Steamed rice (or barley, or soybeans) grown through with Aspergillus oryzae, a mould whose enzymes turn starch to sugar and protein to savoury amino acids. It's the starter behind miso, soy sauce, sake, mirin and amazake.
sweet · chestnutty · mushroomy · floral
Kinds
Rice koji (kome-koji)
The all-rounder — for shio koji, amazake, white miso and curing meat.
Japanese grocers, fresh in the chiller or dried in bags, and online. Dried keeps for months; fresh is best frozen.
A little history
It descends from Chinese qu, the mould cultures behind Chinese wines and sauces. Japan's brewing society has called it the country's national fungus.
Worth knowing
To grow it, inoculate steamed rice with spores and keep it warm and humid — around 30°C — for about two days, until the grains are bound in white fuzz and smell of chestnuts. Rubbed on a steak for a couple of days in the fridge, it gives something like the flavour of a long dry-age; the sugars brown fast, so sear with care.