Igaya Syuzou, founded in 1853 in Nakano City, Nagano Prefecture, produces Iwashimizu through the deep cold of the Shinshu winter, where temperatures drop below minus ten degrees Celsius and snowpack exceeds a meter, enabling natural low-temperature fermentation. The brewery uses a notably high koji ratio of roughly fifty percent, rather than the conventional twenty percent, generating its distinctive balance of sweet and sour. Limited distribution keeps Iwashimizu largely confined to a select network of specialty retailers nationwide.
