Determination of freeze-drying process variables for strawberries

The production of high-quality freeze-dried strawberry pieces was studied by the response surface method (quadratic model). Working pressure (P) and heating plate temperature (T) were determined as the most important factors affecting the criteria of final product quality (appearance/shape, colour texture, rehydration ratio). Freeze-drying experiments involving a thick layer of strawberry pieces were conducted in different operating conditions according to a two-factor (P, T) experimental design. The surface response method allowed us to graphically determine optimal working conditions. By superimposing all quality criteria contour plots, the optimal conditions found were P = 30 Pa and T = 50 degrees C. The freeze-drying rime ranged from 60 to 65 h. The experimental values for freeze-drying time, appearance and colour of freeze-dried strawberries obtained with these optimal conditions were very similar to those predicted by the second-order surface response model. (C) 1997 Elsevier Science Limited.