Hydrolysis of plant proteins at the molecular and supra-molecular scales during in vitro digestion

The digestion of plant protein is highly dependent on multiple factors, with two of the most important being the protein source and the food matrix. The present study investigated the effects of these two factors on the digestion of seitan (a wheat-based food), tofu, soya juice, and a homemade emulsion of soy oil and water that was stabilised with pea protein. The four plant matrices and their respective protein isolates/concentrates (wheat gluten, soya protein, pea protein) were subjected to in vitro static digestion following the INFOGEST consensus protocol. We monitored the release of α-amino groups during digestion. We found that food matrix had a strong influence on protein digestion: soya juice was more hydrolysed than fresh tofu (51.1% versus 33.1%; P = 0.0087), but fresh tofu was more hydrolysed than soya protein isolate (33.1% versus 17.9%; P < 0.0001). Likewise, the pea-protein emulsion was better hydrolysed than the pea-protein isolate (P = 0.0033). Differences were also detected between the two solid foods investigated here: a higher degree of hydrolysis was found for tofu compared to seitan (33.1% versus 11.8%), which was perhaps a function of the presence of numerous dense protein aggregates in the latter but not the former. Furthermore, freeze-drying more than doubled the final degree of hydrolysis of seitan (P < 0.0001), but had no effect on tofu (P = 1.0000). Confocal microscopy revealed that protein networks in freeze-dried seitan were strongly altered with respect to the fresh product; instead, protein networks in freeze-dried and fresh tofu were largely similar. Finally, we found that the protease:protein ratio had a strong effect on the kinetics of proteolysis: a 3.7-fold increase in the concentration of the soya protein isolate with respect to that of the soya juice decreased the final degree of hydrolysis from 50.3 to 17.9% (P = 0.0988).