What is the principle of making soy sauce vinegar in Qi Yaomin's Book?

There are 12 koji-making methods in Qi Yaomin's Book, among which "yellow clothes" and "yellow steaming" are used to make sauce and vinegar. It's usually a fragment of Sanqu. The former uses whole wheat grains, while the latter uses wheat flour. After steaming, spread it on the mat foil and cover it with tender leaves until a layer of yellow mold grows. This kind of koji can secrete amylase and protease, which can not only saccharify starch, but also alcoholize starch. More importantly, it can hydrolyze protein in beans into amino acids, which is the main reason why the sauce has a mellow and special flavor. When Qi Yao Min Shu was written, there was quite mature and scientific experience on the culture of Xanthomonas and how to give full play to the activity of the enzymes secreted by Xanthomonas.