Gelation of soy glycinin; influence of pH and ionic strength on network structure in relation to protein conformation

Formation and structure of glycinin gels were studied in relation to protein conformation for two pH values and three ionic strengths. While at I=0.03 the gels were found to be fine stranded, gel coarseness increased when the ionic strength was higher. At I=0.03 finer gel network structures were formed at pH 3.8 than at pH 7.6, whereas for I=0.2 and 0.5 the reverse was found. The observed differences in gel stiffness (rheological dynamical measurements) did not correspond to coarseness of the gels. It was found that the nature of the primary network particles was different at pH 7.6 compared to pH 3.8, since at pH 7.6 only 51–69% of total protein was incorporated in the gel network (predominantly basic polypeptides), while at pH 3.8 all protein was present in the network. The higher water holding capacities observed at pH 7.6 compared to pH 3.8 support the idea that at pH 7.6 the non-network protein resides in the pores. At all conditions studied denaturation coincides with the induction of β-sheet at a secondary level (IR measurements), and with gel formation (except for I=0.03). The largest increase in gel stiffness did not take place directly after denaturation but during the cooling part of the temperature cycle used. This increase in gel stiffness could not be related to changes in secondary structure.