Water activity

Water activity is an indicator of the amount of free water contained in a food or beverage. It is based on the relative humidity at which a food or culture medium is in equilibrium with the surrounding air. Specifically, it is expressed as the water vapor pressure of the product divided by the maximum vapor pressure of pure water at the same temperature. Water activity takes a value between 0 and 1; the closer it is to 1, the more free water is present and the easier it is for microorganisms to use. In the production and storage of sake, water activity affects microbial growth and quality preservation, so appropriate management is important. For example, excessive water activity can encourage the growth of unwanted bacteria. Understanding water activity is therefore essential for maintaining the quality of sake.