Water activity

Water activity measures how available water is in a material for chemical reactions and microbial growth. It is defined as the ratio of the material’s water vapor pressure to that of pure water at the same temperature.

Water potential: Water potential is the potential energy of water relative to a reference state, governing its movement between regions. Both measures describe water availability, but water potential also captures osmotic and pressure effects.
Food preservation: Food preservation comprises methods that slow spoilage and maintain food safety and quality. Controlling water activity is one way to limit microbial growth and chemical deterioration.
Chemical potential: Chemical potential is the change in a system’s free energy when the amount of a chemical component changes. Water activity expresses water’s chemical potential relative to pure water.
Raisin: A raisin is a dried grape with concentrated sugars and reduced water content. Its sugars and drying lower water activity, helping explain its shelf stability despite retained moisture.
Water vapor pressure: Water vapor pressure is the pressure exerted by water molecules in the gas phase above a surface. Its ratio to pure water’s vapor pressure defines water activity.
Minimum water activity for microbial growth: The minimum water activity for microbial growth is the lowest value at which a microorganism can grow under specified conditions. This threshold helps assess whether a food’s water activity permits growth by particular microbes.
Water molecule: A water molecule consists of two hydrogen atoms bonded to one oxygen atom and has a polar structure. Water’s polarity allows it to interact strongly with solutes and material surfaces, changing its availability.
Honey: Honey is a concentrated sugar solution produced by honeybees from floral nectar. Its high sugar concentration creates low water activity that inhibits most microbial growth.
Sorption isotherm: A sorption isotherm relates a material’s equilibrium moisture content to relative humidity at a fixed temperature. It shows how much moisture a material holds at a given water activity.
Hurdle technology: Hurdle technology combines preservation methods to prevent microbial growth while maintaining product quality. Water-activity control can work alongside acidity, heat, or preservatives as a preservation hurdle.

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