Key words
Sake brewing; Ethanol fermentation; Fermograph; Fermentation rates; Fermentation kinetics; Saccharomyces cerevisiae; Aspergillus oryzae
Sake brewing is a complicated process involving the simultaneous saccharification of rice starch by enzymes produced by the koji mold Aspergillus oryzae and fermentation of the resultant glucose by the yeast Saccharomyces cerevisiae. The combination of koji mold, yeast strain, and brewing conditions dictate the finished sake quality and properties. Therefore, small-scale brewing tests are typically performed to ensure that the brewing conditions yield the desired product in advance of industrial use.
To examine fermentation properties, both ethanol concentration and carbon dioxide emission are measured multiple times during the course of brewing tests. Since sake fermentation occurs under hypoxic conditions abundant in glucose 1. and 2., aerobic respiration is almost completely repressed in yeast cells, and thus, ethanol production is well approximated by quantification of evolved carbon dioxide during sake fermentation. Traditionally, carbon dioxide emission is monitored by manual measurements of sake-mash weight losses using an analytical balance; however, this process is tedious and labor-intensive as it requires repeated analysis of individually prepared samples.