Water activity

Food safety
Terms
Foodborne illness Good manufacturing practice (GMP) Hazard analysis and critical control points (HACCP) Hazard analysis and risk-based preventive controls (HARPC) Critical control point
Critical factors
FAT TOM pH Water activity (aw)
Bacterial pathogens
Clostridium botulinum Escherichia coli Listeria Salmonella Vibrio cholerae Cronobacter spp
Viral pathogens
Enterovirus Hepatitis A Norovirus Rotavirus
Parasitic pathogens
Cryptosporidium Entamoeba histolytica Giardia Trichinella
v t e

In food science, water activity (a_w) of a food is the ratio of its vapor pressure to the vapor pressure of water at the same temperature, both taken at equilibrium.^[1] Pure water has a water activity of one. Put another way, a_w is the equilibrium relative humidity (ERH) expressed as a fraction instead of as a percentage. As temperature increases, a_w typically increases, except in some products with crystalline salt or sugar.

Water migrates from areas of high a_w to areas of low a_w. For example, if honey (a_w ≈ 0.6) is exposed to humid air (a_w ≈ 0.7), the honey absorbs water from the air. If salami (a_w ≈ 0.87) is exposed to dry air (a_w ≈ 0.5), the salami dries out, which could preserve it or spoil it. Lower a_w substances tend to support fewer microorganisms since these get desiccated by the water migration.

Water activity is not simply a function of water concentration in food. The water in food has a tendency to evaporate, but the water vapor in the surrounding air has a tendency to condense into the food. When the two tendencies are in balance— and the air and food are stable—the air's relative humidity (expressed as a fraction instead of as a percentage) is taken to be the water activity, a_w. Thus, water activity is the thermodynamic activity of water as solvent and the relative humidity of the surrounding air at equilibrium.