Making Koji, the Base of Miso, Sake, and Shoyu

Jun 19, 2011
Katharine Luebcke

Fully formed koji - Forrest O.

Fermented foods are prominent in many cuisines, and in Japanese cuisine, many common fermented foods require koji--a mold often carried by rice--as a base.

Molds have a bad reputation in the kitchen; they are the last thing one wants to see on a loaf of bread. Molds actually have quite a niche to fill in the culinary arts and produce many of the worlds tastiest foods. If you've ever enjoyed a nice bowl of miso soup, you have the carefully selected and controlled mold koji to thank.

What is koji?

Carrying the mold scientifically known as Aspergillus oryzae, koji is at partly responsible for a wide range of foodstuffs: miso, or shoyu, sake, amazake, and certain pickles are but a few of them. To produce koji for culinary use, cooked rice is inoculated with the mold, then allowed to ferment for two to three days. Like many fermentation based items, the time required is lengthy, but the active time is minimal.

It may sound odd to deliberately contaminate food with mold or bacteria, but it is common kitchen practice. Many of the world's favorite foods are the products of such contamination. If you enjoy wine, beer, many aged cheeses, a number of dry cured sausages, soy sauce, sourdough bread, or yogurt, you are reaping the benefits of these maligned creatures. Not all are beneficial, but they should not be banned from the kitchen on principle.

What is required to make koji?

The most difficult part of producing koji is finding the isolated spores. The best source will be the Internet, through stores such as GEM Cultures or Vision Brewing. As a general rule, if you can find a store which sells various dairy cultures or kombucha mushrooms, you might be able to find the spores rather than prepared koji. But once you've acquired the culture, everything else is either simply to procure or already in your home. This includes a non-reactive dish, a small non-reactive bowl of water to provide humidity, a pot for steaming, a fine sieve, a probe thermometer, and an insulated box or cooler.

How is koji made?

This is a basic recipe for light miso koji, so consult the instructions before beginning the process if you purchase a different variety.

Ingredients:

1.5 cups medium-grain white rice
1 tablespoon all-purpose flour
0.5 teaspoon light miso koji spores

Place the rice in a bowl of clean water. Swirl the grains with your hand until the water becomes cloudy and milky. Drain, fill the bowl with clean water, and repeat. The rice will need to be rinsed several times, until the water is clear when poured off. Let the rice soak in more cold water for several hours, preferably overnight, ideally set in a sieve. If you do not have a sieve, just soak directly in the water and drain carefully through a colander.
After draining the rice, steam it over a pot of simmering water in a sieve or in a fine weave porous cloth stretched across the opening. The rice should be spread as far across the pot's opening as possible, so that the steam has nowhere to go but through the rice. Try to use a pot only slightly wider than the sieve. Once the steam begins to percolate through the rice, cook for an hour; the rice must be translucent. Transfer the rice to the non-reactive dish, then break the mass apart to facilitate even cooling and to prevent a single large mass from forming. Let cool to 90 F.
In a small skillet, toast the flour until it begins to smell nutty, then remove to a clean bowl. When the flour has cooled, mix in the koji spores. Sprinkle part of the flour mixture over the rice, then mix with a spoon, tossing to distribute evenly. Repeat until all the flour mixture is incorporated.
Place the dish of rice in the cooler or box, set the dish of water beside it, and place the thermometer in the middle of the rice. Just setting the thermometer on top can give an inaccurate reading and possibly kill the spores. Close the lid loosely to avoid crushing the wire, then ferment for 24 hours. Over the 24 hours, stir the rice with a clean spoon every 8 to 10 hours. If the fermenting koji is left alone, it will eventually form a single large mass which will be difficult to break apart for further use.
The temperature should be somewhere between 85 and 95 F for the first 12 hours to prevent the mold from dying or taking on a bad taste. For the second 12 hours, the temperature can range from about 75 to 100 F without killing the culture; the time may be altered based on the temperature.
After the second fermentation, the koji should be fully formed with a white layer encompassing the rice kernels. If it has not appeared, repeat the stirring and wrap to ferment for another few hours. As it is perishable, koji should be used immediately or frozen. To freeze, spread the koji on a sheet pan, place in the freezer until completely hard, then transfer to a container. Thaw in the refrigerator.

As with many things in the culinary world, fermentation is both an art and a science. This is but a basic description of how to produce koji in your own kitchen; the procedure can be altered, but the lengthy and warm incubation period is crucial. Once you understand the process, koji can be produced from a wide variety of grains, depending on what you wish to produce at the end.