Water Activity is the amount of unbound water in a sample. Water that is not bound to the ingredients themselves can be used by unwanted microorganisms which could lead to one of the contributing factors for food spoilage. Water activity is a thermodynamic measure of water expressed as the vapor pressure of water in a sample divided by vapor pressure of pure water at a given temperature.
The more unbound water we have the more likelihood we have for microbial spoilage. Water Activity (aw) Meters measure the unbound water vapor pressures to determine microbial spoilage, chemical and physical stability.
Pure water that is unencumbered has more energy and therefore can be available and used to do bad things by microorganisms. As water comes in contact with a material substance the water's energy and water activity (aw) decrease. Water reacts differently with different material substances and can have a higher or lower water activity rating* (aw). A water activity (aw) number closer to the top of the chart closer to 1.00 has a higher water activity while those lower have less.
Water Activity in food
The water activity (a w) of a food is the ratio between the vapor pressure of the food itself, when in a completely undisturbed balance with the surrounding air media, and the vapor pressure of distilled water under identical conditions. A water activity of 0.80 means the vapor pressure is 80 percent of that of pure water. The water activity increases with temperature. The moisture condition of a product can be measured as the equilibrium relative humidity (ERH) expressed in percentage or as the water activity expressed as a decimal.
Total water activity is more closely related to the chemical, physical and biological properties of foods than their moisture content. Water activity in food affects changes in color, structure and stability.
All foods contain a certain amount of water. Most of the chemical and biological changes in food are due to high water content. especially before or after food processing (storage, transportation)
Most foods have a water activity above 0.95 and that will provide sufficient moisture to support the growth of bacteria, yeasts, and mold. The amount of available moisture can be reduced to a point which will inhibit the growth of the organisms.
