Margaret Balfour, Managing Director of Integrity Compliance Solutions did a Linkedin post on Water Activity. She said, “water activity (Aw) is a common method to manage microbial growth in foods. Water activity is defined as the partial vapor pressure of water in a substance divided by the standard state partial vapor pressure of water. All microorganisms require water to live – at varying levels. By manipulating the Aw of a food, we can control the growth of the target pathogen. The Aw of distilled water is 1.0, raw meat 0.99, milk and juice 0.97, saturated NaCl (salt) solution 0.75, dried fruit 0.6, indoor air 0.5-0.7 and honey 0.5 – 0.7. The physical presence of water itself does not support growth unless the water is chemically available to the pathogen. By adding ingredients such as salt and sugar, we can manipulate the Aw and extend shelf life.”
She then asked a very provocative question. “The challenge is what to do when consumers don’t want any added sugar, salt, preservatives etc.!”
(Ref. Barbosa-Canovas App. D 2007).
Reduction of water activity in foods prevents the growth of vegetative microbial cells, germination of spores, and toxin production by molds and bacteria. A decrease in water activity increases the lag phase of microorganisms and decreases the growth rate. The water activity can also be reduced by using three basic methods, namely dehydration, crystallization, and addition of solutes. Inadequate hygienic conditions during drying, transport, and storage can cause contamination by molds, which can result in the formation of mycotoxins
Water activity Diagram
The main objective of drying is to decrease the water activity (aw) of various perishable materials to values <0.5, in order to enable their storage at ambient temperature. The importance of aW in controlling the shelf-life of foods by suppressing the growth of micro-organisms, by reducing the rates of chemical reactions, and by inhibiting enzymatic deterioration is well established. The respective relationships have been summarized by Labuza et al. (1970) in a diagram similar to that of Fig. 1.1. Water activity is more important to the stability of a food than the total amount of
water present, and it makes it possible to develop generalized rules or limits for the stability of foods. For most foods, the critical point below which no micro-organism can grow is in the 0.6–0.7 water activity range. A food product is most stable at its monolayer moisture content, which varies with the chemical composition and structure. Water activity is also useful for predicting the final moisture content at equilibrium versus drying conditions (temperature and relative humidity), and for the selection of ingredients and packaging method. However, limitations of the water activity concept have also been identified.
A water activity of 0.80 means the vapor pressure is 80 percent of that of pure water. The water activity increases with temperature. 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.
Most foods have a water activity above 0.95 and that will provide sufficient moisture to support the growth of bacteria, yeasts, and mould. The amount of available moisture can be reduced to a point that will inhibit the growth of the organisms. 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. (Water activities in foods)
Food Preservation by Reducing Water Activity
Water Activity (aw) in Foods; DEPT. OF HEALTH, EDUCATION, AND WELFARE PUBLIC HEALTH SERVICE FOOD AND DRUG ADMINISTRATION; Date: 4/16/84, Number: 39, Related Program Areas: Foods