Miso is a fermented paste that’s made by inoculating a mixture of soybeans with a mold called koji (Aspergillus oryzae) that’s been cultivated from rice, barley, or soybeans. Over weeks (or even years!), the enzymes in the koji work together with the microorganisms in the environment to break down the structure of the beans and grains into amino acids, fatty acids, and simple sugars.
Source: Boneappetit.com