Process–quality optimization of the vacuum freeze-drying of apple slices by the response surface method

Summary Working pressure (P) and heating plate temperature (T) were considered to be the most important factors affecting the kinetics of the freeze-drying operation for apple slices as well as the criteria of final product quality (appearance/shape, colour, texture, rehydration ratio). Freeze-drying experiments involving a thick layer of slices under different processing conditions were designed according to a central composite rotatable system with two factors (P, T). Results were analysed by response surface methodology (RSM). By superimposing all quality criteria contour plots, the optimum levels of processing conditions yielding the best quality freeze-dried apple slices were determined to be an operating pressure of 50 Pa, and a heating plate temperature of 55 °C. For a product loading of 17 kg m^?2, the freeze-drying time ranged from 48 to 50 h. The experimental values of the freeze-drying time, appearance and colour of freeze-dried apple slices obtained with these optimal conditions were very similar to those predicted by the corresponding second order polynomial model. The rehydration ratio of freeze-dried apples was about 0.55 g g^?1 of water removed and the texture loss of rehydrated apples was estimated to be more than 85%.