Effects of vacuum drying assisted with condensation on drying characteristics and quality of apple slices

Abstract

Vacuum drying is a preferred drying technology in food industry due to the higher quality of the final products than conventional hot air drying. However, the low drying efficiency significantly limits its application. The low heat transfer and high humidity under vacuum conditions are the main causes for the low drying rate. The present study innovatively focused on dehumidification inside the vacuum chamber to improve the drying rate, and created novel vacuum drying device assisted with a condensation unit (VD + C) by putting a cooling plate into the vacuum chamber to condensate the water vapor into liquid. The innovative device was used to study the drying rate on apple slices, the nutrients degradation and color changes comparing to hot air drying, vacuum drying and vacuum freeze drying. The results showed that VD + C significantly enhanced the drying effect of apple slices, the drying rate (Deff value) was the highest and the energy consumption (SEC value) was the lowest among the tested drying methods. Compared with hot air drying and vacuum drying, the water rehydration rate, the content of ascorbic acid, total phenol content and antioxidant activity were improved, and the color was obviously better than that of hot air drying and vacuum drying. Besides, the microstructure of apple slices by VD + C was better than that by hot air drying.