A CRITICAL-REVIEW OF SOME NONEQUILIBRIUM SITUATIONS AND GLASS TRANSITIONS ON WATER ACTIVITY VALUES OF FOODS IN THE MICROBIOLOGICAL-GROWTH RANGE

Some facts concerning the effect of non-equilibrium situations and gla ss transitions on the water activity (A(w)) values of foods in the mic robiological growth range are reviewed. Analysis of literature data su ggests that although some amorphous intermediate moisture foods are no n-equilibrium systems (e.g. very slow rate of attainment of equilibriu m), the 'stability' (time dependency) of measured water sorption isoth erms, or the prediction of water activity from equilibrium data corres ponding to aqueous solutions of main food solutes (NaCl, sugars), are little affected. However, crystallization of amorphous components (sug ars), which is also a consequence of non-equilibrium situations, may l ead to significant modifications of A(w) values in foods. As far as th e prediction of microbial growth in foods is concerned, changes in non -equilibrium semi-moist foods may be in many situations slow (within t he time frame of the food shelf-life) and/or so small that they do not seriously affect the application of A(w) concept as a predictor of mi crobial growth. However, the use of A(w) should always consider the po ssible influence of some non-equilibrium situations.