Understanding the mechanisms of whey protein isolate mitigating the digestibility of corn starch by in vitro simulated digestion

Exogenous proteins in starchy foods may alter the digestibility of the starch. In this study, the effect of whey protein isolate (WPI) with different ratios on the digestibility of co-gelatinized corn starch (CS) was studied, and the possible mechanisms were investigated. The rapidly digestible starch (RDS) content was reduced from 66.89% to 59.01% and the resistant starch (RS) content was increased from 14.47% to 22.78% when 4.5% WPI was added during co-gelatinization. This exogenous protein also suppressed the reduction of the average molecular weight of starch during digestion. WPI had no effect on CS digestibility when starch and protein were gelatinized separately and mixed during digestion. The inhibition of starch digestion by WPI was closely related to the protein-starch interaction. Rheological property analysis indicated strong hydrogen bonding interactions were formed between WPI and CS during co-gelatinization. The physical barrier of WPI also had a certain inhibitory effect on the digestion of starch, while WPI only had a transient effect on the viscosity of the digestive juice. Meanwhile, WPI restricted the hydrolysis of CS by inhibiting the activities of α-amylase and amyloglucosidase, and the inhibition of amyloglucosidase activity had a special effect on the rapid digestion of starch at the early stage.