Steamed soybeans are wrapped in rice straw, held at around forty degrees, and left for a day. The bacteria living on the straw move into the beans and fermentation begins. For most of its history that was the entire method, and no one had to be told what the organism was called.

Mito's beans are small. The domain encouraged an early-ripening soybean that could be harvested before the typhoons, and it turned out to be too small to be much use for tofu or miso. What was left over went into straw. A great many local specialities begin exactly here, with the crop that nobody else had a use for and somebody refused to waste.

The reputation came later, after the railway arrived in the Meiji years and a merchant began selling the wrapped bundles at the station as a souvenir. Modern production holds the beans near forty degrees for six hours, lets the fermentation heat carry the temperature up on its own, and stops the process with cold air after about a day.

Ibaraki produces the most of it. Fukushima, next door, eats the most. The place a food is famous for and the place it is actually eaten are more often different than you would think, and nobody in either prefecture seems troubled by it.