Structural properties of dehydrated products during rehydration


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The following structural properties of dehydrated apple, banana, carrot and potato were examined during rehydration: true density, apparent density, porosity and specific volume. Samples were dehydrated using five different drying methods: conventional, vacuum, freeze, microwave and osmotic; after which they were rehydrated in an air dryer at 50 °C and 80% air humidity. Structural properties were examined during rehydration to various moisture contents, ranging from 0.01 to 3.5 kg kg–1 (dry basis). The dehydrated products did not recover their structural properties after rehydration as a result of structural damage that occurred during drying and the hysteresis phenomenon which took place during rehydration. Porosity of the rehydrated products was higher during rehydration than during dehydration. A generalized structural model was used to describe the structural properties, and of the four parameters that were incorporated, only the shrinkage coefficient, which represents volume expansion, changed on rehydration.