Koji: The Mould Behind Miso, Soy Sauce and Sake
Koji is a grain or bean, most often steamed rice, that has been inoculated with the mould Aspergillus oryzae and allowed to grow for a couple of days. The fluffy white mould produces powerful enzymes that break down starch and protein, and it's the foundation of miso, soy sauce, sake, mirin, rice vinegar and amazake.
The Brewing Society of Japan has even named A. oryzae Japan's "national fungus."
How koji is made
- Steam rice (or barley, or soybeans) until cooked but not mushy.
- Cool it to about 35 °C / 95 °F and sprinkle with koji spores (tane koji).
- Hold it warm and humid, around 28–35 °C / 82–95 °F, for about 36–48 hours.
- The grains become covered in a white, sweet-smelling bloom. It's harvested before the mould turns green and spores.
What the enzymes do
Koji's value is its enzymes:
- Amylases break starches into sugars. In sake brewing, those sugars feed yeast, which makes alcohol. In amazake, they simply make a sweet rice drink.
- Proteases break proteins into amino acids, especially glutamate, the source of umami. This is how soybeans become savoury miso and soy sauce.
- Lipases break fats into flavourful fatty acids.
In other words, koji does a long, slow pre-digestion of food, building sweetness and savouriness that wasn't there before. It's not itself the fermentation organism in most of these foods; it's the enzyme factory that sets up yeast and bacteria to do their work.
Koji foods
| Food | Koji base | What happens next |
|---|---|---|
| Miso | Rice, barley or soy koji | Mixed with soybeans and salt, aged months to years |
| Soy sauce | Soy and wheat koji | Fermented in brine, pressed |
| Sake | Rice koji | Yeast converts sugars to alcohol |
| Mirin | Rice koji | Fermented with shochu, sweet and lightly alcoholic |
| Amazake | Rice koji | Held warm in water, becomes sweet |
| Shio koji | Rice koji | Salt and water, fermented 1–2 weeks |
Cooking with koji at home
Shio koji is the easiest way in. Use about 10% of the food's weight as a marinade:
- Chicken or pork: marinate a few hours to overnight. It tenderizes and browns beautifully.
- Fish: 30 minutes to a few hours.
- Vegetables: a spoonful in dressings or roasted vegetables for depth.
Because of its sugars and amino acids, shio-koji-marinated food browns fast through the Maillard reaction. Cook over moderate heat and watch for burning.
Chef Fizz says: Koji is a tiny, fuzzy sous-chef that works the night shift turning rice into flavour.
Questions
Is koji mould safe to eat?
Yes. Aspergillus oryzae has been domesticated in Japan and China for well over a thousand years and is considered safe. Its toxin-producing wild relatives are different, which is why you should buy koji spores (tane koji) from a reputable supplier.
What is shio koji?
Shio koji is koji rice fermented with salt and water for about a week or two. It's a sweet-salty, umami-rich paste or liquid used as a marinade or seasoning, often at about 10% of the food's weight.
What does koji taste like?
Fresh koji rice smells sweet, floral and a bit like chestnuts or mushrooms. Its real power shows up after it ferments or works on other food, adding sweetness and savoury depth.
How is koji used to age meat?
Coating meat in koji rice or shio koji lets its enzymes break down proteins and fats on the surface. In a few days it achieves some of the tenderness and nutty, savoury flavour of long dry-aging.