Thermodynamic Approach to Osmo-dehydration

A thermodynamic approach is proposed to describe the osmo-dehydration treatment of apple pulp. The experimental evidence comes from isothermal desorption trends directly drawn from traces of Knudsen thermogravimetry and is strictly related to the water activity, aW, that is continuously recorded during the dehydration progress, thanks to the specific equipment used. The trends recorded concern three kinds of samples, namely, the nontreated and the partially osmo-dehydrated apple pulp and the sugar syrup used for the treatment, the investigations being applied to three different sugar syrups. It was recognized that these desorption trends actually reflect real equilibrium states of the systems considered at every dehydration level. Since the apple pulp may practically be referred to as a two aqueous phase system, namely, intra- and extracellular, the equilibrium condition imposes aW to be the same in either phase. This basic statement supports the view that the desorption trend of the nontreated apple pulp and that of the sugar syrup are actually related to the intra- and extracellular aqueous phases, respectively. The desorption trend of any partially osmo-dehydrated apple pulp remains between them and reflects the bi-phasic nature of the system. The approach also allows a quantitative description of properties and phenomena related to the dehydration, namely, extent of the cell shrinkage, proportion between intra- and extracellular mass, water diffusion coefficient through the cell membrane, expected sensorial differences related to the kind of dehydration process, and heat drying or osmo-dehydration performed with different sugar syrups.