Water activity in biological systems - A review

Abstract

Water deserves a major attention by researchers dealing with biological systems and related materials, like food, since it is ubiquitous and can be used like a "native" probe to garner information about the hosting system, provided it may be freely displaced across. Its thermodynamic potential, namely, the water activity, a W, is related to that of the other compounds of the system considered via the Gibbs-Duhem relationship reflecting the extent of the residual availability of water to solvate further solutes and sustain the molecular mobility of the bio-polymeric compounds. As for the experimental approaches to a W, this short review re-addresses the reader to other publications, while devotes a section to the Knudsen thermo-gravimetry that was used by the authors to determine the desorption isotherms of many food systems and related aqueous compounds. The paper remarks the importance of a preliminary assessment of water mobility and recalls the concept of "critical a W" that takes into account the reduced mobility of water molecules in the vicinity of the glass transition. This opens the question of the reliability of sorption isotherms which encompass a wide a W range and the interpretation of the observed adsorption/desorption hysteresis. The multi-phase character of many biological systems is another issue of interest related to the reliability of the experimental approaches to a W. As examples of the role of a W on the stability of bio-systems and on the practice of a technological treatment, protein unfolding and osmo-dehydration of fruit pulps are reported. © Copyright by Institute of Animal Reproduction and Food Research of the Polish Academy of Sciences.