By Keiko Kuroshima, the soy sauce sommelier.
Soy sauce brewing starts with koji — and koji starts with tane-koji. Steamed soybeans and roasted wheat are combined and inoculated with tane-koji, a seed culture that allows koji mold to propagate across the grain mixture. Tane-koji is, quite literally, the seed from which koji grows.
The full koikuchi production process, with tane-koji highlighted at the koji cultivation stage — the first decision that shapes everything that follows.
The mold at the heart of this process has shaped Japanese fermentation culture for centuries. It’s used in sake, miso, and countless other fermented foods — a contribution significant enough that the Brewing Society of Japan designated it the country’s “national mold” (kokkin) in 2006. The most widely used species in Japan is Aspergillus oryzae. In soy sauce production, Aspergillus sojae is also used, and plays a specific role in developing the flavors distinctive to shoyu.
Within these two species, there is remarkable variety. Some strains excel at breaking down starches into sugars. Others are more efficient at converting proteins into amino acids. Still others differ in how quickly they propagate. Each strain brings a different set of characteristics to the brewing process.
Four koji types used across Japanese fermented foods: white koji (shochu), soybean koji, rice koji, and barley koji. Each substrate produces a distinct enzyme profile.
Some large manufacturers develop and cultivate their own proprietary strains in-house. Most soy sauce breweries, however, source their tane-koji from specialist suppliers known as moyashi-ya. These suppliers maintain collections of hundreds — sometimes thousands — of koji strains, and work with breweries to match the right strain to the flavor profile they’re aiming for.
Tane-koji is nothing more than a fine powder. But within it lies the capacity to produce the umami, sweetness, and aroma of soy sauce. The choice of which tane-koji to use is the very first decision in the brewing process — and it determines the direction of everything that follows. Tane-koji is the blueprint of soy sauce.
