There is an expression in sake brewing: “Ichi koji, ni moto, san tsukuri.” It means: “Koji first, moto (starter culture) second, brewing third.” When it comes to brewing sake, making the koji rice is the most important first step in the process. In this deep dive into koji, we’ll look at its history, the different types and their uses, how it is used, and describe how you can make it yourself.

What is koji?

Koji is a culture of a filamentous fungus prepared on cooked grains or beans. Most often, when we talk about koji we mean Aspergillus oryzae (A. oryzae) and we are referring to koji rice — the rice upon which A. oryzae is cultured. A. oryzae is fast growing, and a strong secreter of enzymes that primarily break down starch into sugar and proteins to amino acids. This process of breaking down foods with enzymes is what we rely on in fermentation and brewing. Koji enables us to produce umami-rich foods (miso, soy sauce), sake and other fermentations.

A brief history of koji

The use of koji dates back to 300 B.C. in China, and is mentioned in “Rites of the Chinese Dynasty.” Rice, barley and soybean koji were used to make soy sauce, miso, Huangjiu (Chinese yellow wine) and Japanese sake. The first mention of koji outside of China is in the “Harima no Kuni Fudoki” in the 8th century A.D.

Possible health benefits

I’ll start by saying I am not a nutritionist nor a physician. But there are many that claim potential health benefits of koji. My Japanese friends talk about the amazing powers of “amazake” (a sweet drink made from koji). Amazake is a source of B vitamins, amino acids, digestive enzymes and kojic acid. Here is a recipe for making your own amazake: “The Power of Koji & The Health Benefits of Amazake”.

Koji is used in primary and secondary fermentation, and fermented foods are thought to have many positive benefits for the gut. The enzymes produced by koji help break down proteins into amino acids, potentially increasing their availability to us and our gut biome (My Healthy Japan). Think of this as pre-digested food. We break down proteins to amino acids in our stomach, but this way, we are ingesting readily available amino acids. Amino acids are the building blocks our body uses to grow and repair itself. The National Library of Medicine also reports that koji can work as a kind of prebiotic that helps maintain healthy bacteria in the gut.

Another byproduct of koji fermentation is kojic acid. You can find it in many skin treatment products and cosmetic products. It is used to treat melasma (pigmentation spots) and can lighten skin by preventing the formation of melanin. (Healthline: Kojic acid)

What is koji rice?

Koji rice is rice on which Aspergillus has been cultured. It helps to disambiguate by referring to it as koji rice because koji spores can be cultured on many solid substrates such as rice, beans or root vegetables. Koji rice is one of the four ingredients in sake brewing and has a key role in fermentation.

Its place in the sake brewing process

Yeast is like a fussy child — it will only eat simple sugars, such as glucose. Unfortunately, rice is made up of starches (polysaccharides), which are inedible to yeast. Koji rice supplies the enzymes to break down starch into glucose for the yeast to gorge on. Then, in the same tank, yeast turns sugar into alcohol. In sake brewing, this is called multiple parallel fermentation.

Best rice types for sake

When brewing sake there are sake-specific rice strains (“sakamai”) and table rice strains. The best rice type for brewing sake depends on the type and flavor profile you intend to make. These rice types can broadly be categorized as hard and soft rice. Soft rice, such as Omachi or Yamadanishiki rice, can dissolve more quickly during fermentation; harder sake rice, such as Gohyakumangoku, is generally slower to dissolve during fermentation.

Koji on hard rice or soft rice are decisions brewers make in order to control aspects of fermentation. Soft rice is easier for koji mold to penetrate and can facilitate long, slow fermentation. This type of fermentation often results in the beautiful aromatics of ginjo and daiginjo sake. Brewers can combine a soft rice for the koji rice and a harder rice for fermentation to slow down fermentation and create beautiful sake from harder rice types.

Types of koji mold

Koji mold is differentiated by the color of the spores and its uses. The common types of Aspergillus are black koji, white koji and yellow koji.

