Aspergillus oryzae produces two keystone enzymes
Koji is the Aspergillus oryzae mold grown on steamed rice or grain. As it colonizes the grain, it secretes amylase (which breaks down starch into glucose) and protease (which breaks down proteins into amino acids). These two enzymes are the foundation for every major fermented soy and rice product: miso, soy sauce, sake, mirin, and koji seasoning all depend on amylase-protease action to unlock sugar and umami from their raw ingredients.
The mold's mycelium spreads through the grain, breaking down cell walls and making nutrients bioavailable. Simultaneously, the mold respires and produces heat, raising the temperature of the koji batch. This combination of enzyme activity and metabolic warmth is why koji makers carefully manage koji temperature (25-30C) and humidity to maximize enzyme production.
Umami compounds emerge from protein hydrolysis
Protease breaks proteins into free amino acids, especially glutamate and alanine. Glutamate is umami (savory flavor). The more thoroughly koji proteases work, the higher the free amino acid content and the more umami-forward the final product. Miso gains umami from both koji (via amylase-released sugars and protease-released amino acids) and from salt-tolerant bacteria that continue breaking down proteins during months of aging.