Water Activity Meter (aw meter)
Water activity meter
Japanese: aw計（水分活性測定器）
Dedicated instrument measuring water activity. An indicator predicting microbial growth potential.
An aw meter measures water activity in food. Water activity indicates the proportion of water available to microorganisms, expressed from 0 to 1.0. Miso aw is typically 0.80-0.85.
Lower water activity makes microbial growth more difficult. Salt and sugar lower aw by binding water molecules, making them unavailable to microbes. The mechanism by which miso salt enhances preservation is precisely this aw control. The aw number visualizes the relationship between salt and microorganisms.
Chilled mirror dew point method is recognized as a high-precision aw measurement technique. For predicting food preservation, water activity provides a more accurate indicator than moisture content. Miso's typical aw of around 0.80 creates an environment inhibiting most pathogenic bacterial growth.