A solid substrate fermentation process involving the fermentation of grain and soybeans by the Aspergillus fungus. The cooked substrate is inoculated with a culture of the filamentous fungi and grown as a shallow layer in which amylase and protease enzymes break down the substrate to form koji, which is a proteolytic enzyme used for treating flour proteins. It is also the basis of other similar fermentations such as the production of citric acid, and rice fermentations including the rice wine saké.