Study on Drying Characteristics and Quality Properties of Longan （Dimocarpus longan Lour.） during Intermittent Heat Pump Drying

In order to understand the change of water state and quality characteristics of longan during practical drying production and discover the pivotal drying stages, the dynamic changes of moisture content and state, color, 5-HMF, main nutritional and functional qualities, and antioxidant capacity were monitored through whole intermittent heat pump drying and equilibrium stages. The results showed that the moisture content of longan decreased rapidly during drying stages, and decreased slowly during the equilibrium stages. After through drying stage 2, the free water of longan was removed and transferred coupled with the immobilized water increased. With the drying and equilibrium progress going, the color, browning degree and 5-HMF of longan increased in a step manner, color changes were most significantly occurred in drying stage 3, and the content of 5-HMF was dramatically increased to 55.40 μg/g DW after drying stage 1. On the contrary, the total phenols, total flavonoids, total free amino acids were decreased in a step manner, and decreased to 35.23%, 41.05%, and 42.13% after drying stage 3, respectively, when compared with fresh sample. Moreover, the degradation of total free amino acids was mainly decreased in drying stage 1 and 3. According to antioxidant analysis, the antioxidant capacity of longan was mainly decreased in drying stage 2, which was in conformity with the change of total phenols. Overall, drying stage 1 and 3 were the key stages during the intermittent heat pump drying process, which affected quality characteristics of longan mostly.