RETARDING EFFECT OF ACETIC-ACID ON GROWTH OF CONTAMINATED BACTERIA DURING SHOYU-KOJI MAKING PROCESS

The retarding effect of acetic acid on the growth of various microorganisms was studied in a shoyu (Japanese fermented soy‐sauce)‐koji‐substrate composed of precooked whole or defatted soybeans and roasted cracked wheat during the cultivation of a koji‐mold, Aspergillus sojae or Aspergillus oryzae. During the initial acetic acid concentration over the range of 0.4 to 0.8% based on the water content of the koji‐substrate, the growth of a strain of Micrococcus sp., which had been isolated from shoyu‐koji, was effectively suppressed, but the growth of various strains of koji‐molds was not inhibited. Acetic acid showed a pronounced retarding effect on the growth of some tested strains of contaminated bacteria belonging to Micrococcus and Bacillus species. The acid also effectively retarded the growth of some strains of bacteria belonging to Staphylococcus species, Gram‐negative aerobes and Enterobacteriaceae which were artifically added to the koji‐substrate. It was shown that the effect of acetic acid is not merely due to action to lower pH of the koji‐substrate. However, the growth of lactic acid bacteria tested were not retarded or retarded slightly. Copyright © 1979, Wiley Blackwell. All rights reserved