Abstract: In order to improve the drying efficiency and the product quality and at the same time reduce energy consumption, alkali (2%, 4% and 6%) pretreatments coupled with heat pump drying (HPD) or hot air drying (HAD) were compared for their application to the drying of Goji berries (Lyciumbar barum). Drying rate, color, rehydration ratio and major nutrients (polysaccharides, carotenoids and flavonoids) were determined. Experimental results showed that the surface wax layer of Goji became gradually thinner and finally ruptured after treatment with sodium carbonate solutions, thereby leading to formation of water channels and consequently reducing drying time. The appropriate sodium carbonate concentration was 2%. In terms of drying efficiency, there was no significant difference between HAD and HPD. However, the Goji dried by HPD showed better color, higher rehydration ratio and higher retention ratio of nutritional components. Compared with drying at constant air temperature and relative humidity, multi-stage drying at varying temperature could not only maintain the quality of dried product, but also was time-saving. The optimum drying conditions of Goji were pretreatment with 2% sodium carbonate solution, and drying at 40 ℃ for 2 h, at 45 ℃ for 4 h, at 50 ℃ for 6 h, and at 55 ℃ for 10 h and at 40% relative humidity. Under these conditions, the drying time was 21 h, and the dried product had a total color difference value of 15.09 and a rehydration ratio of 2.35 g/g, and contained 10.29 g of polysaccharide, 0.43 g of flavonoid per 100 g of sample and 1503.13 μg/g of total carotenoid.

Key words: fresh Lycium barbarum, heat pump drying, alkali solution treatment, temperature, drying quality