Water activity and microorganism control: Past and future

Abstract

The influence of water or moisture content of foods on shelf-life has been recognized since early ages; most primitive cultures found a convenient way to reduce moisture content in foods to a level that prevents or delays microbial spoilage, such as drying, salting, and adding sugars. In some cases, the spoilage of products subjected to these procedures has also been referred to in the following ways: halophilic bacteria can grow in salted products; osmophilic yeasts may ferment sugar in preserved foods; and some dried foods can be spoiled by xerophilic molds (Mossel 1975). Substantial interest in the influence of water activity (aw) on food product quality and stability, promoted by empirical observations between total moisture content and product stability, began during the middle of the last century. Scott (1957) introduced the concept of aw as a quantitative approach to define the influence of moisture content on microbial response in foods. Microbiologists recognized that aw, rather than moisture content, controlled microbial response, as well as sporulation and/or toxin production (Jay et al. 2005). The relationship between aw and food-borne microorganisms has been the topic of study by food researchers over the past several decades (Christian 2000; Chirife 1995; Lenovich 1987). Microbiologists have investigated how microorganisms respond under different conditions of temperature, pH, additives, atmosphere composition, and aw (Hocking and Pitt 1987; Beuchat 1996). The influence of aw in microbial death, survival, sporulation, and toxin production in food has been extensively studied by food microbiologists (Lenovich 1987; Beuchat 1983, 1987; Hocking and Christian 1996; Gutierrez et al. 1995). The aw principle has been included in various government regulations, with the recognition that control of aw as an important critical control point for risk analysis, as defined by the HACCP concept, sets up aw limits on food products.