In 1910, when the current brewmaster Toshio Sumiya’s grandfather started the family shop, the production of mirin was a complex, exacting process requiring years of experience to master. After serving a long apprenticeship, Sumiya’s grandfather chose the perfect location to begin his own business, an area in central Japan known as Mikawa, where three great rivers flow into the Bay of Ise.

Now part of Aichi Prefecture, this region is known for its mild climate, high-quality rice, and excellent water. As a result of these ideal conditions, Sumiya’s grandfather was able to produce a mirin that was thick and rich beyond everyone’s expectations. It was named Mikawa Mirin, literally “three river mirin”, in honor of its birthplace.

The labor-intense fermentation methods still used by the Sumiya family are steeped in the history and culture of pre-industrial Japan. More than a process, the family’s work represents a way of life that, like Mikawa Mirin, is rare in the modern world.

At Sumiya Bunjiro Shoten, the year-long cycle begins in the fall with the making of koji, the 1,000-year-old fermentation catalyst, ubiquitous in Japanese cuisine, that kick-starts the fermentation of many important foods such as sake, rice vinegar, miso, shoyu, and tamari. The making of koji begins with one thousand pounds of locally grown rice, polished to remove the oily outer bran (preventing an off-taste), then soaked overnight in spring water. The following morning, the rice is steamed and then cooled until it is warm to the touch. Next, the producers sprinkle Aspergillus mold spores over the rice, mixing them in carefully so that each rice grain comes into contact with a microscopic spore. Finally, the warm inoculated rice is hurriedly carried to a uniquely constructed chamber called a muro. This traditional koji incubation room has three-foot-thick cedar-lined walls that are insulated with rice hulls.

Through the night, in the warm, humid conditions of the muro, the spores of the starter culture germinate, extending enzyme-laden filaments into each individual grain of rice. These filaments break down the complex carbohydrates, transforming them into sweet sugars. By morning, the 1,000-pound mound of rice is fused together into a dense, damp mass. Using their hands and wooden shovels, the Sumiyas work through the morning breaking up the huge mound of rice into individual grains. They work in temperatures of over 100°F with 100% humidity. While most visitors cannot stand the stifling air of the muro for more than a few minutes, the Sumiyas, after decades of acclimatization, work at a relaxed pace, stopping briefly to gossip or to wipe the perspiration from their faces. After lunch, the rice is placed in dozens of wooden trays and left to ferment for a second night.

Through the night, the Sumiyas visit the muro often to check on the developing koji and to regulate the muro temperature by opening or closing the windows, which are located in the ceiling. After decades of making koji, Brewmaster Sumiya notes, “It’s a world of mystery, which is better left to intuition than to modern technology.” Early the next morning, he enters the warm, misty muro to taste the fluffy-white, glistening koji. With a confident nod, he signals to indicate that the next phase of production can begin.

Although most mirin manufacturers, past and present, buy inexpensive shochu that has been distilled from molasses, the Sumiya family prepares its own from hand-made koji, premium rice, and spring water. These ingredients are mixed together and stirred each day for about a month. The resulting alcoholic mash, called sake moromi, is placed into cotton sacks, pressed, filtered, and distilled into clear rice shochu. This completes the first phase of authentic mirin processing.

Next, 2,000 pounds of sweet glutinous rice are soaked and steamed. Stripped to the waist, Sumiya’s youngest brother mounts a platform beside the rice steamer. Here he begins the backbreaking, hours-long task of shoveling the cooked sweet rice onto a cooling table. Before the day is over, he will repeat this process twice more, shoveling a total of three tons of rice. The cooked rice is then added to the shochu together with more koji. This second mash, called mirin moromi, is placed in 1,000-gallon enamel vats that are insulated with rice-straw mats. Apart from some occasional stirring, the mirin moromi is left alone to ferment for about three months. Gradually, the koji enzymes break down the complex carbohydrates and protein of the glutinous rice into sweet simple sugars and amino acids that blend with the shochu to form a delicious alcoholic rice pudding that, unfortunately, only traditional mirin manufacturers ever get to sample.

Standing over the huge vats, Sumiya sniffs the sweet rising vapors to judge the progress of the developing mash. A quick taste confirms what his nose has already discovered: It’s time to pump the mash into cotton sacks and press out its sweet essence. (The remaining flavorful pressed moromi is used to make delicious mirin pickles.) Finally, this sweet essence (immature mirin) is returned to the enamel vats and left to age for about 200 days. During the hot days of the long Aichi summer, the subtle color and flavor of the mirin develops further. In the fall, Sumiya and his brothers eagerly sample their 70,000-gallon golden harvest and confirm that grandfather’s recipe, unchanged after almost eighty years, yields mirin that is as delicious as ever. The mature mirin is then filtered through cotton and bottled, unpasteurized, for shipment to customers around the world. Continuing his grandfather’s commitment to quality while adapting to modern times, Sumiya now offers Organic Mikawa Mirin.

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