Substrate
Japanese: 基質
The substance upon which an enzyme acts. Protein is the substrate for protease; starch for amylase.
A substrate is the substance an enzyme acts upon. The enzyme-substrate relationship is often compared to a lock and key—a specific enzyme acts only on a specific substrate, a property called substrate specificity.
Protease's substrate is protein, amylase's is starch, lipase's is lipid. Koji mold secretes all three simultaneously, enabling parallel breakdown of multiple substrates. Miso's complex flavor results from multiple enzymes acting on multiple substrates at once.
The type and amount of substrate profoundly influence fermentation outcomes. More soybeans mean more protein substrate and stronger umami; more rice means more starch substrate and greater sweetness. Recipe design is essentially substrate balance design.