Bioactive compounds, biological activity, and starch digestibility of dried soy residues from the soybean oil industry and the effects of hot-air drying

BACKGROUND: Soybean residue is a by-product from the soy industry. It is rich in protein, lipids, carbohydrates, and bioactive compounds but it is still underutilized as a food ingredient. The aim of the present study was to investigate the bioactive components, biological activity, and starch digestibility of soybean residues collected from the soybean oil industry (SOI) and the laboratory (SOL), and the effects of hot-air drying on the treated residues. RESULTS: The predominant phenolic acids were protocatechuic, chlorogenic, and ferulic acids, and the major flavonoids were rutin, daidzein, and genistein. After hot-air drying, the total phenolic, flavonoid, and isoflavone content of dried SOI and SOL increased significantly, in comparison with untreated whole soybean (WSB), SOI, and SOL. Daidzein of dried SOI and SOL increased from 63 to 510 mg kg(-1) dry weight. The 2,2-diphenyl-1-picryldrazyl (DPPH) and ferric reducing antioxidant power (FRAP) values increased almost twofold in comparison with SOI and SOL. The antioxidant compounds in untreated and treated soy-residue extracts were the most efficient in inhibiting alpha-amylase. The phenolics and flavonoids caused starch digestion to reduce from 6.6% to 2.7%. The correlations, involving phenolic compounds (including phenolics, flavonoids, and isoflavones), demonstrated that the different phenolic extracts showed different antioxidant properties, which played an important role in soy oil-residue phenolic fractions. CONCLUSIONS: We suggest that soy oil residues could be used as ingredients in functional foods because of their health-promoting attributes. (C) 2021 Society of Chemical Industry.

Automated summary

This study found that the bioactive components in soybean residues significantly increased after hot-air drying, with improved antioxidant properties and reduced starch digestion. These findings suggest that soybean residues could be utilized as functional food ingredients.