Yellow koji and its uses

Yellow koji (Aspergillus oryzae), is the koji most used in sake brewing. When used for brewing sake, the koji is carefully grown at a slightly higher temperature in order to coax it to produce more enzymes for breaking down starches. For miso and soy sauce, the koji is propagated at a slightly lower temperature to promote the production of enzymes that break down proteins to amino acids. This control and the fact that yellow koji is a low acid producer makes it a flexible koji mold for many different applications, but it is ideally suited for sake.

White koji and its uses

White koji, or Aspergillus kawachii (a mutation of Aspergillus awamori), is used in shochu fermentation. It produces citric acid that lowers the pH of the fermentation mash. The lower pH means that the mash can be fermented at a higher temperature than yellow koji without risk of spoiling. As shochu is distilled, the citric acid does not make it into the final product and doesn’t impact the flavor profile. Try making amazake with white koji!

Interestingly, sake brewers have started to use a small amount of white koji in addition to the usual yellow koji to add some citric acid to sake. The sake listed below are all brewed with some white koji.

Sake made with white koji:

• Fukucho “Seaside” ($38)
• Mutsu Hassen “Pink Label” ($44)
• Mutsu Otokoyama “Chokara” ($39)

Black koji and its uses

Black koji, Aspergillus awamori, is specifically used to brew “awamori,” a type of Okinawan liquor made from Thai rice. It produces a lot of citric acid which helps protect the fermentation mash at high temperature without spoilage. It has a stronger flavor profile than white koji, and is actually the progenitor from which white koji mutated. As with white koji, some black koji can be used when brewing sake. Try Kuro Kabuto “Junmai Daiginjo,” which is brewed with some black koji to increase the acidity and umami of the sake.

Other ways to use koji mold

Koji mold eats starch and needs water. Steamed rice is a great substrate for cultivating koji, but other starches can be used to cultivate koji such as barley, soybeans, potatoes and other vegetables. Koji grows really well on pearl barley, and koji barley is often used to make miso. Growing koji mold on brown rice, grains and beans often requires that they be milled first to allow the koji spores to grow into the substrate.

Koji can also be used to age or cure meats, but meats are low in carbohydrates. By grinding koji rice to a powder, mixing it with flour and using this mixture to coat the meat, you can introduce the enzymes to its surface.

Can you eat koji rice?

Yes, you can eat koji rice, and it is delicious. It is slightly sweet and becomes sweeter the longer you chew it. When it’s fresh it has a chestnut aroma. Noma restaurant once served koji barley as a dessert by leaving it as a cake rather than breaking it down into individual grains. But for the most part, koji is used as an ingredient in marinades, and to fuel secondary fermentations in food such as miso.

Shio koji

Shio koji is a salt (“shio” means salt) koji marinade. To make it, you ferment koji in water with salt. The ratio of koji to water is 1-to-1. Then, add 5% to 10% of the weight of water and koji in salt. (Exactly how much salt you use depends on how you normally cook; I tend to use around 5% salt.)

For example, 100 grams of water + 100 grams of koji rice = 200 grams. Five percent of this would be 10 grams of salt; 10% would be 20 grams of salt. Let it ferment until the koji dissolves and then use this as a marinade to tenderize and add umami to meats, fish, vegetables and even rice crispy treats. Shio koji can also be made from koji barley rather than koji rice.

Miso

If you’ve seen miso paste in the store you’ll know that it comes in different styles: red, yellow, white, sweet, Kyoto, barley, etc. The different styles indicate how long the miso was aged and how much salt was used to make it.

Miso made from koji barley (“mugi” miso) is not gluten free, but has some great texture and umami. (I like it as a dip for raw vegetables.) A Kyoto style miso, sometimes called white or sweet miso, is made with cooked soybeans, koji, water and salt. It ferments at a warm room temperature for about a week. “Aka” miso (red miso) ferments for six months or more, and has a much higher salt content.

And miso doesn’t have to be made from soybeans. I make miso from peas, garbanzo beans, black beans, cheddar jalapeno focaccia bread and other carbohydrate-rich foods. Miso can separate to give you a flavorful, umami-rich liquid on the surface called tamari. Some uses for miso paste include soups, stews, pasta sauces, caramels, dips, salad dressings, cookies and butters. Miso paste adds salt, umami and depth to dishes. Piñon miso caramels are a favorite in my house.

