Acid-induced cold gelation of globular proteins: effects of protein aggregate characteristics and disulfide bonding on rheological properties

The process of cold gelation of ovalbumin and the properties of the resulting cold-set gels were compared to those of whey protein isolate. Under the chosen heating conditions, most protein was organized in aggregates. For both protein preparations, the aggregates consisted of covalently linked monomers. Both types of protein aggregates had comparable numbers of thiol groups exposed at their surfaces but had clearly different shapes. During acid-induced gelation, the characteristic ordering caused by the repulsive character disappeared and was replaced by a random distribution. This process did not depend on aggregate characteristics and probably applies to any type of protein aggregate. Covalent bonds are the main determinants of the gel hardness. The formation of additional disulfide bonds during gelation depended on the number and accessibility of thiol groups and disulfide bonds in the molecule and was found to clearly differ between the proteins studied. However, upon blocking of the thiol groups, long fibrillar structures of ovalbumin contribute significantly to gel hardness, demonstrating the importance of aggregate shape.