Esterase activity in soy sauce Moromi as a factor hydrolyzing flavor esters

In order to elucidate the reason for the meager occurrence of volatile esters in soy sauce, the ester-decomposing activities of microorganisms concerned in soy sauce fermentation were examined. Soy yeasts showed at least 10 times higher esterase activity than the other yeasts used for fermented beverages. The yeast esterase was not greatly affected by the pH or NaCl concentration. Soy koji cultured with Aspergillus sojae or A. oryzae showed very high ester-splitting activity. By gel-filtration of koji esterase, the i-amylacetate (i-AmAc) decomposing fraction was obtained. This fraction showed a decrease of activity at lower pH or higher NaCl concentration. Koji esterase decreased its activity in moromi but remained over the entire moromi period. Koji esterase exhibited a higher activity than yeast esterase in fermenting moromi. These strong esterase activities are thought to be one of the causes of the low concentration of ester flavor in soy sauce.