Editor's Note: Welcome to Sake School! Your professor is Monica Samuels, who trained with American Sommelier Association and the Sake Education Council. She is a Sake Educator for New York Vintners in Tribeca. Before her current role as Sake Ambassador for Southern Wine & Spirits of New York, Monica was the National Sake Sommelier for the SUSHISAMBA restaurant group.
In order to brew beer, barley (or another grain) goes through a malting process, where enzymes within the barley help to break down starch molecules and begin converting them into sugars. One of the fundamental differences between sake and beer is this: sake rice does not contain the kinds of enzymes that barley does, so an additional ingredient is needed to help convert the rice's starch into sugar. Some believe that the first alcohol made in Japan was kuchikami-no-sake (chewing in the mouth sake), made by chewing grains and spitting them into a pot. The enzymes in saliva helped to break down the starch in the rice, so the mixture in the pot would eventually ferment. The resulting image isn't exactly appetizing, and there was no sake boom as a result of kuchikami-no-sake.
It wasn't until the period of the Asuka rule (from 538 to 710) that koji-kin came into play. Koji-kin is a mold that's commonly used in Chinese, Korean, and Japanese culture to ferment soybeans as well as to make alcohol. Its scientific name is Aspergillus oryzae. There are many varieties of koji-kin, but Japanese sake is almost always made from yellow koji-kin (the greenish-brownish-yellowish mold seen above).
To convert sake rice into sugar that can be fermented, koji-kin is delicately distributed over steamed sake rice in a very hot, humid room. Over a period of 48 to 72 hours, the mold is carefully cultivated to grow evenly onto the sake rice. When the "moldy rice" is ready to be incorporated with the other elements of the sake, there is a noticeable, sweet chestnut-like aroma coming from the rice. To the naked eye, each grain of rice appears to be coated with a white frost.
This cultivation of koji (once koji-kin is fully grown onto sake rice, it is simply called "koji") is often considered the most important part of the sake brewing process. Although the method of koji-making varies greatly from one brewery to another, in each brewery that I have had the opportunity to visit, the koji muro (room for making koji) is the most sacred and revered part of the brewery.
It should be noted that not all sake rice becomes koji in the koji muro. In fact, only an average of 20% of sake rice used is developed into koji (the remaining 80% of steamed sake rice is referred to as the kake-mai). As the sake-making process continues, the sake yeast eats the sugars and begins to produce alcohol, but the koji also continues to work and converts the remaining rice (the kake-mai) into sugar. This "multiple parallel fermentation" is one of the elements that make sake production so complex.
In addition to converting the starch molecules of the rice into sugar, koji-kin also affects the presence and perception of umami in sake. While it's busy converting starch molecules into sugar, koji-kin also releases a number of amino acids, notably glutamate, which imparts umami, a savory quality or depth of flavor with a long lasting, mouthwatering sensation on the tongue.
The addition of koji-kin is essential for converting sake rice into sugar, which is what makes sake possible; that it also results in glutamate is an extra benefit that helps make sake more delicious.