Anaerobic conditions
Japanese: 嫌気的条件（けんきてきじょうけん）
An environment with no or very little oxygen. Alcoholic and lactic acid fermentation proceed under these conditions.
Anaerobic conditions mean an oxygen-free environment. Packing miso into a container, sealing with plastic wrap, placing a weight on top—this sequence is also a procedure for creating anaerobic conditions.
Alcoholic fermentation by yeast proceeds only anaerobically. With oxygen present, yeast switches to aerobic respiration, producing CO₂ and water instead of alcohol—a phenomenon called the Pasteur effect, marking the fork between fermentation and respiration.
Most lactic acid bacteria also prefer anaerobic conditions. The oxygen-poor interior of miso is precisely the habitat for lactic acid bacteria and salt-tolerant yeast. Excluding oxygen is a design act that guides desirable fermentation.