Akazu was the red vinegar used in Edo-period sushi, made by fermenting sake lees.
Akazu, literally “red vinegar,” developed as a by-product of sake production. Sake lees are the solids left after fermented rice mash is pressed to extract sake. Fermenting those lees produced a dark, aromatic vinegar that gave sushi rice a reddish color and a deeper, savory flavor.
Edo-style sushi developed in the 1820s as a quick food in what is now Tokyo. Cooks used vinegar to shorten or eliminate the long fermentation process found in earlier forms of sushi. The rice was also more strongly salted than many modern versions, while sugar was not yet used in the same way.
Akazu is sometimes confused with su, the generic Japanese word for vinegar, or with ponzu, a citrus-seasoned sauce. Mirin is a sweet rice wine, not the vinegar used for this historical sushi style. Nakano Matazaemon, founder of Mizkan, is associated with the development of this red vinegar.