By Harold McGee
The Japanese koji, by contrast, is made fresh for each particular sake brewing, is based only on polished, unground rice, and is inoculated with a selected culture of Aspergillus oryzae alone, with no other molds. The mold preparation for sake therefore doesn’t provide the complexity of flavor that the Chinese preparation does, with its roasted wheat, variety of microbes, and period of drying.
Because the koji contains no yeasts, the Japanese system requires a separate source of yeast. The traditional yeast preparation, the moto, is made by allowing a mixture of koji and cooked rice gruel to sour spontaneously with a mixed population of bacteria, mainly lactic acid producers (Lactobacillus sake, Leuconostoc mesenteroides, and others) that contribute tart and savory tastes and some aroma. A pure yeast culture is then added and allowed to multiply. Because this microbe-soured moto takes more than a month to mature, it has been largely replaced by the simple addition of organic acids to the moto mash, or by the addition of acids and concentrated yeasts directly to the main fermentation. These time-saving methods tend to produce lighter, less complex sakes.