Marie Ebersolt

22nd July, 2023

Are you familiar with kôji? If you're a fan of healthy cooking, it's time you found out more about this culinary ingredient that's coming back into fashion in Japan. It has long been an important part of the Japanese diet. In condiments such as soy sauce or miso, and in sake, kôji plays an essential role in their manufacture. Today, rather than resorting to industrial sauces full of preservatives and flavour enhancers, many Japanese households have introduced kôji into their homemade seasonings. But what exactly is kôji?

Kôji is a fermented cereal produced by a fungus called kôji-kin. This fungus transforms rice starch into sugar and protein into amino acids, which is why kôji always has a sweet note and a rounded flavour that gives a depth of flavour, the famous "umami", to the food it accompanies. Unlike many fermented products, such as cheese or alcohol, kôji has no unpleasant smell or taste. 
There are three types of kôji: rice kôji, the most commonly used, barley kôji and soy kôji. They are used in most Japanese seasonings and spirits. By adding them to rice, glutinous rice, soya beans or barley, it is possible to create a wide variety of food products. Rice kôji can be used to make sake, vinegar, mirin and pickled vegetables; barley kôji can be used to make miso and shôchû (which can also be made with the other two); and soy kôji can be used to make a wide variety of soy sauces. Apart from the raw material, only water and salt are added to the ingredients. All these products are obtained both by human hands and through the cooperation of the three major families of fermentation agents: moulds, bacteria and yeasts. The latter are micro-organisms that act on organic substances to produce organic compounds. This is called fermentation.

To make kôji, you need a raw material and kôji-kin, a fungus that belongs to the mould family and whose scientific name is Aspergillus. The fungus is brought into contact with the cereal to be fermented. It feeds on the starch, breaking it down and transforming it into sugar, which attracts yeast and lactic acid bacteria. The former produce alcohol and carbon dioxide, while the latter produce lactic acids that repel undesirable bacteria. This is how you get really good sake. The fermentation process can continue: the sake becomes vinegar when acetic bacteria are added.

Such a sophisticated process is the result of a long history. Like many fermented foods, sake came into being thanks to the marvellous phenomenon of serendipity, and it was this alcoholic beverage that revealed the existence of the mould we are interested in, since it is the essential ingredient. The discovery of the unnamed Aspergillus is mentioned in an official text from the early Nara period (710-794). It mentions that rice given as an offering and which had begun to mould was brewed to produce sake. After harvesting, the rice was parboiled and offered to the gods in the hope of a bountiful harvest in the future. Living in a damp environment near the rice fields, it attracted the kôji spores that had appeared on the rice stalks and were flying through the air. Farmers would then enjoy the taste and euphoric effect of rice transformed into sake. Even today, sake is presented as an offering during Shinto rituals.
More specifically, the first specialists appeared around the 10th century, when they began to cultivate kôji-kin. These craftsmen were known as kôji-za or moyashi-ya and had a job in their own right, growing tane-kôji, which were sprinkled over a cereal to activate the fermentation process. Several shops opened in Kyoto during the Muromachi era (1336-1573), but following a violent confrontation between these growers of tiny mushrooms and wealthy sake makers, the profession was banned and integrated into the brewing industry. In effect, the brewers integrated the know-how of their suppliers to rationalise their production. Today, only a dozen moyashi-ya exist in Japan, including one that is three hundred years old and another that survived the battle mentioned above. Even today, these moyashi-ya grow tane-kôji, true "domesticated mushrooms". In the past, parboiled rice was mixed with wood ash to create an alkaline environment favourable to the development of Aspergillus and repellent to its other congeners. Today, our knowledge has improved and our methods are less rudimentary. As a result, many species of Aspergillus are selected and developed using technology.

After organic food, homemade food is on the up. Like many Japanese, wouldn't you like to try your hand at making a kôji-based seasoning? It couldn't be simpler: mix raw rice with freeze-dried kôji, which you can find in a Japanese grocery shop or on online retailers, salt and water. If you have a yoghurt maker that goes up to sixty degrees, you'll have your shio-kôji overnight. If not, leave to ferment for a week, stirring the mixture every day. You can add a flavouring of your choice and you'll be able to season or marinate your food in an original and tasty way. Exceptionally, I'm sharing some of my recipes with you. Don't worry: research has confirmed that kôji does not produce any toxins harmful to the human body.

First of all, it's very important to sterilise your container before making your kôji. I recommend using household spirit sprayed on a clean cloth. 
The basic recipe is as follows: 100 grams of freeze-dried kôji, 165 ml of filtered water, 35 grams of salt. After fermentation, you'll have your own homemade shio-kôji. Add a few rosemary leaves and a clove of garlic and you've got rosemary kôji!  
Are you more sweet than savoury? Why not make yourself a drink that's refreshing in summer and energising in winter? In a bottle holding at least 350 ml, mix 80 grams of rice kôji with 220 ml of water filtered at 70 degrees (or 150 ml of boiling water and 70 ml of filtered water). Shake the bottle for ten seconds and leave to ferment at room temperature for seven hours, shaking again halfway through. You can drink this ama-zaké hot or cold, and it will keep for ten days in the fridge.

The kôji also appears to be versatile. It is used in fields other than cooking. Let's go back to the chemical phenomenon first. When kôji-kin spores attach themselves to their host, they sporulate and spread mycelium, synthesising enzymes that break down the substrate: proteins are transformed into amino acids, starches become sugars and fats, fatty acids. Today, these enzymes are industrially isolated to produce consumer goods such as digestive medicines, detergents and soap. Researchers in the field of sustainable development are also interested in kôji-kin to decompose food waste, manure or wood from clearings and transform it into biofuel, as the organic matter obtained is a source of energy.

Marie Ebersolt