Actividad De Agua

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DECAGON DEVICES

Fundamentals of Water Activity
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t is now generally accepted that aw is more closely related to the physical, chemical, and biological properties of foods and other natural products than is total moisture content. Specific changes in color, aroma, flavor, texture, stability, and acceptability of raw and processed food products have been associated with relatively narrowaw ranges.”
—Rockland LB & Nishi SK, Food Tech 34:42-59 (1980).

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CONTENTS
What is Water Activity? Page 2-3
n Definition n Measurement Methods

Water activity for product safety and quality Page 4-5
n Microbial Growth n Chemical/Biochemical Reactivity n Physical Properties n Moisture Migration

Government Compliance Page 6–7 Microbial Growth Limits Table Page 8

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What is WaterActivity?

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ater is recognized as being very important, if not critical, to the stability of most products. Controlling the water within a product, by some method of drying or by chemically/structurally binding (salting or sugaring) has long been used by man for preservation. This not only controls microbial spoilage, but also chemical and physical stability. Water Content Alone is Not aReliable Predictor Traditionally, discussions about water in products or ingredients focus on moisture or water content, which is a quantitative or volumetric analysis that determines the total amount of water present. Water content of a product is a familiar concept to most people. One measures the water content by loss on drying, infrared, NMR or Karl Fisher titration. Moisture contentdetermination is essential in meeting product nutritional labeling regulations, specifying recipes and monitoring processes. However, water content alone is not a reliable predictor of microbial responses and chemical reactions in materials. Chemically Bound Water is Unavailable to Microbes The limitations of water content measurement as an indicator of safety and quality are attributed to differences in theintensity which water associates with other components in the product. The water content of a safe product varies from product to product and from formulation to formulation. One safe, stable product might contain 15% water while another containing just 8% water is susceptible to microbial growth. Although the wetter product contains proportionally more water, its water is chemically bound byother components, making it unavailable to microbes. Using only water content values, it’s impossible to know how “available” the water in the product is to support microbial growth or influence product quality.

Water Activity is Most Relevant for Quality and Safety Issues Another more important type of water analysis is water activity (aw). Water activity describes the energy status or escapingtendency of the water in a sample. It indicates how tightly water is “bound,” structurally or chemically, in products. Both the water content and the water activity of a sample must be specified to fully describe its water status. However, water activity is the property most relevant for quality and safety issues. Water activity is closely related to the partial specific Gibbs free energy of thesystem. Thus, water activity is a thermodynamic concept and has requirements for measurements. These requirements are that the system be in equilibrium, the temperature defined, and a standard state specified. Pure water is taken as the reference or standard state from which the energy status of water in food systems is measured. The Gibbs free energy of free water is zero; thus, the water activityis 1.0. Water Activity is a Ratio of Vapor Pressures Water activity is the ratio of the vapor pressure of water in a material (p) to the vapor pressure of pure water (po) at the same temperature. Relative humidity of air is the ratio of the vapor pressure of air to its saturation vapor pressure. When vapor and temperature equilibrium are obtained, the water activity of the sample is equal to the...