied by the method of differential adiabatic scanning calorimetry. SBGF thermograms have two maxima. The low temperature maximum is consistent with denaturation of 7S component, while the high temperature maximum with denaturation of 11S components of this fraction. In the investigated range of protein concentrations the thermodynamic parameters (temperature and enthalpy) of denaturation of SBGF and its main components are constant. This fact suggests that differential adiabatic scanning calorimetry gives information purporting a change in the protein state at molecular level. The temperatures and enthalpies of denaturation of the main SBGF components linearly rise with increase of NaCl concentration. The slope of dependences of denaturation temperature on salt concentration,Ks, is extremely large (nearly 20 K · l/mole). The elementary thermodynamic theory of lyotropic effects in thermal denaturation of proteins has been developed based on the two-state model and linear approximation of protein-salt interactions by means of the corresponding second virial coefficient. It shows that the dependences of thermodynamic parameters of thermal denaturation on salt concentration should be linear in the initial section. This conclusion is consistent with the experiment. The differences of enthalpies and entropies of transferring denatured and native forms of the main SBGF components from water into NaCl solution have been determined. They are positive and their quantity increases linearly with salt concentration. This fact is consistent with the concept to the effect that the main factor of salt influence on thermal denaturation of SBGF is confined to a decrease of protein hydration. The effect of protein nature on the quantity of lyotropic effect in thermal denaturation has been considered. Using simple considerations as a basis, the dependence of the ratio betweenKs and the denaturation temperature in water has been obtained, which characterizes the lyotropic effect, on the molar fraction of hydrophobic residues in the protein molecule. This dependence is linear and the lyotropic effect rises with increase in the content of hydrophobic residues. It is satisfactorily consistent with the experimental data on NaCl effect on thermal denaturation temperature for ichthyocol gelatin, ribonuclease, lysozyme, 7S and 11S SBGF components. An extraordinary strong influence of NaCl on thermal denaturation temperatures for the main SBGF components can be accounted for by a relatively high content of hydrophobic residues.