Enzymatic Activity and Amino Acids Production of Predominant Fungi from Traditional Meju during Soybean Fermentation

Abstract

To investigate the effect of the predominant fungal species from Korean traditional meju and doenjang on soybean fermentation, the enzymatic activity and amino acid production of twenty-two fungal strains were assessed through solid- and liquid-state soybean fermentation. Enzymatic activity analyses of solid-state fermented soybeans revealed different enzyme activities involving protease, leucine aminopeptidase (LAP), carboxypeptidase (CaP), glutaminase, γ-glutamyl transferase (GGT), and amylase, depending on the fungal species. These enzymatic activities significantly affected the amino acid profile throughout liquid-state fermentation. Strains belonging to Mucoromycota, including Lichtheimia, Mucor, Rhizomucor, and Rhizopus, produced smaller amounts of total amino acids and umami-producing amino acids, such as glutamic acid and aspartic acid, than strains belonging to Aspergillus subgenus circumdati. The genera Penicillium and Scopulariopsis produced large amounts of total amino acids and glutamic acid, suggesting that these genera play an essential role in producing umami and kokumi tastes in fermented soybean products. Strains belonging to Aspergillus subgenus circumdati, including A. oryzae, showed the highest amino acid content, including glutamic acid, suggesting the potential benefits of A. oryzae as a starter for soybean fermentation. This study showed the potential of traditional meju strains as starters for soybean fermentation. However, further analysis of processes such as the production of G-peptide for kokumi taste and volatile compounds for flavor and safety is needed.

Keywords: Meju soybean fermentation; amino acid; enzyme activity; fungi.

Fig. 1. Contents of fungal biomass in twenty-two types of fermented soybean during the fermentation.

For each strain, five samples are gathered at different time points (2, 4, 6, 8 and 10 days). The error bars indicate standard errors; data were measured in triplicate.

Fig. 2. Profiles of protease (A) leucine aminopeptidase (B) and carboxypeptidase (C) activities in fermented soybean during solid-state fermentation process.

For each strain, five samples are gathered at different time points (2, 4, 6, 8 and 10 days). The enzyme activity was determined by measuring the change in optical density (OD) as each enzyme degraded its corresponding substrate. The error bars indicate standard errors; data were measured in triplicate.

Fig. 3. Profiles of glutaminase (A) gamma-glutamyltransferase (B) and amylase (C) activities in fermented soybean during solid-state fermentation process.

For each strain, five samples are gathered at different time points (2, 4, 6, 8 and 10 days). The enzyme activity was determined by measuring the change in optical density (OD) as each enzyme degraded its corresponding substrate. The error bars indicate standard errors; data were measured in triplicate.

Fig. 4. Amino acids contents from fermented soybean solution prepared from fermented soybean on the 2 (A) and 4 (B) days of liquid-state fermentation.

The amino acids contents were analyzed using an amino acid analyzer.