Kōji (麹) is a mould grown on steamed grain, and it is the engine of Japanese fermentation. Its enzymes break grain starch into sugar and protein into savoury amino acids, which is what makes sake, miso, soy sauce, mirin and shochu possible. The species behind it, Aspergillus oryzae, was named Japan's "national fungus" in 2006.
This is our main guide to kōji, written for chefs and curious cooks alike: what it is and what it does, the three colour types and their uses, how it is made, and how a kitchen puts it to work through shio koji, amazake and the wider fermented pantry. It sits inside our Fermented Masterclass.
What Is Kōji?
Kōji (麹/麴) is a filamentous mould of the genus Aspergillus, deliberately grown on steamed grain such as rice, barley or soybean. As the mould grows through the grain it secretes a battery of enzymes that digest it. That enzyme activity, rather than the mould itself, is the entire point of kōji, and it is the first organism in almost every Japanese ferment.
The reason it comes first is simple: yeast and lactic-acid bacteria cannot feed on raw grain starch and protein directly. Kōji's enzymes break carbohydrates into sugars and proteins into amino acids, and supply vitamins, so that yeast and bacteria can then turn rice, barley and soybean into miso, soy sauce, vinegar, mirin and sake. For this reason yellow kōji is described as the origin of washoku culture. Its standing is formal: long used and regarded as both important and safe, Aspergillus oryzae was designated Japan's national fungus (国菌, kokkin) in 2006.
For a chef, the useful way to hold kōji is as a biological tool rather than a flavour. You rarely taste kōji on its own; you taste what its enzymes unlock, the sweetness, the umami and the tenderising that show up in everything from a miso to a shio-koji-cured fish.
Source: National Research Institute of Brewing (NRIB) newsletters and CAoGDX press material; facts extracted and reworded. The Aspergillus oryzae enzyme mechanism is established food science.
The Three Types of Kōji
Kōji moulds divide into three by colour and use: yellow kōji for sake, miso and soy sauce; black kōji for shochu and awamori; and white kōji, a mutant of black, also for shochu. Black and white kōji are together called "shochu kōji" and share one defining trick: they pour out citric acid, which protects the ferment in warm climates.
| Type | Japanese | Species | Principal use |
|---|---|---|---|
| Yellow kōji | 黄麴菌 (Ki-kōji-kin) | Aspergillus oryzae | Sake, miso, soy sauce |
| Black kōji | 黒麴菌 (Kuro-kōji-kin) | Aspergillus luchuensis | Shochu, awamori |
| White kōji | 白麴菌 (Shiro-kōji-kin) | Aspergillus kawachii | Shochu |
White kōji is a mutant strain of black kōji. Black kōji was long used for Okinawan awamori and spread to Kyushu shochu around the end of the Meiji era; both are nationwide now. The science was tidied up in 2013, when an international group established that black kōji is not a strain of Aspergillus niger but a distinct species, correctly named Aspergillus luchuensis. That matters for safety as much as taxonomy, as covered in the FAQ below.
The chef's takeaway: the colour of the kōji is really a map of the drink or food it builds. Yellow sits under the sake-and-seasoning world you cook with daily; black and white sit under the distilled world of shochu and awamori.
Source: NRIB newsletters (koji-mould feature issues) and the Story of Shochu material; the 2013 reclassification and the shochu-kōji grouping are reported there. Facts extracted and reworded.
What Does Kōji Do?
Kōji's value is its enzymes, produced all at once: amylases that turn starch into sugar, proteases that turn protein into amino acids, and others alongside. The balance between these enzymes, not just their presence, is what decides the quality of the resulting sake, miso or shochu, which is why kōji-making is treated as a craft in its own right.
The other key output is citric acid, the speciality of black and white kōji. By making the fermenting mash strongly acidic it suppresses stray microbes and lets the working yeast ferment cleanly even in warm weather, which is exactly why these kōji are essential to shochu and awamori in Japan's hot south. Modern research has read kōji down to the gene: the full genome of the reference strain RIB40 was published in 2005, and NRIB has since built genome databases and DNA tools to study how kōji behaves during fermentation.
For a kitchen, the practical reading is that kōji is doing three jobs you can taste: making things sweeter, making them more savoury, and making proteins more tender. A shio koji marinade leans on all three at once.
Source: NRIB newsletters and the CAoGDX genome-database press release (RIB40 genome 2005; enzyme balance; citric acid). Facts extracted and reworded; gene-research detail is from NRIB's own published material.
How Is Kōji Made?
Kōji is made by scattering kōji-mould spores over steamed rice or barley and letting the mould grow through the grain, digesting it with the enzymes it secretes as it goes. A batch takes roughly two days, worked in a warm, humid room so the mould develops evenly without overheating. The grain is not cooked again; it is colonised.
Improving kōji strains is genuinely hard, which is worth knowing because it explains why good kōji is prized. Kōji is multinucleate, with several nuclei per cell, so older mutation methods rarely changed every nucleus, and no mating type exists for conventional cross-breeding. Modern genome editing now lets researchers edit a chosen gene directly, and NRIB applies it to kōji breeding.
