WATER ACTIVITY, WATER GLASS DYNAMICS, AND THE CONTROL OF MICROBIOLOGICAL-GROWTH IN FOODS

Water is probably the single most important factor governing microbial spoilage in foods, and the concept of water activity (a(w)) has been very valuable because measured values generally correlate well with th e potential for growth and metabolic activity. Despite some drawbacks (e.g., solute effect), the concept of a(w) has assisted food scientist s in their effort to predict the onset of food spoilage as well as to control food-borne disease hazards in food products. In the last decad e the concept of a(w) has been challenged. It has been suggested that reduced-moisture food products (e.g., low and intermediate) may be non equilibrium systems and that most of them are in the amorphous metasta ble state, which is very sensitive to changes in moisture content and temperature. It has been proposed that the glass transition temperatur e T-g (temperature at which the glass-rubber transition occurs), is a parameter that can determine many product properties, the safety of fo ods among them. The concept of water dynamics, originating in a food p olymer science approach, has been suggested instead of a(w) to better predict the microbial stability of intermediate-moisture foods. The us age of a(w) to predict microbial safety of foods has been discouraged on the basis that (1) in intermediate-moisture foods the measured wate r vapor pressure is not an equilibrium one, and because a(w) is a ther modynamic concept, it refers only to equilibrium; and (2) the microbia l response may differ at a particular a(w), when the latter is obtaine d with different solutes. This review analyzes these suggestions on th e basis of abundant experimental evidence found in the literature. It is concluded that nonequilibrium effects (e.g., inability of water to diffuse in a semimoist food) appear to be in many cases slow within th e time frame (food's shelf life) of the experiments and/or so small th at they do not affect seriously the application of the a(w) concept as a predictor of microbial stability in foods. The claims that a food s cience polymer approach to understanding the behavior of aqueous sugar glasses and concentrated solutions may be used to predict the microbi al stability of food systems is not substantiated by experimental evid ence. This approach does not offer, at the present time, a better alte rnative to the concept of a(w) as a predictor of microbial growth in f oods. It is also recognized that a(w) has several limitations and shou ld be always used carefully, and this must include precautions regardi ng the possible influences of nonequilibrium situations. This aspect m ay be summarized by simply saying that anyone who is going to employ t he term water activity must be aware of the implications of its defini tion.