Enzymatic Production of Amino Acids and Antioxidant Peptides from Soybean-Derived Protein Isolate

Soybean meal (SBM) is a major by-product of soybean processing and represents an abundant protein-rich resource for value-added applications. To enhance its utilization, SBM can be converted into protein isolates, free amino acids, and bioactive peptides with antioxidant activity. In this study, proteins were first isolated from SBM using 0.1 M NaOH, followed by acid precipitation with HCl and drying. This process achieved protein recovery of up to 90%. Fourier-transform infrared spectroscopy showed that the SBM protein isolate exhibited intensity profiles comparable to those of bovine serum albumin, suggesting that the secondary structural features were largely retained after extraction. The protein isolate was subsequently hydrolyzed using proteolytic enzymes to produce amino acids and peptides. Thin-layer chromatography analysis of the hydrolysate supernatant revealed the presence of several amino acids, including leucine, valine, glutamic acid, and glycine. The hydrolysate was further fractionated by ultrafiltration into three peptide fractions with molecular weight cut-offs of <3 kDa, 3–10 kDa, and >10 kDa. These fractions were evaluated for antioxidant activity using DPPH and ABTS radical scavenging assays. Among the fractions, the <3 kDa peptide fraction exhibited the strongest antioxidant activity, with the lowest IC₅₀ value of approximately 0.5 mg/mL. These findings demonstrate that alkaline extraction combined with enzymatic hydrolysis is a promising strategy for producing protein isolates, amino acids, and antioxidant peptides from SBM. This approach provides a practical valorization pathway for soybean-derived by-products and supports the development of integrated biorefinery processes in the soybean processing industry.