How was flour fermented in ancient times?

The fermentation method of ancient flour is as follows:

1. Put the dough into a container filled with flour and bury it with dry flour. Lactic acid bacteria in flour will naturally ferment. Leave some fermented flour when making steamed bread, and then use the flour left for the first time as yeast to make dough, leaving some at a time.

2. Fermentation with koji is a block made of Aspergillus and its culture medium, and the culture medium is a mixture of wheat, bran and soybean. The taste of koji is similar to that of distiller's grains, and koji is also used to make wine and soy sauce.