Microstructure and in vitro digestion of mixed protein gels of soy and whey protein isolates

This study aimed to evaluate the microstructure and in vitro digestion of soy protein isolate (SPI) and whey protein isolate (WPI) mixed gels. Heat-set gels with different SPI/WPI mass ratios (0/100, 25/75, 50/50, 75/25 and 100/0) were produced. Regarding microstructure, the presence of SPI led to the formation of protein agglomerates resembling microgels, with micrometric average sizes. SPI proteins arranged in a highly branched structure, not particulate as WPI-only gel, probably due to a gelation process highly limited by diffusion of protein chains. The variety of network morphologies indicated the high complexity of the interactions among different proteins. The results of small deformation rheology showed the WPI proteins tended to interact preferably with soy proteins, resulting in the early increase in G'. However, this results in a poor soy protein network formation, as reflected in a lower G' at the end of the cooling process. As for in vitro digestion, data of size exclusion chromatography showed the gastric phase was insufficient to break up the proteins and peptides present in the gel protein networks. The gastric conditions probably caused the "opening" of the protein structures but was limited by the mass transfer due to steric hindrance in the gel network. Regarding the intestinal step, a more rapid digestion was observed, which may be related to the structure of the protein matrices containing SPI, i.e. more porous than the strands of particulate WPI proteins, which probably made the proteins in SPI-containing gels more accessible to the pancreatic enzymes.