The Koji Fermentation Boom: Shio Koji and Amazake at Home

Koji, the quiet mold behind miso, soy sauce and sake, has become a kitchen favorite in Japan, with shio koji and amazake leading the way.

Published October 26, 2026 at 12:02 PM JST | By Tokyo Paul

Quick answer: The koji boom is Japan's growing enthusiasm for cooking with koji, a rice or grain culture made with Aspergillus oryzae. Shio koji is koji mixed with salt, used as a seasoning and marinade, while amazake is a sweet drink made with koji. Market firm Intage reported that amazake sales reached 14.7 billion yen in 2025, according to a report from August 2026.

Koji sits behind many of Japan's best-loved flavors, from miso to soy sauce to sake, yet only in recent years has it moved from the brewery into home kitchens and supermarket shelves. People search for the koji boom because they want to know what koji is, how shio koji and amazake differ, and how to use them. This guide explains the basics, the recent trend, home use in plain terms, and the cautions that matter when you ferment food yourself.

What koji actually is

Koji is rice, barley or soybeans that have been cultured with a mold called Aspergillus oryzae. The Japan External Trade Organization explains that koji originated in China and is central to Japanese food culture, used in sake, shochu, awamori, amazake, miso and soy sauce. Wikipedia notes that this mold is sometimes called the national fungus of Japan, a title that the Brewing Society of Japan approved in 2006 because of its importance to the country's foods.

In everyday terms, koji works like a set of natural enzymes. As it grows on grain, it breaks starches into sugars and proteins into amino acids, which is why koji-based foods taste sweet, savory or deeply umami without extra additives.

Shio koji and amazake: the two stars

Shio koji is koji combined with salt and water. JETRO describes it as a seasoning that has recently become very popular in Japan, one that can make foods more flavorful and soften meat and fish. Cooks use it to marinate chicken, pork and fish, to season vegetables and soups, and to replace some plain salt in recipes.

Amazake is a sweet drink with koji at its heart. Wikipedia explains that there are two main types: a koji-based version, in which enzymes turn the carbohydrates of rice into sweet sugars, and a version made from sake lees, which can contain a little alcohol. The koji type is normally non-alcoholic. Amazake has a long history, with mentions dating back to the Nihon Shoki, and it is traditionally enjoyed warm with a pinch of grated ginger.

Signs of the boom

A report published in August 2026 by The Star, citing Japan News, says amazake has become a year-round drink rather than a winter-only one, and that sales peak in both winter and summer. It cites the market research company Intage, which says sales reached 14.7 billion yen in 2025. The report adds that interest revived in the 2010s partly because koji was growing popular as a fermented seasoning.

Other examples show the same direction. The report describes a koji-based amazake vending machine installed with the city of Kumagaya in Saitama, a local amazake brand developed in Nagoya, and a venture company from Utsunomiya University that began exporting amazake to the United States. In March 2026 the miso producer Hanamaruki announced plans to sell a liquid version of its shio koji seasoning to American consumers, with sales expected to begin around the summer.

Why people love it

Cooks like koji because it is a simple way to bring depth to everyday meals. A spoonful of shio koji can season a marinade, a salad dressing or a pot of rice without a long ingredient list. Amazake offers a gentle sweetness that appeals to people who want a drink that is not too sugary, and it can be served hot, chilled or blended.

The Star report mentions that koji-based amazake is alcohol-free and that makers market it overseas as a healthy drink. Claims about health benefits vary by product and are not medical advice, so readers with health conditions or dietary restrictions should read the label and consult a professional.

Making shio koji at home, in general terms

The Japanese government's trade promotion organization offers a basic outline: mix salt with rice koji, add water, keep the mixture in a lidded container at room temperature away from direct sunlight, and stir it daily. According to JETRO, it is ready in about two weeks, when it develops a sweet, banana-like aroma.

Recipes differ in their salt ratios, container sizes and temperatures, so it is wise to follow the instructions that come with your koji or those from a trusted source. Use clean containers and utensils, and keep the finished seasoning in the refrigerator once it is ready. If anything smells or looks wrong, do not taste it.

Making amazake at home, in general terms

Koji amazake is made by adding koji to cooled cooked rice and letting it incubate, during which enzymes convert the starch into sugars and sweetness develops, as Wikipedia describes. The exact temperature, timing and equipment depend on the recipe, so follow instructions from your koji supplier or a trusted source.

Because the process involves sustained warmth, hygiene matters. Wash containers carefully and store the finished drink in the refrigerator. The sake lees version is simpler because it is mixed from ingredients like sake lees, water and sugar, but it may contain a little alcohol, so check before serving it to children.

Buying koji products in Japan

Visitors can find shio koji, amazake and other koji products in supermarkets, specialty food stores and department store food halls. Look for pouches of amazake, jars or bottles of shio koji, and packs of rice koji in the refrigerated or fermented food sections. Regional shops sell local amazake as a specialty, and shrines and temples often serve it around New Year, according to Wikipedia.

Labels will list the ingredients, which vary by maker. If you have a food allergy or follow a vegetarian or vegan diet, check the label or ask the shop, since some products include added sugar, alcohol or other flavorings. Dried koji and bottled seasonings travel well, but check the import rules of your home country before packing food.

How koji connects to other Japanese ideas

Koji is a thread running through washoku, Japanese cuisine, which prizes umami, seasonality and fermentation. The same mold that makes your shio koji also shapes the miso in your soup and the soy sauce on your table. Learning about koji therefore helps explain why Japanese pantry staples taste the way they do.

It also links to regional pride. The Star report notes that amazake is increasingly used for regional revitalization and sold as a local specialty. Tasting a local amazake or koji seasoning is a delicious way to learn about a region.

What to remember

The koji boom is a story of an old technique finding a new audience. Shio koji and amazake are easy entry points: one is a seasoning, the other a drink, and both show what koji can do. Start with a ready-made product, read the label, and enjoy a taste of Japan's fermentation tradition.

Want to go deeper? Tokyo Paul's book Japan for Vegetarians is available on Amazon.

Frequently Asked Questions

What is the difference between shio koji and amazake?

Shio koji is koji mixed with salt and water and used as a seasoning or marinade. Amazake is a sweet drink made from koji and rice, or in another version from sake lees. Both use koji, but one is salty and savory while the other is sweet. Check labels for ingredients and alcohol.

Is amazake alcoholic?

It depends on the type. Koji-based amazake is generally non-alcoholic, while versions made from sake lees can contain a little alcohol, according to Wikipedia. Labels should tell you which kind you are buying. If you are serving children or avoiding alcohol, check the packaging or ask the maker.

How long does homemade shio koji take?

JETRO describes a basic method of mixing salt, rice koji and water, stirring daily at room temperature, with the seasoning ready in about two weeks. Recipes vary, so follow the instructions from your koji supplier or a trusted source, use clean containers and refrigerate the finished product.

Keep Exploring

More guides from Tokyo Paul: Koji: The Friendly Mold Behind Japan's Fermented Foods, Amazake and Mugicha: Traditional Japanese Drinks for Every Season and Natto: Japan's Sticky Fermented Soybeans.