Study On The Relationship Between Shape Change And Technology Parameter Of Materials During Special Drying Process

Retaining the shape is an important indicator to value the quality of dried fruits and vegetables. It seriously affects the product's commercial value. The traditional hot air drying is with the characteristics of the low costs, long dehydration time, much energy consumption and low volume retention rate. In order to maximizing the original shape of sliced fruits and vegetables, the combination of microwave vacuum drying and freeze drying instead of freeze drying, coupled hot air with microwave drying instead of air drying were studied using the carrot slices as tested material, and the relationship between shape change and process parameters was also investigated.To find the best combination of microwave vacuum and freeze-drying the shrinkage and its mathematical model of microwave vacuum drying was studied. The experiments showed shrinkage coefficient was a negative correlation with power density, while shrinkage coefficient and moisture were positive correlation with the thickness of sample.To decrease the energy consumption during freeze drying (FD) of vegetables, the study was conducted using carrot slices as tested material. Vacuum microwave drying (VMWD) was introduced and applied before or after FD, which are named combination 1 and combination 2. The parameters of combination 1: the microwave power density was 2.85W/g, 40 percent water content was removed during MWVD, then sublimation drying for 4 h and desorption drying for 3 h during FD. The parameters of combination 2: sublimation drying for 7 h, then VMWD at power density less than 1.0W/g and temperature control mode was adopted. Moreover, the color, the retention ofβ-carotene and rehydrated ratio of combined drying carrot slices were very close to these of FD. Compared FD, energy consumption of two current combined drying methods was saved by 47.0% and 54. 2%, and the drying time was saved half.To reduce the energy consumption in drying process and improve the volume retention ratio of the product, coupled microwave and hot air coupling drying method was used, response surface methodology(RSM) was used to analysis the impact factors. The optimization parameters are: microwave power density 1.9 W/g, air temperature 65℃, the thickness of carrots 5mm. Energy consumption in this way is as 21.27% times as that of traditional hot air drying, and the volume retention rate is raised by 10%.The research showed that vacuum-microwave drying, combination of vacuum microwave and freeze-drying and coupled hot air and microwave drying all can produce high quality dehydrated fruits and vegetables, not only with better color, flavor and nutrition, but also with better shape and texture, compared with traditional methods, energy consumption was greatly reduced.