ELSEVIER zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA JournalofFoodEngineering25 (1995) 531-552 Copyright Q 1995 Elsevier Science Limited Printed in Great Britain. All rights reserved 0260-X774/95/$9.50 0260-8774(94)00033-6 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPO A Critical Review of Some Non-equilibrium Situations and Glass Transitions on Water Activity Values of Foods in the Microbiological Growth Range zyxwvutsrqponmlkj Jorge Chirife & M aria P. Buera* Departamento de Industrias, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, 1428 Buenos Aires, Argentina (Received 24 November 1993; revised version received 20 April 1994: accepted 18 June 1994) ABSTRACT Some facts concerning the effect of non-equilibrium situations and glass transitions on the water activity (A3 values of foods in the microbio- logical growth range are reviewed. Analysis of literature data suggests that although some amorphous intermediate moisture foods are non-equili- brium systems (e.g. very slow rate of attainment of equilibrium), the ‘stability ’ (time dependency ) of measured water sorption isotherms, or the prediction of water activity from equilibrium data corresponding to aqueous solutions of main food solutes (NaCl, sugars), are little affected. However, crystallization of amorphous components (sugars), w hich is also a consequence of non-equilibrium situations, may lead to significant modifications of A, values in foods. As far as the prediction of microbial growth in foods is concerned, changes in non- equilibrium semi-moist foods may be in many situations slow (within the timeframe of the food shelf-life) and/or so small that they do not seriously affect the application of A, concept as a predictor of microbial growth. However, the use of A, should always consider the possible influence of some non-equilibrium situations. INTRODUCTION The solids of many foods (either biopolymers or low molecular weight carbo- hydrates) are often in an amorphous state which is sensitive to changes in temperature and moisture content (Slade & Levine, 1987; Roos & Karel, *Member of CONICET, Argentina. 531