Dissolution and swelling of soy protein isolate hydrogels in alkali

Abstract

The chemical dissolution of protein hydrogels, in particular the rate limiting mechanism, differs in the only two systems studied, whey and egg white proteins. With the aim to develop mechanisms general to globular protein networks, we study here the dissolution of soy protein isolate (SPI) hydrogels for the first time. SPI gels dissolve in alkali in a similar manner than egg white gels, but much faster, showing traits expected when the cleavage of interprotein disulfide crosslinks is rate limiting. SPI dissolution is reduced when swelling is inhibited by the addition of salts, although less profoundly than in whey protein gels. Equilibrium swelling experiments of SPI gels show a sudden increase at pH ≥ 11.5, which cannot be predicted from the swelling data at lower pH. Dissolution is evident in swollen SPI gels at pH > 12 but only at low salt concentrations. A double dissolution threshold is therefore also observed like in β-lactoglobulin gels, although its origin is different. The pH threshold in whey proteins at pH ∼11.5 is due to the destruction of non-covalent interactions, but in SPI it only causes a large increase in swelling. Dissolution only occurs at higher alkali concentrations where disulfide crosslinks start to be cleaved quickly. The importance of this reaction in SPI gels, where disulfide crosslinks are not extensive in the gel network, is explained by the large size of most soy proteins that will not detach meanwhile a gel network still exists. The size of the initial proteins becomes a new parameter when analyzing the dissolution of different protein systems, in addition to the chemistry involved in the formation of hydrogels.