Abstract: In this paper, composite powders of walnut peptide, buckwheat and quinoa at different ratios were prepared. The optimal ratio was determined by evaluating their protein nutritional value based on the calculated score of ratio coefficient of amino acid (SRCAA). The conditions for liquefaction and saccharification of buckwheat and quinoa before being blended with walnut peptide were optimized by one-factor-at-a-time and orthogonal array design methods. The composite powder of walnut peptide, buckwheat and quinoa was obtained by spray drying and its digestbility and antioxidant activity in vitro were studied. The results showed that the optimal ratio of walnut peptide to buckwheat to quinoa was 0.7:13:5 (m/m), and the SRCAA of this combination was 95.15%, which was closed to the reference protein pattern. The optimal liquefaction conditions were α-amylase 7 U/g, 60 ℃, and digestion time 50 min, and the optimal saccharification conditions were β-amylase 150 U/g, 55 ℃, and digestion time 2.5 h. Under these conditions, the concentration of reducing sugar was 20.03 mg/mL. The contents of peptide, reducing sugar, flavonoid and polyphenol were increased during simulated in vitro digestion; especially during simulated intestinal digestion, the content of rapidly digestible starch (RDS) was 64.19% and the content of slowly digestible starch (SDS) was 27.12%, while the content of resistant starch (RS) was only 8.70%, which indicated that starch was more thoroughly hydrolyzed after gastrointestinal digestion. The antioxidant activities in vitro of the composite powder was proved by its reducing power (IC50 = 5.04 mg/mL), scavenging activity against DPPH radical (IC50 = 0.67 mg/mL), hydroxyl radical (IC50 = 62.32 mg/mL) and superoxide anion radical (IC50 = 13.07 mg/mL) and total antioxidant capacity (27.63 U/mL). To conclude, the composite powder of walnut peptide, buckwheat and quinoa was not only easy to digest and absorb but also had antioxidant activity.

Key words: amino acid evaluation, liquefaction, saccharification, in vitro digestion, antioxidant activity