RESUMEN

The main objective of this work was to analyze and to deepen in the study of hot air drying pears and to analyse the effects of the osmotic pretreatment  in the drying kinetics and in the quality of the final product, through the analysis of rehydration process of the dried product. Experimental results allow to model the kinetics of the different process stages (drying and rehydration) throughout a diffusional model. Obtained results showed that the higher the drying temperature, the greater the water effective diffusivity. Osmotic pre-treatment provoked a reduction of the water effective diffusivity and an increment of the total drying time and yield of the process. These effects were more marked when longer osmotic pre-treatments were used.  
Rehydration of dried samples showed that the influence of the rehydration temperature varied  according to the temperature used in the drying process. A slightly more  hydrated product was obtained when lower temperatures were used in the drying and rehydration process. The osmotic pre-treatment hardly affects mass transport of water and solutes, although a greater solute content was reached when longer pre-treatment were used because of  the superficial impregnation of samples. In these pre-treatments the calculated water De values were also lower. 
Interactions water-structure was analysed throughout the water sorption isotherms, determination of the glass transition temperatures of the amorphous fraction of the liquid phase and measurement of the mechanical properties of samples with different water content. No differences were found in the capacity of retention of water of pear when samples were subjected to adsorption- desorption processes. The plastification effect of water was shown in the glass transition temperatures as well as the mechanical properties of samples, which become weaker and more resistant to fracture when the water content decreased.
Dried samples stability was analysed during storage, throughout the changes of colour samples. The obtained results showed that colour of the dried samples remained stable during storage. Among the DO/VI treatments, the vacuum impregnation or long osmotic treatments was recommended to improve  the final quality of dried product.

key Words: drying by hot air, osmotic pre-treatment, rehydration, pear, glass  transition, mechanical properties, colour.