SUMMARY: The paper deals with the relationship betweeen the water content of washed and the thermodynamic activity of water, , over the range 0 < < 1.0. Two overlapping types of measurement are used: osmotic water exchange and water-vapour sorption. The data are given in terms of the equation = where is water content, is external osmolality (actual or effective, depending upon the technique used), is a constant and has values which can be interpreted in terms of the type of process involved: e.g. in the initial stages of osmotic dehydration, of unity would indicate ideal solution behaviour; in the final stages of drying, any constant value of would conform to an adsorption function like that of Halsey (1948). The data obtained by the two techniques used fall on a single curve with several zones including () non-ideal solution behaviour at high , () two intersecting ‘co-operative adsorption’ zones at very low , and () a region of variable where processes such as dissolution of small molecules and ‘saturated’ swelling of interlinked polymeric structures are presumed to occur. It is thought that detailed interpretation of the unified water uptake curves would be valuable in understanding mechanisms of humidity-dependent loss of viability, in particular the zones of exceptionally rapid attenuation.


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