vandaktivitet

Vandaktivitet, or water activity (aw), is a measure of the availability of water in a product for microbial growth and chemical reactions. It is defined as the ratio between the vapor pressure of water in the product and the vapor pressure of pure water at the same temperature. Unlike moisture content, which is an amount of water present, vandaktivitet reflects how much water is free to participate in reactions.

Water activity is a key determinant of microbial stability. Most bacteria require an aw above about 0.90 to grow, while molds can grow at lower levels, often around 0.65–0.80, and yeasts typically tolerate roughly 0.60–0.85. Because of this, products are often processed or formulated to maintain a low aw in order to extend shelf life and reduce spoilage, using drying, humectants, or packaging strategies.

Measurement of vandaktivitet is performed with specialized instruments called water activity meters. These devices may use chilled-mirror dew point techniques or capacitance/hygrometric sensors. Measurements are typically performed at a controlled temperature, as aw is temperature dependent. Calibration against known standards is common to ensure accuracy.

Factors affecting vandaktivitet include composition (salts, sugars, polyols, and other humectants bind free water), temperature, and interactions with food constituents such as fats and proteins. Practical applications span dried foods, jams, fermented products, and beverages, where controlling aw helps inhibit microbial growth and slow enzymatic and chemical reactions. It is important to distinguish aw from water content, since two products with similar moisture can have very different vandaktivitet and stability.