Water
Japanese: 水（みず）
The medium binding ingredients and supporting microbial activity.
In miso making, water connects ingredients and provides the environment for microbial activity. Soaking, steaming, mixing — water is involved at every step.
Hardness, pH, and minerals can affect fermentation and flavor. In sake, there is a saying about 'choosing your water.' For home miso, clean tap water works well; filtering for chlorine is advisable. Mind the water temperature at mixing. Too cold slows enzyme activity; too warm encourages unwanted bacteria. Around 20 degrees Celsius is ideal.