Optimization of vacuum freeze-drying technologies of longan fruits

In order to confirm the optimum technological parameter in vacuum freeze-drying of longan, experiments were conducted with longan cultivar “Wulongling”. The eutectic point and the consolute point of longan were determined by electrical resistance method in the vacuum freeze-drying. The technologies of the vacuum freeze-drying of longan were optimized with both the single-factor and the orthogonal experiments, and according to evaluating the effects of the comprehensive indexes, including shape-keep rate, colors of the products and the energy consumption in the process. The results showed that the eutectic point and the consolute point of longan’s pulp materials were -26℃ and -20.0℃ in the vacuum freeze-drying, respectively. The optimized technology was to cut the longan’s pulp into half or slices,with the temperature phases set as 95℃(7 h)→85℃(1 h)→75℃ (1.5 h)→65℃(3 h)→55℃(to the end), the vacuum value of drying chamber ranged from 60 Pa to 70 Pa, and the maximum temperature of 50℃. With the optimal technology, the energy consumption could be saved as far as possible,the shape-keep rate of freeze-drying product could be improved, and the product color was maintained from white to light yellow.