Temperature
Japanese: 温度
The most fundamental environmental condition governing the speed and direction of fermentation. Each microorganism has its own optimal temperature range.
Temperature is the most fundamental parameter for controlling fermentation. Each microorganism has an optimal temperature at which it is most active.
Koji mold prefers 30–35°C, lactic acid bacteria range from 20–45°C depending on species, and most yeasts thrive at 25–30°C. Too cold and reactions slow; too hot and enzymes denature or microbes die.
The tradition of winter preparation (kan-jikomi) for miso uses low temperatures for slow fermentation, developing complex flavors. Managing temperature is essentially the act of designing a fermented food's taste.