Water activity is a better index for microbial growth that water content. The closer a food products value is to 1.000, the more water that product has available to participate in various chemical and physical reactions. Water activity is measured in a range from 0.030 to 1.000. The weight of water desorbed from the food into the vapor phase (P), and its ratio to the vapor pressure of pure water (Po) at the same temperature, describing the waters escaping tendency of fugacity ratio. Water activity is defined as the ratio of the water vapor pressure of a food (P) to that of pure water (Po). This creates an imbalance in osmotic pressure which draws the water from cells. The second method involves tying up the free water by the addition of solutes, usually sugars or sodium chloride (salt). The simplest way to reduce water is with a process which drives off water, i.e. The amount of water removed from or added to a food depends on the nature and amount of water-soluble substances (water binding capacity) present in the product. Hence, texture, non-enzymatic browning reactions, enzymatic activity, lipid oxidation, and other aspects of foods may be influenced by manipulation of aw levels. The water activity of a food product describes the degree to which water is “bound” in the food, and its availability to participate in chemical/biochemical reactions and growth of microorganisms. Water activity’s usefulness a food quality and safety measurement was suggested when it was obvious water content (% or total moisture) could not adequately account for microbial growth fluctuations because the amount of water present often does not adequately characterize the capacity to find water or to limit its escaping tendency. Water activity, or a w, is a measure of the free moisture in a product and is defined as the ratio of vapor pressure of a substance to that of pure water at a specified temperature. Measurement of Water Activity, or Available Water
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