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  

Acidification of food products

The United States Food and Drug Administration (FDA) has defined an “acidified food” as a low-acid food to which acid(s) or acid food(s) has been added to the product to produce a finished equilibrium pH of 4.6 or below and a water activity greater than 0.85.

In acidified foods, the addition of low-acid ingredients significantly alters the pH of the acid food ingredients in the product. The acidified food regulations apply to shelf-stable foods that are sold without refrigeration in sealed containers.

Certain dairy products, such as sour cream and fermented vegetables, such as sauerkraut are preserved through lactic acid produced by the growth of bacteria together with the holding of these products refrigerator temperatures above freezing.

When sauerkraut is canned, it is given a heat process sufficient to destroy all spoilage and disease microorganisms.

Acidified, low acid foods must be acidified to a pH of 4.6 or less. Preferably the pH should be less than 4.6 to allow a safety margin. Optimal flavor profiles for most products are reached at approximately pH 4.2. If the flavor is slightly acidic the flavor profile may be balanced with the addition of a small quantity of sugar.

The manufacturer of acidified foods must register their establishment and file a scheduled process established by a competent process authority. In addition, the operation must be under the supervision of an individual who has successfully completed an FDA approved course on processing acidified foods.
Acidification of food products