The formula of oi-koji is to add extra koji to the miso during its maturation process in order to bring out its gorgeous aroma, mild sweetness, and deep umami flavor. Koji malt breaks down the rice and convert them into sugar, giving it a sweet taste during the fermentation process. However, the sweetness gradually diminishes as the yeast eats up the sugar during the aging process. Adding extra koji helps compensate the sugar to bring out a sweet and mild flavor. Because the yeasts stay active in this oi-koji process, it is also characterized by a gorgeous, fruity aroma. This traditional method was implemented around 1945 but once discontinued due to production efficiency and other reasons. It was revived in 2021 with a present-day technology and new investments in equipment.

At the HANAMARUKI factory, most miso is first prepared in 90-ton tanks, then opened into hoppers to mix it thoroughly before transferring it into 5-ton tanks for maturation. The mixing helps activate the yeast to eliminate uneven fermentation which allows the miso to mature evenly. In the "kowake-jikomi”" process, the yeast is divided into smaller tanks with one-tenth the capacity of regular tanks during the maturation process. By dividing the soybeans into small portions, an environment is created in which the yeast bacteria can move more actively and evenly. Back in the old days, many households used to prepare homemade miso in small barrels. It can be said that the kowake-jikomi is one of the traditional methods used for miso manufacturing.

We use Japan-grown soybeans, "Toyomasari" and "Enrei", to make miso. They are characterized by their low oil and high carbohydrate content, soft when boiled, and smooth when mashed. They are also beautifully white in color and are known among technicians as a cultivar suitable for making miso. Since the state of the soybeans can easily affect the quality of miso, our technicians adjust the heat and steaming time according to the year's soybeans to maintain quality. Some soybeans may be a little tough or too soft even if they are boiled in the same way. We also carefully select the appropriate rice and salt to bring out the full flavor of the soybeans.

In general products, the temperature of the fermentation room is artificially heated and kept at about 30 degrees celsius which is the best temperature for koji enzymes and yeast to work. The fermentation process takes 50 days for short-lived products and 90 days for long-lived products. Natural brewing is a method of production in which fermentation is left to nature without artificial heating. It is brewed in the cold season and allowed to mature over the summer, taking about one year to slowly mature in the climate of Shinshu. In order to maintain quality during the long ripening period, the salt content is kept high using a traditional method. The degree of maturity is determined by the coloring of the miso, making this a time-consuming and labor-intensive process that requires a great deal of skill and sensitivity on the part of the technicians.