Reducing energy consumption of carrot slices dehydration by combined vacuum microwave and freeze drying

In order to reduce the energy consumption during freeze drying (FD) of vegetables, vacuum microwave drying (VMWD) was introduced and applied before or after FD, which is referd to combination Ⅰ and combination Ⅱ respectively. For combination Ⅰ, the optimized microwave power density, moisture content removed during VMWD, sublimation drying time and desorption drying time during FD were 1.6 W/g, 40%, 4 h, and 3 h, respectively. For combination Ⅱ, the optimized sublimation drying time and microwave power density of VMWD were 7 h, and 1.0 W/g respectively; and temperature control mode was adopted for VMWD. Moreover, the color, the retention of b-carotene and rehydrated ratio of carrot slices by combined drying were very close to those of FD, but the volume retention ratio was slightly less than that of FD while still maintaining an acceptable appearance. Compared with FD, energy consumption of the two combined drying methods was saved by 47.0% and 54. 2% respectively, and the drying time was saved by 50%.