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Water activity or aw is the Energy State of Water in a Substance . It is defined as the Vapor Pressure of water divided by that of pure water at the same Temperature ; therefore, pure Distilled Water has a water activity of exactly one. As the temperature increases, aw typically increases, except in some product with crystalline Salt or Sugar . Higher aw substances tend to support more Microorganisms . Bacteria usually require at least 0.91, and Fungi at least 0.7. See Fermentation . Water migrates from areas of high aw to areas of low aw. For example, if Honey (aw ā 0.6) is exposed to humid air (aw ā 0.7) the honey will absorb water from the Air . FORMULAE Definition of aw: : where p is the vapor pressure of water in the substance, and pā is the vapor pressure of pure water at the same temperature. Alternate definition: : where lw is the Activity Coefficient of water and xw is the mole fraction of water in the aqueous fraction. Relative Humidity : : Estimated mold-free shelf life in days at 21° C: : USES FOR WATER ACTIVITY Water activity is an important consideration for food product design and food safety. Food Product Design Food designers use water activity to formulate products that are Shelf Stable . If a product is kept below a certain water activity, then mold growth is inhibited. This results in a longer shelf-life. Water activity values can also help limit Moisture Migration within a food product made with different Ingredients . If raisins of a higher water activity are packaged with bran flakes of a lower water activity, the water from the raisins will migrate to the bran flakes over time, resulting in hard raisins and soggy bran flakes. Food formulators use water activity to predict how much moisture migration will affect their product. Food Safety Water activity is used in many cases as a Critical Control Point for Hazard Analysis And Critical Control Points ( HACCP ) programs. Samples of the food product are periodically taken from the production area and tested to ensure that water activity values are within a specified range for food quality and safety. Measurements can be made in as little as five minutes, and are made regularly in most major food production facilities. For many years researchers tried to equate bacterial growth potential with Moisture Content . They found that the values were not universal, but specific to each food product. WJ Scott in 1953 first established that it was water activity, not Water Content that correlated with Bacterial Growth . It is firmly established that growth of bacteria is inhibited at specific water activity values. FDA regulations for Intermediate Moisture Foods are based on these values. Lowering the water activity of a food product should not be seen as a Kill Step . Studies in Powdered Milk show that viable cells can exist at much lower water activity values but that they will never grow. Over time bacterial levels will decline. WATER ACTIVITY MEASUREMENT Water activity values are obtained by either a Capacitance or a dew point Hygrometer . Capacitance Hygrometers Capacitance hygrometers consist of two charged plates separated by a Polymer Membrane Dielectric . As the membrane adsorbs water, its ability to hold a Charge increases and the capacitance is measured. This value is roughly proportional to the water activity as determined by a sensor-specific Calibration . Capacitance hygrometers are not affected by most Volatile chemicals and can be much smaller than other alternative sensors. They do not require cleaning, but are less accurate than Dew Point Hygrometers (+/- .015 aw). They require regular calibration and can be affected by residual water in the polymer membrane. Dew Point Hygrometers The temperature at which Dew forms on a clean surface is directly related to the Vapor Pressure of the air. Dew point hygrometers work by placing a mirror over a closed sample chamber. The mirror is cooled until the dew point temperature is measured by means of an Optical Sensor . This temperature is then used to find the Relative Humidity of the chamber using Psychometric charts. This method is the most accurate (+/- .003 aw) and often the fastest. The sensor requires cleaning if debris accumulates on the mirror. The method can also be sensitive to compounds such as Propylene Glycol and Ethanol which can co-condense with water on the mirror resulting in an inaccurate dew-point determination. These compounds are not common and are not usually present at sufficient amounts to pose a problem. Equilibration With either method, vapor Equilibrium must occur in the sample chamber. This will take place over time or can be aided by the addition of a fan in the chamber. Thermal Equilibrium must also take place unless the sample temperature is measured. WATER ACTIVITY AND MOISTURE CONTENT Water activity is related to Moisture Content in a Non-linear relationship known as a moisture sorption isotherm curve. These isotherms are substance and temperature specific. Isotherms can be used to help predict product stability over time in different storage conditions. SELECTED A<SUB>W</SUB> VALUES Example Foods aw Values of Microorganism Inhibition REFERENCE
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