Thermally induced protein gelation: gelation and rheological characterization of highly concentrated ovalbumin and soybean protein gels

In 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 heat-set at high pH(pH～10), 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.