The best koji requires perfectly-steamed rice, with a hard outer and soft inner part of each grain. The “Koshiki” steamer is designed to facilitate this by first producing a “dry” steam to harden the outside of the rice grains.
Thirty years ago, Kanzawagawa Sake Brewery was in the position to purchase a new boiler to feed steam to the koshiki. The modern boilers at that time boiled water in pipes warmed by high pressure steam, boiling water quicker and taking up less space. These features make these boilers the more common choice for sake breweries even today. Despite the apparent inconveniences, Kanzawagawa Sake Brewery elected to purchase the less convenient Wagama boiler.
In contrast to the modern boilers, the Wagama is heated directly by fire and needs one hour to be boiled. However that one-hour process is valuable for koji production. In winter time, the modern boiler heats the inside of the koshiki very rapidly, while the contents (the rice) are still cold. The hot steam goes through the koshiki and is cooled by the cold rice. As a result, the steam often condenses back to water, making “koshiki-hada”(mushy rice). Koshiki-hada takes much longer to turn into the “perfect” rice with a hard outer but soft inner, and the quality of each batch is not always stable. However in the hour that the Wagama takes to reach boiling point, the rice and the air inside the koshiki are also warmed and koshiki-hada is rare.
Kanzawagawa Sake Brewery does not rely on the significant benefits of the Wagama and instead pursues the ideal steamed rice. Their ideal has the rice softened entirely and intensely dried on the outside of each grain. This approach and attention to detail highlights the brewery’s pursuit of quality.
Usually in sake production, rice absorbs water and gains additional weight of over 30% during steaming, and an additional 10% (of the original) after steaming, a gross increase of 40%. Sake breweries generally monitor this water absorption closely and a variance of even 0.5% would not be tolerated. However Kanzawagawa Sake Brewery’s method involves the rice absorbing water to a much higher 38% during steaming, finally reaching a 48% increase of weight . This gap of almost 10% (well beyond the very narrow band of acceptability of most breweries) is extremely surprising. This extremely wet steamed rice then takes 4 hours to dry into the “hard outer and soft inner” ideal. This has become known as the Kanzawagawa style of steaming.