When looking at what makes sake special, one thing that comes up often is koji mold. It is the heart of the sake-making process — no beverage in the world uses koji in its production the way that sake does. Of all factors involved in sake brewing, the addition of koji exerts the most influence on the final product. As such, it is deserving of some more detailed attention.
For those that might need it, a quick review: Koji mold is propagated onto about 30 percent of the steamed rice used in sake production. The enzymes produced as this mold grows into the individual grains break down starch molecules into glucose. This is then fermented by the yeast cells.
It takes about two days to prepare the koji for use in a fermenting tank. Once added to the tank, it continues to trickle sugar slowly into the mash; the yeast cells then eat that sugar and give off alcohol and carbon dioxide. The starch-to-sugar conversion, and the fermentation of that sugar, take place simultaneously in the same tank. This makes sake unique; in other beverages the conversion to sugar occurs first, and fermentation later.
This also makes the whole process massively complicated. Too much sugar at one time chokes the yeast cells, too little curbs them. To further complicate the matter, numerous factors affect how sprightly the yeast is, such as the temperature, the chemistry of the water and the desired flavor profile (sweet or dry).
So, obviously, how fast the koji works is of paramount importance. How quickly it converts starch to sugar must be balanced with how soon that sugar will be needed later. And how fast the mold will work is determined by countless subtle adjustments during its two-day preparation in the special koji-making room of the kura (brewery).
A quick break for some terminology: koji itself is steamed rice that has had koji mold cultivated upon and into it. That mold itself is known as koji-kin or tane koji; its formal name in the scientific world is Aspergillus oryzae. Its nickname in the brewing world is moyashi, since it resembles sprouts (at least under a microscope).
The mold is commonly found in more humid regions, especially Asia. Different strains of koji are also used in making natto, soy sauce and miso paste.
Back to the koji-making room: If the mold (in powdered spore form) is applied generously and allowed to propagate haphazardly, it will create sugar from starch at a fast rate. If it is restrained, however, it will trickle the glucose in at a slow speed. Note that neither of these is inherently better than the other, but rather, getting it just right for the million conditions of that batch and day is the key.
But wait. The plot thickens! It is not just about glucose. As the koji is prepared, other compounds that wield considerable leverage over the final flavor profile are also created. These include various vitamins, amino acids, peptides (combinations of amino acids), malic acid, succinic acid and others. It is precisely these compounds that make each sake unique in terms of the myriad of fruit, herbal and textural tones.
There are machines that make koji, but almost without exception, the best koji is made by hand and calls for constant attention. Even the room itself (called the koji muro) has a big effect on things. Once a brewer, while serving me a sample, felt the need to apologize since the brewery had just rebuilt the koji muro, and the smell of the new cedar walls came through in the final product.
Yeah, right, I thought. She’s paranoid; her product is always good. But when we tasted it, amazingly, she was correct. It was faint and not totally unpleasant, but the essence of cedar had come through. That’s how much influence koji-making can have.
Eiko Fuji (Yamagata Prefecture)
Eiko Fuji makes a truly wonderful array of sake, all of them eminently drinkable and unique, but this is likely my favorite of their products. Fragrant, light and crisp, this thoroughly enjoyable sake is wonderful alone, yet is balanced and modest enough to go well with meals as well. It can be a bit hard to find, but in Tokyo, try Keio department store in Shinjuku.