Ingredient

Koji

Koji is grain — usually rice — that has been grown through with the mould Aspergillus oryzae. It is not eaten as it is. It is an ingredient for making other ingredients, and it sits underneath miso, soy sauce, sake, mirin and rice vinegar, which is to say underneath most of what makes Japanese food taste the way it does.

What the mould is actually doing

It is making enzymes. That is the whole job.

Left alone, a grain of rice is starch and protein locked up in forms nothing much can use. The koji mould grows through it and floods it with two families of enzyme: amylases, which cut starch into sugars, and proteases, which cut protein into amino acids — including a great deal of glutamate, which tastes savoury.

So koji converts a bland grain into a source of sugar and savouriness. Add yeast to the sugar and you get sake. Add salt and soya beans and time and you get miso or soy sauce. Add nothing but warmth and water and you get amazake, a sweet drink made without any sugar at all.

One organism, one trick, and an entire cuisine's worth of ingredients downstream of it.

What is made from it

Miso

Koji plus cooked soya beans plus salt, aged. The koji provides the enzymes that break the beans down over months.

Soy sauce

Koji grown on a soya and wheat mixture, then brined and fermented, then pressed. The liquid is the sauce.

Sake

Koji converts rice starch to sugar while yeast simultaneously converts that sugar to alcohol — both at once, in the same vessel, which is unusual among the world's alcoholic drinks.

Mirin

Koji, glutinous rice and distilled spirit, left to sweeten. Real mirin is sweet because of the enzymes, not because sugar was added.

Amazake

Koji and rice and water, held warm. Intensely sweet, non-alcoholic, no added sugar.

Shio koji

Koji, salt and water blended into a slurry. A modern kitchen staple and the easiest way for a home cook to use koji directly.

Using it directly in the kitchen

Most cooks meet koji through shio koji, which is the practical entry point.

• Shio koji as a marinade for meat and fish. The proteases work on the surface proteins, so it seasons and tenderises at the same time. A few hours does noticeable work on chicken, pork or a firm fish.
• As a seasoning in place of some of the salt. It brings savouriness that salt alone does not, so a dressing or a vegetable dish made with it tastes fuller at the same salinity.
• On vegetables, especially mushrooms, aubergine and cabbage, before roasting.
• Watch the sugar. Koji-marinated food browns fast, because the enzymes have made sugars available. Cook it a little gentler than you would the unmarinated version.
• Do not over-marinate. Left far too long, proteases turn the surface of a delicate fish mushy. This is the one failure mode worth knowing.
• Amazake as a sweetener in baking and drinks, where its sweetness is mild and its flavour faintly of rice pudding.

Buying it, and a word about the mould

Dried rice koji is sold in packets in Japanese and East Asian grocers and keeps for months in a cool dry cupboard or the freezer. Shio koji is sold ready-made in bottles and is the easiest thing to start with.

Aspergillus oryzae is a domesticated organism. It has been selected over a very long period for use in food and is not the same as the wild moulds that turn up uninvited on food at home, which is worth saying because "growing a mould deliberately" alarms people reasonably enough. Cultivating koji from spores at home is done, and it needs controlled temperature and humidity and a properly sourced culture; growing it by accident is not a thing that happens.

FoodHQ does not publish salt percentages, pH figures or keeping times for home preserving. Those are safety numbers, they are set by a food safety authority rather than by a cookery writer, and the right ones depend on what you are fermenting and where you are. In the UK the Food Standards Agency is the authority — use a recipe from them or from another body with the same standing, and treat this page as the explanation of why that recipe is shaped the way it is.

Common questions