Kōji is a fungus. In fact, a few fungi. The Japanese character for kōji means 'rice flower', which is exactly what it looks like when it's growing—a landscape of white, fluffy, tiny flowers blooming from the surface of the grains.
There are a few related species called kōji with many strains (specific variants of species). Aspergillus Sojae, as the name suggests, is often grown on soybeans to make shoyu (soy sauce). Aspergillus awamori is black and is used to make Okinawan rice wine called awamori. But perhaps the most widely used kōji is Aspergillus Oryzae, and when we talk about kōji we often mean this one.
Kōji is foundational to Japanese cuisine. It is the basis for miso, shoyu, tamari, sake, mirin, and many other cornerstones of Japanese cooking. Kōji's relationship with humans is even a key part of its evolutionary history. As far as scientists can tell, Aspergillus Oryzae emerged some thousands of years ago from its closest relative, Aspergillus flavus, alongside early human brewing practices in East Asia. While A. flavus produces aflatoxin, one of the most potent naturally occurring fungal toxins known, A. Orzyae doesn't produce any—instead, it makes delightful aromas and, crucially, enzymes that turn starches into sugars. Scientists think that these traits were selected by humans to produce alcohol from starchy grains, helping A. flavus transform into A. oryzae. In other words, kōji came to be because it helped our ancestors get drunk.
Kōji has analogs around East Asia. In Korea, it is called nuruk, and is largely grown on cakes of cooked grains like wheat, rice, or barley. In China, it goes by qu (pronounced 'chü') and takes quite a few different forms as one would expect in such a large and diverse land. One of the main differences between kōji and its analogs is that, while qu and nuruk involve mixed cultures of many different fungi and bacteria together, including Aspergilli, kōji cultures have been selected over many generations to contain an individual species or even strain of Aspergillus. This is why, when making kōji, we use pure spores.
While kōji is pretty special, we can also loosely compare it to products and processes that might be more familiar. Bloomy-rinded cheeses, like Camembert and St. Marcellin, also involve white fluffy fungi grown on the surface to break down proteins and fats in the curd and produce new flavors. Malting has often been compared to kōji too, in that malting also involves transforming a grain's starches into sugars so it can be brewed into alcohol. The main difference here though, is that the transformation in malting happens with enzymes inside the grain, while with kōji the enzymes are added from outside the grain by the fungus.
There are many features that make kōji so useful. These have to do mainly with the enzymes and flavors kōji yields.
Enzymes are small proteins that make specific chemical reactions happen. Kōji produces a range of enzymes that make it super powerful for fermentation. It makes amylases, which help cut up long starches into smaller sugars, making them available to other microbes like yeasts and bacteria that can then turn them into alcohols, acids, and aromas. This is why kōji is used in making sake, and why we use it in our brewing process.
It also makes proteases, which help break down long proteins into single amino acids, unlocking flavors like umami. This is why it is used in making miso, shoyu, and tamari. Kōji can also form different levels of these enzymes depending on how it grows. Higher temperatures can encourage more amylase production, while lower temperatures can encourage more protease. In addition to these enzymatic qualities, kōji also produces a range of aromas like tropical fruits, nuts, honey, or flowers, and often a delicate note of raw mushrooms.
How to make kōji?
Kōji likes quite specific growth conditions-between 30-40°C, 70-75% relative humidity, and good ventilation for introducing oxygen and dissipating the heat produced by the fungus as it grows.
Unless one lives in the tropics, creating these conditions requires a bit of infrastructure. Traditionally kōji is made in a kōji-muro, or kōji room, lined with wood to help regulate humidity. Pine or cedar are often used for their antimicrobial properties, to help inhibit the growth of other microbes. Our kōji-muro is lined with Douglas fir. But one can also make a home kōji set-up with little more than an insulated chamber or box of some kind, a simple thermostat, a small heater, and a humidifier. Under these conditions making kōji takes just under two days.
Soak pearled barley for a few hours at room temperature, or overnight in the fridge.
Turn on the heater and humidifier in the kōji chamber, set to 30°C and 70%RF (retention factor).
Steam barley until cooked through but still firm, around 40 minutes depending on set-up.
Remove barley from the oven. With gloved hands break the barley up into individual grains.
Once the barley has cooled to about 35°C, inoculate with kōji spores (1g of spores per 1 kg of grain is sufficient). Mix to distribute, and spread the grains evenly to a depth of no more than a few centimeters.
Cover the tray with a damp cloth to help retain humidity.
Place the tray into the koji chamber. Place the temperature probe into the barley, ensuring that it cannot come out as this could make the kōji overheat and die.
After 24 hours, remove the tray. The grains will be lightly held together with tiny white fibers - the mycelia of the kōji. It should smell fruity and fragrant. With gloved hands mix the grains to aerate and redistribute to ensure even growth. At this point you can also create two furrows in the grains, creating more surface area to help the kōji dissipate excess heat. Return the probe, re-dampen the cloth, and return to the chamber.
After 16 more hours, the grains will be firmly bound together by the white mycelia. It will taste sweeter than grains and smell very fruity and fragrant. Use the kōji immediately, or store it in the fridge for imminent use (a few days) or in the freezer for later use.
If you let the kōji keep growing, it will produce spores. Sporulation changes the flavor of the kōji, so best to use it before this point. Depending on your setup, you might want to check on the kōji more frequently as it grows, to prevent overheating.
Learn more about kōji
In recent years kōji has become popular in the food world. Check out a recent book called Kōji Alchemy for a glimpse of current kōji experiments, or The Noma Guide to Fermentation for how the restaurant uses it not only in their fermentations but in their cooking.