Structural changes in apple tissue during glucose and sucrose osmotic dehydration: shrinkage, porosity, density and microscopic features

Micro- and macrostructural changes in apple slices during osmotic dehydration were studied. Samples were immersed into 25.0% (w/w) glucose or 34.6% (w/w) sucrose aqueous solutions at 30 degreesC until water and solid contents were almost constant (congruent to350 min). They were removed after selected times, and examined for thickness, volume, bulk and solid-liquid densities, porosity, water activity, water loss, solids gain and microscopic aspects. At relatively short times, significant weight and volume losses were observed for both treatments arising from the fast water loss. The solid-liquid density of glucose or sucrose treated samples increased slowly along the process. However, the bulk density increased up to a certain value and then fluctuated with increasing time, showing in some cases another pronounced increase for long times of treatment. Fruit porosity values decreased in line with the increase in bulk density values. At the end of the osmotic treatments in glucose or sucrose solutions, there was a recovery in porosity, although the values were lower than for fresh fruit. The changes in bulk density, porosity and volume of apple tissue along osmotic process were closely supported by microstructural and structural observations (using light microscopy and environmental electron scanning microscopy) and explained by considering osmosis as a multicomponent diffusion process through porous media and due to the relaxation of the viscoelastic shrunken cell walls. (C) 2003 Elsevier Ltd. All rights reserved.