Effect of Proteases in Fraction in Commercial Shio‐Koji on the Proteolysis of Bovine Myofibrils

Rice koji is produced by fermenting rice with Aspergillus oryzae , and shio‐koji, a seasoning derived from rice koji, is known for its meat‐tenderizing properties. In this study, protease fractions were isolated from commercially available shio‐koji, and their effects on the hydrolysis of bovine myofibrils and the release of free amino acids were examined. A crude protease fraction (CP‐F) was obtained by dialyzing shio‐koji to remove low–molecular‐weight components, while an aspergillopepsin I (AP)–enriched fraction (AP‐F) was prepared by ammonium sulfate precipitation. Treatment of bovine myofibrils with CP‐F or AP‐F resulted in a temperature‐dependent increase in free peptides, demonstrating the proteolytic activity of shio‐koji proteases. CP‐F generated a greater amount of peptides than AP‐F. SDS‐PAGE analysis confirmed that both fractions promoted myofibrillar degradation. Proteolysis at 4°C was limited relative to that at 37°C, and partial hydrolysis of myosin heavy chains was observed. These findings indicate that AP in shio‐koji exhibits proteolytic activity toward myofibrillar proteins and has the potential to contribute to meat tenderization.