Rehydration of apple dried by different methods

Abstract

Apple var. Idared cut into cubes was dried by convection and freeze-dried. Dried cubes were equilibrated to variable water activities by storage over saturated salt solutions (range 0-0.33). Rehydration was done for one hour in water at 20°C. Increase in both mass and volume of cubes was followed during rehydration. Changes in height of a cube were recorded using a minimal displacement gauge. Electron transmission microscopy was used to measure cell wall thickness in raw and rehydrated material. Modes of drying affected the behavior of apple cubes upon rehydration. Convective dried apple imbibed water and swelled during rehydration, and a collapse of structure was observed at the beginning of the wetting process. Analysis of both mass and volume increase showed that at the beginning of rehydration capillaries and pores were filled with water and thereafter swelling of biopolymers occurred. Cell walls imbibed water, but their thickness after one hour of rehydration was half of that of raw apple. Freeze-dried apple increased in mass much faster than these dried by convection. Rehydration caused collapse of structure, which was not rebuilt during further wetting. On the other hand, cell walls swelled significantly during rehydration and reached thickness almost that of raw apple. It was inferred that freezing injured tissue, which after drying contained numerous discontinuities. Hence, swelling of domains, which did not form a continuous structure, was not propagated and expressed by the increase in volume on a macroscopic scale. Initial water activity of both investigated materials affected collapse of structure during rehydration. The higher the water activity, the faster the collapse.