Effects of heat treatment on protein molecular structure and in vitro digestion in whole soybeans with different moisture content

The effects of heat treatment on protein structure and in vitro digestibility in whole soybeans with different moisture content (10.68%, 29.70%, 46.29%, and 62.05% wet basis) were investigated. Scanning electronic microscopy presented that thermal treatment destroyed the subcellular structure of soybean seeds and resulted in formation of protein aggregates. When beta-conglycinin (7S) was heat-denatured, the protein aggregates were maintained mainly by hydrogen bonds and hydrophobic interactions (non-covalent) for each moisture content. Also, the decrease of the protein solubility and increase of in vitro digestibility were observed. However, when glycinin (11S) was denatured in soybeans with 10.68% and 29.70% moisture content, the insoluble and indigestible protein aggregates with protein oxidation-induced crosslinking and high content of beta-sheet were presented; in contrast, for 46.29% and 62.05% moisture content, mild protein oxidation, low content of beta-sheet, non-covalent interactions and increased protein digestibility were shown. Non-covalent interactions were shown a positive correlation with gastrointestinal digestibility (r = 0.59, p < 0.05). Meanwhile, protein oxidation or beta-sheet content was significantly negatively correlated with in vitro protein digestibility (r = -0.69 and -0.61, respectively, p < 0.05). Protein structure rather than solubility contributed to difference of in vitro digestibility. The optimum thermal conditions to obtain high-quality digestible protein in whole soybeans are 160 ? for 10.68%, 145 ? for 29.70%, 160 ? for 46.29% and 115 ?/140 ? for 62.05% moisture content.

Automated summary

This study investigated the effects of heat treatment on protein structure and in vitro digestibility in whole soybeans with different moisture content. The results showed that thermal treatment destroyed the subcellular structure of soybean seeds and resulted in the formation of protein aggregates. The protein aggregates were mainly maintained by hydrogen bonds and hydrophobic interactions. Decrease in protein solubility and increase in in vitro digestibility were observed. The denaturation of glycinin in soybeans with lower moisture content resulted in insoluble and indigestible protein aggregates with protein oxidation-induced crosslinking and high content of beta-sheet, while milder protein oxidation, low content of beta-sheet, non-covalent interactions, and increased protein digestibility were observed in soybeans with higher moisture content. Non-covalent interactions showed a positive correlation with gastrointestinal digestibility, while protein oxidation or beta-sheet content showed significant negative correlation with in vitro protein digestibility. Protein structure, rather than solubility, contributed to the difference in in vitro digestibility. The optimum thermal conditions to obtain high-quality digestible protein in whole soybeans varied with moisture content.