Studies on the Effects of Microwave Power and Temperature Control on the Quality of Whole Lychee (Litchi chinensis Sonn.) Fruit during Microwave Vacuum Drying

A batch‐type microwave vacuum dryer was used to study the effects of different microwave power densities on the drying characteristics and quality of whole lychee (Litchi chinensis Sonn.) fruits. Lychees were treated at different power levels and it was observed that the drying process included two drying stages when power density was above 0.3 W/g, and drying process was consisted of three drying stages when power density was 0.1 W/g. The optimal power density was found to be 0.6 W/g. A typical pulp temperature profile of lychees was found during microwave vacuum drying (MVD), including four stages: increasing, decreasing, gently increasing and sharply increasing. Moreover, the decrease in quality was negatively correlated with an increase in temperature at every stage in the temperature profile. Temperature control was applied to improve the quality of MVD lychees. The optimal temperature range value in terms of drying time, vitamin C content, bulk density and sensory evaluation was found to be 60–65C. Practical Applications Air‐dried (AD) lychees are popular as traditional food in South China and its sales volume was about 80% of all processed lychee foods. AD lychees have deep brown pulp with much shrinkage and poor flavor. However, microwave vacuum drying (MVD) could produce high‐quality dried lychees with light golden yellow color, less shrinkage, better taste, shorter drying time and lower energy. The information provided in this work will sever as the basis for further mass production. MVD lychees are much more promising as a snack food than AD lychees.