Rehydration capacity and rate are, in relation to drying, the quality attributes that are the most assessed in the literature. However,few papers deal with the rehydration of freeze-dried products. Most of works is concentrated on air-dried materials. In these studies, the rehydration rate of dehydrated materials has been extensively discussed. On the other hand, limited information is available on the quality attributes related to the fluxes that characterize the process,namely the water uptake and the leaching of solutes, which are essential for better understanding the rehydration ability of the dried materials. Rehydration behaviour has been considered as a measure of the induced damage in the material during drying , such as integrity loss and reduction of hydrophilic properties, which decrease the rehydration ability. Theoretically, if there are no adverse effects on the integrity of the tissue structure, it should absorb water to the same moisture content as the initial product before drying. In this sense, it is expected that the high porosity developed in freeze-dried products plays a significant role in both rehydration capacity and rate, but beyond the freeze-drying conditions, pre-drying treatments as freezing and rehydration itself can induce structural and compositional changes in the food tissue which affect product quality.
Nevertheless, not much knowledge about the effects of rehydration itself on the water absorption capacity is currently available and even less comparative studies involving rehydration behaviours of highly porous freeze-dried materials have been done. Due to the lack of information on the structural changes under rehydration conditions, investigation on the glass transition temperatures of materials subjected to rehydration becomes increasingly important to obtain information on the extent of interaction between the porous matrix and the reconstitution medium, so that its effect on the water absorption capacity can be understood and the rehydration optimized. Obtaining an in-depth understanding, description and characterization of rehydration process in terms of both water uptake and loss of solutes may also lead to an improvement in product quality control and to a better process design.Particularly, for pineapple, guava, mango, papaya and acerola,the effects of freeze-drying on quality attributes, such as structural properties (density and porosity) and nutritional characteristics(vitamins and minerals) have already been examined in previous studies.However, detailed information on rehydration properties of the freeze-dried fruits is still lacking.