Koji: The Secret Ingredient of Japanese Fermented Foods

Centuries before modern refrigeration was invented, Asians have discovered and perfected their own fermentation methods to preserve foods as well as create their own unique flavour ingredients and alcoholic drinks. The Japanese miso, shoyu soy sauce, mirin cooking wine and sake rice wine are all among the products of fermentation. And the key ingredient in their making is the Koji mould.

What is Koji?

Scientifically known as Aspergillus Oryzae, the Koji is a naturally occurring mould in grains like rice and soybeans. In the Meiji era, the traditional technique to isolate and propagate the mould was perfected, and is still used to this day with modern technology. Broadly speaking, the mould is added to steamed rice or soybeans and stored in warm, humid conditions on wooden trays for up to 50 hours. During which, the mould’s enzymes would break down the grains’ carbs and proteins, thus producing the Koji starter culture for fermentation.

Koji in making Shoyu

To brew shoyu soy sauce, soybeans are soaked in water and steamed, then mixed with roasted wheat and Koji, fermented for up to 3 days. The resulting mash is blended with brine and left to brew and ferment for at least 6 months. Next, the mash is poured into cloths folded in layers and pressed to release the raw Shoyu, which is brewed in a clarifier tank for another 3 days to separate the oil and sediments. Finally, the filtered portion is heated to halt the fermentation, and once cooled, results in the delicate shoyu soy sauce. Discover more about shoyu and its variants from this process in our shoyu deep-dive!

Koji in making Miso

To make miso, mashed soybeans are mixed with salt and Koji-cultured rice or barley, then fermented for months, resulting in the umami miso paste. Different lengths of the fermentation period also produce unique miso variants with nuanced degrees of salty and earthy flavours. Come discover more about Miso in our special feature!

Koji in making Mirin

To brew Mirin, glutinous rice is steamed in high heat, then mixed with Koji and Shochu, and left to ferment for about 2 months. Shochu is a Japanese spirit distilled from mashed rice. Essential in making Mirin as the Koji and Shochu comes together in the fermentation process to break down the steamed glutinous rice, and produce the mirin sweetness and aroma. The fermented mash is transferred into bags and pressed to release the resulting liquid, which is then left to mature in storage tanks over months to produce the authentic Mirin. The sweet rice wine was once solely enjoyed as a spirit during the Edo period. But over the centuries, Mirin had also made its way into kitchens and became a cooking wine to enliven dishes with its unique mild sweet flavour.

Koji in making Sake

To brew sake, large grain rice is first polished and washed to remove the husks and brans, then soaked in water before steaming in a special vat. Koji is added next, and the blend is stored in high heat and humidity for about 36 to 45 hours. During which, the rice starch converts to glucose. After that, yeast is added to the mixture and pressed into a puree, then transferred into a fermentation tank and kept for 4 days, with more steamed rice, water and Koji added periodically.

The fermented mash is wrapped in a cloth bag and pressed or left to drip, resulting in raw sake. After filtering through active charcoal to remove sediments, the sake is passed through a hot water pipe to pasteurize, then left to mature in tanks for around 6 to 12 months. Finally, water is added to the matured sake to dilute and smoothen the alcohol, then bottled and ready for consumption.

Come discover more about the traditional sake brewing art in our deep-dive at Fuji Takasago Brewery!

More Asian Fermented Wonders

Asian fermented foods pack a tonne of health benefits and bold flavours. Some are also signature staples in their cuisines and have many tasty uses. Come learn more about them!