Water activity (Aw)
Japanese: 水分活性（Aw）
An indicator of the proportion of free water available to microorganisms in food. Expressed from 0 to 1; the lower the value, the harder it is for microbes to grow.
Water activity (Aw) represents the proportion of 'free water'—water actually available to microorganisms—in food. Pure water has Aw of 1.0; water bound by salt or sugar is unavailable.
Most bacteria cannot grow below Aw 0.90; even molds bottom out around 0.65. Miso's Aw is about 0.80—a sweet spot where most bacteria cannot grow, but salt-tolerant yeasts and certain lactic acid bacteria remain active.
Adding salt lowers Aw; adding sugar does too. Jam's resistance to spoilage and the longevity of salt-preserved foods both stem from reduced water activity. Understanding water activity, rather than moisture content, is the key to reading the preservation of fermented foods. A domain where scientific perspective and practical knowledge intersect. Expert understanding becomes the key to expanding fermentation's possibilities.