The step-level detail of a working kōji room, the exact temperatures, humidity and the turning of the grain, belongs to the brewers and is covered in our sake, shochu and miso guides rather than here. The principle for a chef is enough: kōji is a living culture grown on grain, not a powder or an extract.
Source: NRIB newsletters and Preservation Society material on kōji-making; the two-day cultivation and the breeding constraints are reported there. Facts extracted and reworded; producer-level process detail sits in the sake, shochu and miso records.
Popular Products
Four ways to bring kōji into a kitchen, from the raw culture to its finished seasonings:
Tsurumiso Dried Rice Koji, 500g
Kōji itself: rice grown with Aspergillus oryzae, dried for the shelf. The starting point for making your own shio koji, amazake or miso, and the purest way to understand what kōji is.
Hanamaruki Liquid Shio Koji, 500ml
The modern, pourable shio koji: a smooth liquid kōji seasoning that brushes straight onto fish, meat and vegetables to season and tenderise. The easiest entry to cooking with kōji.
How Chefs Use Kōji
Kōji is not eaten for its own flavour; it is used as the enzymatic starter that makes a whole family of foods possible. Because its enzymes unlock grain starch and protein, kōji underlies miso, soy sauce, vinegar, sake, mirin and shochu, as well as the seasonings a chef can use directly, shio koji and amazake. The simplest way to put kōji to work is to let one of those do the heavy lifting.
Shio koji is the gateway: a paste or liquid of kōji, salt and water that seasons and tenderises in one move. Brush or rub it onto fish, chicken or vegetables an hour or a day ahead, and the proteases break down protein for a softer texture while the amylases add a rounded, savoury sweetness, a cleaner result than salt alone. Amazake brings kōji's natural sweetness to dressings, marinades and desserts without added sugar. And if you want to understand the category from the ground up, dried rice kōji lets you make your own shio koji or amazake.
Our steer for a working kitchen: keep a liquid shio koji for everyday seasoning and curing, and treat it as a tenderiser as much as a seasoning, watching the timing so delicate fish does not over-soften. (Chef-level application detail beyond this is our own kitchen practice; the record flags culinary specifics as a gap, so treat quantities and timings as a starting point to dial in.)
Source: NRIB material on kōji's role across fermented foods; the specific kitchen guidance is SushiSushi practice. No sourced chef-application detail is held yet.
Frequently Asked Questions
What is kōji?
Kōji is a mould, Aspergillus, grown deliberately on steamed grain such as rice, barley or soybean. As it grows it produces enzymes that break the grain's starch into sugar and its protein into amino acids. That enzyme activity is the point of kōji: it is the starter that makes sake, miso, soy sauce, mirin and shochu possible. Aspergillus oryzae was named Japan's national fungus in 2006.
What is the difference between kōji and shio koji?
Kōji is the mould-grown grain itself, the raw culture. Shio koji is a finished seasoning made from kōji combined with salt and water and left to mature into a paste or liquid. So kōji is the ingredient and the engine; shio koji is one of the easiest ways to cook with it, used to season and tenderise fish, meat and vegetables.
What does kōji do in cooking?
Kōji's enzymes do three things you can taste: they make food sweeter by turning starch into sugar, more savoury by turning protein into amino acids, and more tender by breaking protein down. This is why a shio koji marinade both seasons and softens, and why kōji underlies the deep flavour of miso, soy sauce and sake rather than being eaten on its own.
What are the three types of kōji?
Yellow kōji (Aspergillus oryzae) is used for sake, miso and soy sauce. Black kōji (Aspergillus luchuensis) is used for shochu and awamori. White kōji (Aspergillus kawachii), a mutant of black, is also used for shochu. Black and white kōji produce large amounts of citric acid, which protects the ferment in warm climates; together they are known as shochu kōji.
Is kōji safe to eat?
Yes. Yellow kōji has long been regarded as a safe, important micro-organism, part of why it was made Japan's national fungus. Black kōji was reclassified in 2013 as Aspergillus luchuensis, a distinct species that, unlike Aspergillus niger, does not produce the mycotoxins associated with that mould. Kōji has been central to the Japanese diet for centuries.
Where to Buy Kōji in the UK
If you are looking to buy kōji in the UK, we keep a range with fast delivery across the UK, from dried rice kōji for making your own to ready-to-use liquid and paste shio koji and amazake. Start with a liquid shio koji if you simply want to cook with kōji tonight.
For more, browse the Fermented Masterclass, or read about shio koji, miso and shochu, all of which kōji makes possible.
Provenance: built from the SushiSushi Knowledge Base record store/koji/koji.md. The authority is the National Research Institute of Brewing (NRIB / 酒類総合研究所), Japan's official national brewing-science institute; its website, genome databases and press releases are official-body sources. A few published editorial works (NRIB's printed newsletters, the Story of Shochu booklets, and the Preservation Society for Traditional Japanese Koji and Sake-Making leaflet) are separately copyrighted publications, so their facts are extracted and reworded here, never reproduced. Enzyme and fermentation mechanisms are established food science.