Shoyu

Shoyu (soy sauce) is made by cultivating koji directly on cooked soybeans. When we make miso, we add koji rice or koji barley to cooked soybeans with water and salt. To make shoyu, the cooked soybeans are mixed with barley and salt, then left to ferment. Soybeans retain a lot of water which can drown out the koji mold as it grows, but by adding barley, you can balance out the water content, allowing the koji to grow.

During fermentation, the mixture is pressed to extract the dark liquid. It is rich in umami and salt, and adds caramel color to dishes. Shoyu is used in marinades, soups, stir-fries and shoyu ramen broth. Shoyu caramel sauce on vanilla ice cream pairs well with “koshu” (aged sake).

Mirin and rice wine vinegar

Mirin and rice wine vinegar are essential Japanese condiments used in marinades, sauces, soups, pickling, and in the preparation of meats, vegetables and seafood. Much of the mirin found in stores is made from “sake-kasu” (the lees from making sake), but traditionally, mirin is made by mixing steamed glutinous rice, koji rice and alcohol (minimum of 40% ABV) stored for several weeks at around 30 C/86 F.

The enzymes in the koji rice break down the steamed glutinous rice, adding sweetness and umami to the alcohol. The mixture is strained and the resulting clear liquid is mirin. You can eat the reserved as an adult rice pudding, but be warned that it tends to be boozy.

Rice wine vinegar is mirin that is left longer, with the addition of a vinegar mother to convert alcohol to acetic acid. Rice vinegar is generally the least acidic of the vinegars you can buy in the store, and it has a touch of sweetness from the broken down glutinous rice.

How to make koji rice

There are a ton of great publications that will help you explore koji mold. If you’re interested in cultivating your own rice koji, I would suggest following the instructions in “Koji Alchemy: Rediscovering the Magic of Mold-Based Fermentation (Soy Sauce, Miso, Sake, Mirin, Amazake, Charcuterie)” by Rich Shih and Jeremy Umansky, or “Koji for Life” by Nakaji.

Making koji at home takes practice, but starting with koji barley (mugi koji) using pearl barley is a lot easier than starting with rice koji, since barley retains more water after steaming. The website Pants Down, Aprons On has a walk-through for cultivating barley koji.

Cleanliness, humidity and temperature control are very important to successfully cultivate rice koji. The rice needs to be steamed and not boiled. Think of an armadillo: It should be hard on the outside and soft on the inside. Also, make sure you are able to check on the koji as it grows at 12, 24 and 36 hours (plus regularly after the 36 hour point). If the koji starts to fruit (is no longer white and fluffy but starts to get a colored fuzz) it has gone past the point of being useful, and should be pushed for several days so it may thoroughly fruit to provide spores for your next attempt.

How to store koji rice

Generally, keep your koji cold either in the fridge or freezer. Even koji that has been dried using a dehydrator is best kept in the fridge to help preserve the enzymes and nutrients, while also inhibiting the koji mold from continuing to grow or fruit. The efficacy of the enzymes will diminish over time, so I make only what I know I’ll use in a couple of months. It’s also easier to work with smaller batches.

Where to buy koji rice

Koji is easy to buy online, and if you have a local Asian market, they might carry koji there, too. Where to buy koji will depend on where you are located. (Maybe you’re near a sake brewery that is also selling koji.)

I like IseSo Miyako koji because it comes in 1 kilogram packages that you can find online at Amazon or Gohan Market. When I make a large batch of miso, having the 1 kilogram bags comes in handy. Cold Mountain Dry Rice koji is another great product that is readily available in both grocery stores and online, such as at ChefShop. Another source for koji rice online is Umami Insider.

Koji makes brewing sake possible

Koji mold is an amazing thing. It is an essential part of Asian cuisine, either directly or in secondary fermentation. For making sake, it provides the enzymes that saccharify the starch in rice to feed the yeast in fermentation. In short, koji makes brewing sake possible. So remember, all that is moldy isn’t always bad. And when you’re fermenting, keep everything clean so that you stay safe.