Marukin's natural soy brewery has been designated as a Tangible Cultural Property of Japan.
The Shodo Island brewery, one of Japan's largest natural breweries, includes a natural fermentation cellar dating back to the early 1900s. The soy sauce is fermented in century-old wooden barrels through a balanced combination of resident bacteria and ambient yeasts that live naturally throughout the brewery. Black bacteria that are optimum for soy sauce immediately dies off when soy sauce brewing is halted, so black walls at soy sauce manufacturers are regarded as proof of prosperous business.