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Mass transfer phenomena were qualitatively investigated during osmotic dehydration of fresh apple in concentrated solutions using response-surface methodology. The influence of the main process variables (solute concentration, solute molecular weight, temperature and processing time) were thus determined. Osmotic phenomena, such as plant cell plasmolysis, only had a marked effect on dehydration at moderate temperatures (T < 50°C) and when low molecular weight solutes that could easily penetrate tissues were used. More generally, basic dehydration mechanisms were diffusive and seemed to be closely correlated with the formation of dense solute barrier at the surface of the product. The formation of this barrier layer, which was promoted by high molecular weight solutes and/or high solute content, could reduce loss of natural fruit solutes.

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