Sunday, November 17, 2019

Water activity: concept and definition

Water is the most abundant constituent of food and in terms of food safety the most significant. One of the oldest forms of preserving foods is drying or desiccation. The main purpose of drying foods is to lower their moisture content to a particular level that will exclude the growth of microorganisms (bacteria, mould and yeasts).

For any given moisture content, one food may support the growth of microorganisms will growth in a dried food depends on the water activity. The lower the water activity in food, the less probable that microorganism will grow.

Water is an important basic element in foods. For a long time, the industry has known how important it is to check free water. The water activity (aw) measurement forms the basis of this and provides important information about the quality of a product.

Water activity is a fundamental property of aqueous solutions, and by definition is the partial vapor pressure of water in a substance (p) divided by the standard state partial vapor pressure of water (Po):

                                                             aw = p/Po

Water activity is a measure of how efficiently the water present can take part in a chemical (physical) reaction. If half the water is so tightly bound to a protein molecule that it could not take part in a hydrolysis reaction the overall water activity would be reduced.

The free or available water in a food supports microbial growth, and participates in and supports chemical and enzymatic reactions and spoilage processes. It is this amount of free water which is called water activity, aw.

Water activity (aw) is defined as where p and Po are the partial pressures of water above the food and a pure solution under identical conditions respectively.

It 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 narrow aw ranges.
Water activity: concept and definition  

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