A comparative study on the protein digestion of four different soy beverages: effects of the composition, microstructure, and protein digestibility evaluation method

The increased consumption of soy-based products leads to the incentive for more sustainable soybean processing and more accurate nutritional evaluation. The protein structures and aggregation states of different components vary with different soy products, but their relationship with digestibility is unclear. In order to study the digestion of soy protein in complex food matrices, four soy-based beverages were carefully prepared, including whole component soy beverage (WS), soy beverage with insoluble soybean residue removed (DO-WS), soy beverage with lipids removed (DL-WS), and soy protein isolate beverage (HSPI). During digestion, the microstructure revealed that particles of all soy beverages were reduced and more evenly distributed, but striated fibres (in WS and DL-WS) still remained after digestion. Tricine-SDS-PAGE profiles showed that after in vitro gastrointestinal digestion of the four beverages, almost all the bands corresponding to the complete proteins from soy disappeared, leaving fewer visible bands with a low MW - below 12 kDa. In vitro protein digestibility analyzed by TCA precipitation (strategy B), which ranged from 64.24% to 68.70%, was more accurate, with peptides of MW <1 kDa accounting for over 84% for all the four digested fractions. Moreover, the highest values of in vitro DIAAS (96/80) and digestible protein (29.40/24.41 g per 100 g of soybeans) were achieved by WS. Characterization of the insoluble digesta further elucidated that peptides with a smaller MW (below 12 kDa) and a higher amount of hydrophobic amino acids aggregated more easily, resulting in the occurrence of precipitates and the relatively lower in vitro protein digestibility. This research contributes to the understanding of protein digestibility in whole legume-based diets, which in turn could aid in the development of new whole legume products and more efficient utilization of proteins.