Effects of simulated gastrointestinal digestion on the physicochemical properties, erythrocyte haemolysis inhibitory ability and chemical antioxidant activity of mulberry leaf protein and its hydrolysates

Summary Proteins and peptides must be degraded and modified in the gastrointestinal ( GI ) tract prior to absorption; this process changes their physicochemical and biological properties. Mulberry leaf protein ( MP ) and its hydrolysates ( HMP ) have favourable antioxidant activities. To investigate, in vitro and separately, the effects of GI digestion and intestinal digestion on the stability of MP and HMP , we monitored the changes in secondary structures, amino acids, molecular weights and antioxidant activities. We found that MP was more hydrolysed by pepsin than by pancreatin, unlike HMP . The final digests of MP and HMP were mainly composed of polypeptides (0.5–6.5 kDa) and oligopeptides (<0.5 kDa), respectively. The GI digestion influenced MP and HMP differently; GI digestion increased the antioxidant efficiency of MP and decreased that of HMP . For the intestinal digests, the antioxidant activities of MP and HMP also differed. The 1,1‐diphenyl‐2‐picrylhydrazyl ( DPPH ) and 2,2′‐azinobis (3‐ethylbenzothiazoline‐6‐sulphonic acid) diammonium salt ( ABTS ) radical quenching abilities of MP and HMP at 1 mg mL −1 were comparable to or exceeded that of L ‐glutathione ( GSH ) and 6‐hydroxy‐2,5,7,8‐tetramethychroman‐2‐carboxylic acid (trolox). Meanwhile, the erythrocyte haemolysis rates of MP , HMP and their GI products at 0.05–1.0 mg mL −1 were significantly lower than that of the 2,2′‐azobis (isobutyramidine) dihydrochloride ( AAPH ) control. Both MP and HMP can be used as natural antioxidants and may promote digestive health.