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 the potential for growth and metabolic activity. Despite some drawbacks (e.g., solute effect), the concept of a(w) has assisted food scientists 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 decade the concept of a(w) has been challenged. It has been suggested that reduced-moisture food products (e.g., low and intermediate) may be nonequilibrium systems and that most of them are in the amorphous metastable state, which is very sensitive to changes in moisture content and temperature. It has been proposed that the glass transition temperature Tg (temperature at which the glass-rubber transition occurs), is a parameter that can determine many product properties, the safety of foods among them. The concept of water dynamics, originating in a food polymer science approach, has been suggested instead of a(w) to better predict the microbial stability of intermediate-moisture foods. The usage of a(w) to predict microbial safety of foods has been discouraged on the basis that (1) in intermediate-moisture foods the measured water vapor pressure is not an equilibrium one, and because a(w) is a thermodynamic concept, it refers only to equilibrium; and (2) the microbial response may differ at a particular a(w) when the latter is obtained with different solutes. This review analyzes these suggestions on the 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 the time frame (food's shelf life) of the experiments and/or so small that 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 science polymer approach to understanding the behavior of aqueous sugar glasses and concentrated solutions may be used to predict the microbial stability of food systems is not substantiated by experimental evidence. This approach does not offer, at the present time, a better alternative to the concept of a(w) as a predictor of microbial growth in foods. It is also recognized that a(w) has several limitations and should be always used carefully, and this must include precautions regarding the possible influences of nonequilibrium situations. This aspect may be summarized by simply saying that anyone who is going to employ the term water activity must be aware of the implications of its definition.

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