Microbial biotransformation of proteins into amino acids in unpolished Thai and polished Japanese rice varieties cultivated with distinct industrial strains of <i>koji</i> mold

We previously reported the cultivation of industrial koji mold strains to produce unpolished Thai-colored rice kojis. These kojis, along with those made from unpolished Thai white rice and polished Japanese white rice, showed increased polyphenol content after cultivation, with the highest levels observed in unpolished Thai-colored rice kojis. In this study, an increase in both proteinogenic and non-proteinogenic amino acid contents, particularly γ-aminobutyric acid (GABA) content, was observed in both unpolished Thai and polished Japanese rice kojis, suggesting the ability of koji mold in the biotransformation of proteins. This increase was almost comparable even when using different rice varieties; in contrast, it varied depending on the koji mold strain used. The observed increase in both polyphenol and functional amino acid contents, especially GABA content, highlights the potential of unpolished Thai and polished Japanese rice kojis, particularly unpolished Thai-colored rice koji, as multifunctional materials, benefiting from polyphenol and amino acid functionalities.