Influence of combination drying methods on composition, texture, aroma and microstructure of apple slices

Apple slices dried by two combination drying methods and by freeze-drying (FD) were evaluated for their composition and analyzed for texture, aroma and microstructure. FD and followed by microwave vacuum drying (FD + MWVD) yielded dried samples of higher reducing sugars, total pectin and lower total phenols comparing to FD samples. On the other hand, MWVD + FD samples had lower sugars, total phenols and total pectin than FD + MWVD samples. On the whole, aroma retention in FD + MWVD samples is better than MWVD + FD products and worse than FD samples. Moreover, the crispness of FD + MWVD samples was higher than MWVD + FD samples and the hardness of FD + MWVD samples was lower than MWVD + FD samples. Microwave application had a significant effect on total phenols, pectin and aroma. In addition, longer time high temperature at desorption drying stage and short time higher temperature at MWVD stage during FD + MWVD process resulted in lower reducing sugars and worse aroma retention, respectively. Faster drying rate in MWVD process induced worse cell structure. (C) 2011 Elsevier Ltd. All rights reserved.