Yeast-immobilized SPV device for koji quality control in sake brewing process.

The malted rice, koji, is an indispensable material for the brewing of sake. It saccharifies rice starch and supplies vitamins for the yeast in sake brewing. Since the quality of sake depends strongly on the quality of koji, quality control of koji is very important in the brewing. There are some methods to measure the activity of enzymes and the quantity of vitamins with the quality of koji. None of these methods, however, directly relate to the yeast metabolism. We constructed a sensor system to monitor the yeast metabolism in sake brewing by use of immobilized Saccharomyces cerevisiae and a Surface PhotoVoltage device (SPV). In this system, S. cerevisiae K701 and K9, designed for use in sake brewing by the Brewing Society of Japan, were employed as immobilized microbe. The pH change due to the production of organic acids in sake brewing is measured using the SPV. A linear relationship was observed between decrease in the photocurrent (the metabolism response) and the concentration to less than 60 mM of glucose (r=0.990). Then we measured the koji extract and observed the difference of response between K701 and K9 which corresponded to the productivity of acidic substances by batch test.