White Particulate Formation as a Result of Enzymatic Proteolysis of Protein Substrates in Brine-fermented Foods

Abstract

Rice koji and rice-soy koji (1:1, w/w) were used as enzyme sources in combination with parahydroxyl benzoate (paraben), sulfanilamide, tetracycline, chloramphenicol or glacial acetic acid as antimicrobial agents for sufu fermentation. After 1 year of fermentation, no formation of white particulate was observed in rice koji products and in the rice-soy koji products with 10 mL/L of glacial acetic acid in the brines. Compositional analyses revealed that pH, tyrosine content and total amino nitrogen content of rice koji brines were lower than those of rice-soy koji brines. In a model system, a series of test tubes containing NaCl solution (150 g/L) and autoclaved soy koji or soybean used as protein substrates with supplements of various enzyme sources were fermented for 3 months at 35 °C. When an indigenous strain of Zygosaccharomyces rouxiiisolated from a fermented sufu brine or the sufu brine in which white particulates were present was respectively introduced into the systems using autoclaved soy koji as substrate, no white particulate was observed. However, white particulates were observed in the systems supplemented with soy koji extract, trypsin or pancreatin. When autoclaved soybean was used as the protein substrate, a white particulate was observed only in the system containing soy koji extract. Amino acid analysis of the particulates showed tyrosine to be the major component and independent of protease origin and presence of microorganisms.