pH-regulated mechanisms underlying the digestibility of rice starch-soy β-conglycinin complexes

It is well known that pH environments can enhance the anti-digestibility of starch-protein complexes via strengthening interactions, but how pH regulates the digestibility of rice starch (R)-soy β-conglycinin (7S) complexes, particularly the molecular mechanisms, remains unclear. In this study, the effects of pH on interactions, molecular structures, and the digestive properties of R-7S complexes were explored. This study found that as the pH increased from 3 to 9, the resistant starch content of the R-7S complex gradually increased from 13.20 % to 19.30 %. Consequently, the R-7S complex exhibited significantly stronger anti-digestibility under weak alkaline conditions (pH 9) compared to those under acidic conditions. The molecular simulation results indicated that the electrostatic interactions and hydrogen bonding gradually strengthened when the pH increased from 3 to 9. As pH shifted toward alkaline environments, the molecular conformation (the average interchain distance and molecular size) of amylopectin and the supramolecular conformation (short-range ordered structures and starch aggregate size) transitioned from a disordered and loose structure to an ordered and compact structure. Thus, the higher anti-digestibility of the R-7S complex under weak alkaline environments (pH 9) was attributed to stronger hydrogen bonding, more compact and stable amylopectin conformations, and more compact starch aggregate structures. This study provided a theoretical basis for optimizing the processing of starch-soybean products to enhance their anti-digestibility.