Studies on the quality and moisture distribution of kiwifruit dried by freeze drying combined with microwave vacuum drying

The drying quality of kiwifruit dried by a combination of freeze drying and microwave vacuum drying (FD-MVD) in terms of drying time, appearance quality and microstructure. The results showed that the quality of FD-MVD products was highest with FD 12 h because the lowest volume density and close to the color of single FD samples. The optimal microwave power for the MVD process was found to be 0.30 W/g. The microstructure of FD 8 h samples was maintained very well due to the whole cell structure between the center and the edge of the kiwifruit. This is because that the moisture content at the edges and centers was identical in the FD-MVD samples with FD 8 h. However, in FD-MVD samples with FD 10 h and FD 12 h, the moisture content at the center was significantly higher than at the outer ring, which resulted in obvious shrinking and folding of cell structure. Therefore, FD 12 h was considered the optimal moisture conversion point because it resulted in a good appearance quality. FD 8 h was the optimal moisture transfer point when the microstructure, drying time and drying energy were considered jointly. Practical Applications Kiwifruit is rich in nutrients and is planted in over 80 countries worldwide. FD method used to dry kiwifruit consumes large amounts of energy although results in good quality products. FD-MVD dried kiwifruit requires less energy to produce and its quality is similar to that of kiwifruit subjected to FD only. This study describes a new, low energy method of kiwifruit drying that has possible implications for large-scale commercial production. In addition, effect of different cell structure of center and edge in kiwifruit on microstructure afford a new quality judgment method during drying for similar materials.

Automated summary

The study found that the quality of kiwifruit dried by a combination of freeze drying and microwave vacuum drying (FD-MVD) was highest at FD 12 hours and the microstructure was well maintained at FD 8 hours. When considering drying time, energy consumption, and microstructure, FD 8 hours was identified as the optimal moisture transfer point and FD 12 hours was considered the optimal moisture conversion point.