Abstract:The characteristics of the enzymatic hydrolysate of soybean protein isolate (SPI) emulsion (oil-in-water (O/W) and water-in-oil (W/O) type) in the process of gastric digestion were discussed by using the dynamic gastric model in this study. Microscopic examination and particle size determination showed that the uniformly distributed droplets were separated, flocculated, and combined gradually after being digested by the enzyme, and the liquid droplet size of O/W emulsion became larger, whereas that of the W/O emulsion became smaller. After the emulsion was digested by gastric juice, the potential values of emulsions changed from negative to positive; in addition, with the increase in the enzymatic hydrolysis time, the potential values of O/W emulsion and W/O emulsion gradually decreased and increased, respectively. This finding indicated that the strong acidic environment changed the charged protein residues at the emulsion interface, leading to an imbalance of the electric charges on the surface. The free amino acid contents of both emulsions and the content of small peptides of the O/W emulsion increased first and subsequently decreased, while the content of small peptides of W/O emulsion constantly increased. Therefore, it could be inferred that secondary emulsification occurred in the O/W emulsion but not in the W/O emulsion. Polyacrylamide gel electrophoresis (SDS-PAGE) and two-dimensional electrophoresis were used to analyze the changes in the protein composition of the emulsions. The results showed that 11S protein was more easily hydrolyzed by pepsin than 7S protein during the enzymatic hydrolysis and was hydrolyzed faster in W/O emulsion than in O/W emulsion, while β subunit was not easily hydrolyzed. Furthermore, two-dimensional gel electrophoresis showed that the partial basic and the partial acidic subunits of 7S and 11S protein, respectively, were easily hydrolyzed by pepsin.
