Koji

Koji is the common name for the culture of the filamentous fungus Aspergillus oryzae used in East Asian fermentation. Practically, koji also refers to the grains on which the mold has been grown, most often steamed rice, but also barley or soybeans. The mold provides enzymes that pre-digest starches and proteins, enabling efficient fermentation.

Preparation involves sterilized grains being inoculated with koji-kin (spores) and incubated at warm, humid conditions for about 24 to 48 hours. During growth, the mold secretes enzymes, especially amylases and proteases. Amylases convert starches to sugars; proteases release amino acids. This enzymatic breakdown allows subsequent fermentation by yeasts or bacteria and develops the flavors of products such as sake, miso, soy sauce, and mirin.

Variations in koji arise from different substrates and strains, leading to color differences and distinct enzyme profiles. White koji, yellow koji, and other forms exist, reflecting substrate choice and microbial strains. Koji is a foundational component of traditional Japanese fermentation and has been adopted in diverse modern applications worldwide.

Safety and regulation: Strains used for koji are selected for non-toxigenicity and safety. When produced under controlled conditions and with good manufacturing practices, koji-based fermentation is considered safe; improper handling or contaminated cultures can pose food-safety risks.