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Koji - The culture behind Japanese food production

            Koji - The culture behind Japanese food production

Koji is the culture behind Japanese food production, discover what it is and how it is used.

What is Koji?

Koji is not actually a yeast, as many people mistakenly believe. Koji is cooked rice and/or soya beans that have been inoculated with a fermentation culture, Aspergillus oryzae. This naturally occurring culture is particularly prevalent in Japan, where it is known as koji-kin, which explains why so many Japanese foods have been developed over the centuries using it. It is used to make popular foods like soya sauce, miso, mirin and sake.

The first step in making these products is creating the koji. This involves adding the Aspergillus culture to steamed rice or soya beans or, in the case of shoyu soya sauce, to a combination of steamed soya beans and roasted, cracked wheat. 
The resulting mixture is then placed in a warm and humid place for up to 50 hours, often in wooden trays called koji buta in Japanese. During this time the Aspergillus feeds on the rice or soya beans, using enzymes that are adept at breaking down carbohydrates and proteins.

How it is used?

Once it has been created, the koji is usually added to larger quantities of rice or soya beans, together with a brine solution. In the case of mirin, it is mixed with glutinous rice and the distilled alcoholic beverage shochu. In each case, the enzymes in the koji break down complex carbohydrates and proteins into amino acids, fatty acids and simple sugars. 

When making sake, rice is mixed with koji, which breaks down the carbohydrates into sugars then subsequently fermented by yeast to produce alcohol and carbon dioxide.

The benefits of Koji

The amino acids, fatty acids and simple sugars released by the action of the koji add flavour, depth and, it has been argued, a number of health benefits to foods. For example, the fermentation of soya beans using koji to create miso is known to increase the levels of isoflavones (link to Q & A on isoflavones), which are compounds that are said to be effective in the prevention of cancer.

One of the amino acids released by the action of koji is glutamate, which imparts an intensely satisfying and delicious savoury taste known as umami. This, combined with the simple sugars also released, ensure that foods made using koji have a uniquely rounded and deep flavour.

Fluffy white grains of rice koji, here being used to make sake.

Making koji for Clearspring's mirin.

Clumps of rice are broken up to ensure that the koji develops in a uniform way.

The rice inoculated with kojikin culture is placed in wooden trays in a warm, humid atmosphere to propagate.

Miso is just one of the many traditional Japanese foods that relies on koji...

...as is sake, here served in a traditional wooden container called a masu.

Shop Koji cultured products

Comments (23 Responses)

08 October, 2016

Guillermo Alfonso Padilla

please.teach me. how to produce. culture.the spores of koji-kin. or how to make the powder that is put on the steamed rice to make sake.miso.or amasake.how to get the spores.or that fungi aspergillious oryzae

27 May, 2016


My friend makiko has started making shio-koji and ama-koji, she sells it too :)

12 January, 2021

Dr.Oleg V Anokhin

Good afternoon ! Dear and respectable company! Colleagues.

To you, send letter with proposition Dr. of Chemistry of Natural composition materials from Ukraine. I have , want and ready propose new fundamental technologies for future perspective products. My solutions on a basis of Bast Crops colloid systems. Parts of grape or nuts or … For more information about my propositions and possibilities PLEASE open in GOOGLE – Dr. Oleg V Anokhin. If interesting – my e-mail address – anokhin@voliacable.com

16 March, 2016


Hi Clearspring, I love your Miso. This is another request for Koji. I know you don’t sell it but I think you should! I have a special interest in microorganisms and would love to experiment with making Miso. Fermented foods are an emerging trend, as is DIY…consider it. You could sell a Miso Making Kit!

12 January, 2021

David Higginbotham

I suggest translation of “Barley” be changed to “Malted”.
English: Barley is the Grain to be sprouted and converted into Malt (carbohydrates converted into sugars).
Koji is the Molded Rice converted into usable sugars.

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