SynopsisIn order to optimize the use of proteins as functional ingredients in foods, one needs more insight into the effects of environmental conditions (pH, ionic strength, and temperature) on the functional properties of protein. This paper summarizes the results of an extensive study on heat-induced gelation of ovalbumin (egg-white protein) and soybean protein in the concentration range from 10 to 35 g/100 g. It was the aim of the study to relate the rheological properties of thermally induced protein gels to the microstructure of the gel and the physicochemical properties of the constituent protein.The gelling behavior of the protein was quantified with rheological techniques, and the physical properties of the gels were determined, at small and large deformations. From the swelling/dissolving behavior of the gels in various media, the nature of the crosslinks was determined qualitatively. The microstructure of the gels was determined with electron microscopy. Nmr-spectroscopy was applied in order to elucidate changes in conformation during heating. It was found that the formation of a continuous covalently crosslinked network is not a prerequisite for thermally-induced protein gelation. The properties of a gel strongly depend on the pH at which the gel is formed. When heatset at high pH(pH-lo), a homogeneous, strong, and almost transparent gel is formed, consisting of flexible crosslinked protein gels. Heat-setting at low pH (pH 5) leads to the formation of a heterogeneous and weak gel, which easily exudes water. This gel consists of crosslinked aggregated protein. The ionic strength of the solvent in which the protein is dissolved and heat-set has a much lower effect on gel properties.

SynopsisThe relation between the chemical structure of a protein and the physical properties of a heat-set gel of that protein has been investigated. The physical properties of the gel are determined by means of mechanical experiments in which the viscoelastic properties of the gel are determined in terms of the storage shear modulus, the loss modulus and the stress-strain curve. The storage shear modulus defines the solid (elastic) character of the gel and quantifies the rigidity (hardness), the loss modulus defines the liquid (viscous) character of the gel. The chemical structure of the protein and the nature of the solvent determine the nature and number of cross-links in the gel. The cross-links in gels formed by heating concentrated solutions of ovalbumin in 6M urea solutions were found to be disulfide bridges and the mechanical properties of these ovalbumidurea gels approximated those of an ideal rubber. The latter finding enables one to calculate the number of cross-links per ovalbumin molecule from the value of the storage modulus, using the classical theory of rubber elasticity. This theory, together with the Flory-Huggins lattice model, can also be used to calculate the number of cross-links per ovalbumin molecule from the swelling behavior of ovalbumin/urea gels. The number of cross-links per ovalbumin molecule calculated from these two types of experiments are in mutual agreement and correspond with the number of thiol groups in ovalbumin. We conclude, therefore, that theories of polymer physics can be used to relate the chemical structure of a protein to the physical properties of its gel.

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The subunits AA2 and N2, present in α‐crystallin from the nucleus of young and old bovine lenses, were isolated and characterized. It was found that AA2 is identical to a shorter A‐chain and hence it was designated as A1–101. The subunit N2 turned out to be identical with a shorter B‐chain and‐was designated as B1/2−170. Characterization of the subunit N1, present only in α‐crystallin from the cortex of old bovine lenses, suggested that this subunit is a modified B‐chain, probably a deamidation product; it was designated as Bo. Comparison of the appearance of degraded and deamidated chains in relation to the age of Uhc fiber cells in old bovine lenses with that in calf lenses revealed that the observed specific limited degradation of the subunits of α‐crystallin increased with older age of the tissue. The deamidation process was found not to be related to the aging of the tissue. Eventually, a clear picture concerning the heterogeneity and fickleness of theα‐crystallin subunit structure was obtained.

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