Water activity and the preservation of plant foods

The general constitution of plant foods, namely of vegetables and fruits, may be described as a watery solution of low molecular weight species, mainly sugars, salts and organic acids and of high molecular weight hydrocolloids, contained in a water insoluble cellular matrix of macromolecules, mostly carbohydrates, including insoluble pectic substances, hemicelluloses and proteins. All of these constituents, to different extents, interact with water and have the ability to lower its vapour pressure; with small molecules this is mainly through polar binding, and with large biopolymers, through surface interactions and capillary effects. For a long time, scientists have realised that the relative vapour pressure, i.e. the water activity, could be much more important to the quality and stability of food than the total amount of water present. Over the past few decades, the glass transition temperature, as well as its dependence on moisture content has also gained interest, as it could become the main determinant of the dynamic of changes in many non-equilibrium, low moisture systems. Processed fruits and vegetables are often low moisture, sugar-rich foods, characterised by colour, flavour and structural properties. The relationships between both aw and Tg and some important chemical, physical and structural changes which can occur during processing and preservation of fruits and vegetables and their derivatives, are presented.