Onattō is a traditional fermented food that has been passed down for generations in the Saku region of eastern Nagano Prefecture. Unlike the natto commonly known in Japan as fermented soybeans with a characteristic sticky texture, Onattō is made by fermenting cooked rice with rice koji.
The natural fermentation process allows koji mold to break down the starch in the rice and convert it into sugars. The result is a mildly sweet food with a soft, smooth texture and a distinctive fermented aroma.
There are also regional variations that include ingredients such as black soybeans or sasage beans. Traditionally enjoyed during the colder months, Onattō is a fascinating example of the food culture and fermentation wisdom that developed in the Saku region.
Historically, the Saku region was familiar with two different foods referred to as “nattō.” One was ito-hiki nattō, the familiar fermented soybean dish known for its sticky, stringy texture. The other was a fermented rice dish made with rice and koji—the food now known as Onattō.
Rice has traditionally held a particularly important place among Japan’s staple grains. It is said that the rice-based version came to be called “Onattō,” using an honorific form of the name to distinguish and show respect for this rice-based food.
In the past, many households in the Saku region made their own miso. During the process, there was often extra rice koji left over. Rather than wasting it, people found a way to make use of it by fermenting it together with rice.
During the cold winter months, the mixture could be kept warm overnight, traditionally even inside a kotatsu, the heated table used in Japanese homes. The warmth encouraged fermentation and brought out the rice’s natural sweetness.
Onattō therefore represents more than a traditional food. It reflects the practical wisdom of rural households, where ingredients were carefully used and preserved according to the seasons.
As fewer households make miso at home today, the traditional practice of preparing Onattō has also become less common. Nevertheless, modern kitchen appliances such as rice cookers can make the fermentation process easier, helping this regional food remain accessible to new generations.
Interest in traditional fermentation and fermented foods has also encouraged renewed attention to Onattō. Its simple ingredients, natural sweetness, and connection to local food culture make it an intriguing specialty for visitors interested in traditional Japanese cuisine.
Short-grain rice: 1 rice-cooker cup, approximately 150 g
Rice koji: 100 g
Lukewarm water: approximately 100 ml
First, cook the rice in a rice cooker. Once cooked, allow the rice to cool to approximately 30°C.
Soak the rice koji in lukewarm water for approximately 30 minutes.
Mix the cooled rice thoroughly with the prepared koji. Place the mixture in a rice cooker and use its warming function to ferment it for approximately five hours.
As fermentation progresses, the mixture becomes pale, soft, and slightly creamy. Once it reaches the desired consistency, remove it from the rice cooker.
It can then be combined with cooked black soybeans, sasage beans, sweetened beans, dumplings, or even fruit such as strawberries, depending on personal preference.
Temperature control is one of the most important points when making Onattō. If the rice is too hot, the koji microorganisms may be damaged and fermentation may not proceed properly. If it is too cool, the flavor may become overly sour.
A temperature of around 30°C is recommended. Gently stirring the mixture approximately every 30 minutes can also help monitor the fermentation process.
Using glutinous rice instead of regular rice can produce a sweeter version. Fermentation time may also be adjusted according to the season and room temperature to achieve the desired flavor and texture.
Once prepared, Onattō can be stored in the refrigerator and enjoyed for several days. Its naturally sweet flavor makes it suitable for a variety of occasions, from a small accompaniment to tea to a breakfast dish or a simple alternative to dessert.
Onattō can be enjoyed on its own as a snack or served as a sweet accompaniment to Japanese tea.
During the New Year period and the colder months, it is traditionally enjoyed with mochi, Japanese rice cakes. The gentle sweetness of Onattō pairs particularly well with the chewy texture of freshly prepared mochi.
Another way to enjoy Onattō is to add water and gently heat it, creating a drink similar to amazake, a traditional Japanese sweet fermented beverage.
Unlike commercially produced amazake, this homemade version offers the natural sweetness and aroma created through the fermentation of rice and koji.
Contemporary recipes have introduced new ways to enjoy Onattō, including combinations with fruit, nuts, and yogurt.
Its connection to traditional fermentation has also attracted renewed interest, particularly among people looking for distinctive regional foods and naturally fermented ingredients. These modern variations can transform Onattō into a simple snack or dessert-style dish while retaining its traditional character.
Onattō is a unique traditional food that captures the wisdom of everyday life in Saku. Using two familiar ingredients—rice and koji—people discovered how natural fermentation could transform ordinary cooked rice into a gently sweet and nourishing food.
The dish reflects a culture of avoiding waste, making the most of available ingredients, and adapting food preparation to the changing seasons. Although the custom of making Onattō at home has become less common, its distinctive flavor and cultural value continue to attract attention.
For travelers exploring Saku and the eastern Nagano region, discovering Onattō offers a chance to experience a lesser-known side of Japanese food culture—one shaped not by elaborate ingredients, but by centuries of fermentation knowledge and everyday ingenuity.