Examination of the Causes of Instability of Soy Protein Isolate during Storage Through Probing of the Heat-Induced Aggregation

This study investigated the mechanism of instability of soy protein isolate (SPI) as influenced by thermal aggregation during SPI preparation. Samples with different degrees of aggregation but similar protein solubility were prepared by heating native SPI (5 % w/v) at 80 or 90 °C for different times before spray-drying. The samples were then stored at 37 °C for up to 12 weeks and analyzed periodically by atomic force microscopy, gel permeation chromatography, and SDS-PAGE. All SPI samples underwent remarkable protein solubility decreases during the first 8 weeks of storage. The rates of solubility loss were positively correlated with the amounts and/or sizes of soluble aggregates contained in the initial samples (time zero), suggesting their nucleation and activation effects. Solubility tests in SDS-urea solutions and disulfide analysis indicated that non-covalent interactions were the main driving forces for protein storage instability. Conversely, disulfide bonds and protein carbonyls were abundant in soluble aggregates, and their content increased markedly during storage. This effect suggested that covalent linkages acted as blockers for hydrophobic aggregation.