The influence of drying air temperature on the physical properties of dried and rehydrated eggplant

Drying processes generally cause volume and surface change of foodstuffs. Information on the porous structure and the mechanical properties of dried food products is needed for determining food quality, process design and estimating properties such as density and moisture diffusivity. In this work we investigated the structural changes induced in eggplant by convective air drying at four different temperatures (40, 50, 60 and 70°C) and their effect on the subsequent rehydration process. Drying and rehydration kinetic curves were also measured. The changes in physical properties, such as porosity, pore-size distribution and bulk density were determined by Hg porosimetry, Scanning Electron Microscopy and Optical Microscopy while their effect on the textural characteristics by dynamometric measurements. As expected, the increase of the drying air temperature causes shorter drying times. The drying temperature influences strongly the microstructure of dried samples: the porosity increases with the air temperature, but the structure is better preserved at intermediate temperature (60 °C) as confirmed by the lower firmness values with respect to the other dehydrated samples (40, 50, 60 and 70°C). In these latter, the longer drying time and the higher temperature, respectively, causes the development of a wrinkled structure. In particular, at 70 °C the structure of dehydrated samples appears totally broken with a consequent faster water uptake during rehydration