moromi

Moromi is the main fermentation mash used in traditional sake production. It is the bulk mixture formed by combining steamed rice inoculated with koji mold (Aspergillus oryzae), water, and the enzymes produced by the koji, with yeast that will convert sugars into alcohol. The moromi stage follows the initial starter mash and is where most of the fermentation occurs.

In traditional brewing, moromi is built up gradually in a process called san-dan shikomi. Portions of steamed rice, koji, and water are added over several days to a starter mash until the vessel is full. The completed moromi typically ferments in large tanks for about two to four weeks under controlled temperature to influence flavor and aroma development.

During moromi, enzymes from the koji and yeast break down rice starches into fermentable sugars, which are then converted into alcohol. The process also develops the characteristic flavors and aromas of the final sake. Fermentation is monitored and managed with temperature control and periodic mixing.

After fermentation, the mash is pressed to separate the liquid sake from the solids (the sake lees or kasu). The extracted liquid may be pasteurized, filtered, and aged or bottled for sale and consumption.

The term moromi is also used for the fermenting mash in other Japanese products, such as miso and soy sauce, where a mixture of ingredients inoculated with koji undergoes long-term fermentation to produce the finished foods.