At the equilibrium point, water is neither given off nor absorbed. The amount of vapor in the surrounding air generally is measured as relative humidity. The vapor pressure of a salt or sugar solution is reduced in comparison to that of pure water.
BACTERIAL ACTIVITY SYNONYM FREE
Products containing free water give off moisture in vapor form to the air in the environment, only when the vapor pressure in the air is below that of the product.
A number of saturated salt solutions are available for this purpose. After equilibrium is reached, the moisture content is determined gravimetrically or by other methods. Relative humidity sensors of great variety are available for this purpose, including electric hygrometers, dewpoint cells, psychrometers, and others.Ī second basic method for preparing isotherms is the exposure of a small sample of food to various constant humidity atmospheres. Water activity is equal to equilibrium relative humidity divided by 100: (a w = ERH/100) where ERH is the equilibrium relative humidity (%). In the first method, food of known moisture content is allowed to come to equilibrium with a small headspace in a tight enclosure and partial pressure of water activity is measured manometrically, or relative humidity is measured using a hyqrometer. Two basic methods can be used to obtain the constant temperature sorption curves. The same practice is followed to study curves relating water activity under equilibrium conditions to water content. The most successful method for studying the sorption properties of water in food products has been the preparation of "Sorption Isotherms," or curves relating the partial pressure of water in the food to its water content at constant temperature. The binding action is referred to as the sorption behavior of the food. These sites include the hydroxyl groups of polysaccharides, the carbonyl and amino groups of proteins, and others on which water can be held by hydrogen bonding, by ion-dipole bonds, or by other strong interactions. A portion of the total water content present in food is strongly bound to specific sites and does not act as a solvent. The materials, therefore, must be in soluble form to permeate the cell wall. The bacterial cell can only transfer nutrients in and waste materials out through the cell wall. If the water activity of food is controlled to 0.85 or less in the finished product, it is not subject to the regulations of 21 CFR Parts 108, 113, and 114. The amount of available moisture can be reduced to a point which will inhibit the growth of the organisms.
Most foods have a water activity above 0.95 and that will provide sufficient moisture to support the growth of bacteria, yeasts, and mold. The moisture condition of a product can be measured as the equilibrium relative humidity (ERH) expressed in percentage or as the water activity expressed as a decimal. The water activity increases with temperature. A water activity of 0.80 means the vapor pressure is 80 percent of that of pure water. The water activity (a w) of a food is the ratio between the vapor pressure of the food itself, when in a completely undisturbed balance with the surrounding air media, and the vapor pressure of distilled water under identical conditions. ITG SUBJECT: WATER ACTIVITY (a w) IN FOODS
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