Is the koji mold behind soy sauce a tamed cousin of a toxic mold?

Possibly, say some scholars, and the evidence is in the mold's DNA. The koji mold, Aspergillus oryzae, is the fungus that cooks in East Asia use to turn rice into sugar for sake and soybeans into soy sauce and miso. Genomic analysis has led some scholars to believe that it is a domesticated form of Aspergillus flavus, a close relative, that had mutated and stopped making toxic aflatoxins. Aflatoxin B1, made by strains of A. flavus, is the most toxic and potent hepatocarcinogenic natural compound characterized, a liver poison.

The two fungi share the same cluster of genes for making aflatoxin, but in A. oryzae that cluster does not work. The koji mold has not been found to produce aflatoxins, and in one study its aflatoxin genes were not expressed even under conditions that favour toxin production. So generations of brewers, without knowing any genetics, may have kept the safe version going.

Why would they want it? Because of what the mold does to food. Its enzymes break starches into simple sugars and proteins into amino acids, among them glutamic acid, which contributes to the savoury taste called umami. Soy sauce, which took its current form about 2,200 years ago in the Western Han dynasty of ancient China, is built on this. Maillard reaction products, made from the amino acids and sugars the mold freed, are part of what makes soy sauce taste the way it does.

In Japan the fungus is so valued that a scholar called it a "national fungus", and the Brewing Society of Japan approved the name in 2006. Who first used it, China or Japan, is still debated between two scholarly theories.