Drying characteristics and product quality of Lycium barbarum under stages-varying temperatures drying process

Lycium barbarum is a kind of fleshy berry in bright red, which is one of the well-known traditional Chinese medicines with rich nutrients. Currently, Lycium barbarum is widely dehydrated using sun-drying. This method is simple and low-cost, which are its advantages. But the drying time is long and its application is limited by the climate and sanitary conditions. Also, the dried product quality is difficult to control. In this paper, the multi-stage varying temperature hot air drying was conducted on Lycium barbarum to shorten the drying time and improve the dried product quality. This drying method was successfully applied in the drying processing of rice, cherry tomatoes, honeysuckle, etc. First, the hot air drying characteristics of Lycium barbarum were investigated under constant air temperature (40, 50 and 60℃), relative humidity (40%, 50% and 60%) and material thickness (8, 24, and 40 mm); and the product quality was evaluated through polysaccharide content, color and rehydration ratio of dried Lycium barbarum. The hot air drying experiments were conducted in a lab-scale SY-5 type fruit and vegetable drying room in which the hot air temperature and humidity could be pre-scheduled by programming. The polysaccharide of dried Lycium barbarum was extracted through the ultrasonic-aided water extraction method and then, its percent content was measured using the phenol-sulfuric acid method. The color of dried product was evaluated using the image technology and its red, yellow and blue color value could be obtained. The rehydration ratio was judged through soaking the dried Lycium barbarum in hot water (80℃) for 20 min and then measuring the mass of water absorbed. Experimental results under the constant drying temperature and humidity conditions showed that the drying time of Lycium barbarum decreased largely from above 48 to 15 h with the increase of air temperature from 40 to 60℃, also decreased slightly with the increase of relative humidity. The improvement of air temperature had a negative effect on the product qualities such as color, but the relative humidity had a small positive effect on product quality. When air temperature increased from 40 to 60℃, the contents of Lycium barbarum polysaccharides decreased from 7.76% to 7.20%. The red color value reduced from 13.9 to 12.9, the rehydration ratio varied from 50.0% to 51.4%. Thus, the drying temperature is the governing parameter in the hot air drying of Lycium barbarum. Then, several multi-stage varying temperature drying processings of Lycium barbarum were conducted and their dried product qualities were compared. The optimum drying condition obtained for the multi-stage varying temperature drying technology was the drying temperature of 40℃ (6 h)-50℃ (6 h)-60℃, relative humidity of 40% and three-layer material thickness. Under the optimum drying condition, the dried Lycium barbarum had a red color value of 13.7, polysaccharide content of 9.77% and rehydration ratio of 51.6%. Finally, The drying time and the product quality of Lycium barbarum with/without pre-treatment using 3% alkali solution were also compared and it was found that pre-treatment with 3% alkali solution for 1 min could shorten the drying time from 28 to 25 h, improve the red color value from 13.7 to 15.4 and rehydration ratio from 51.6% to 55.4%, and meanwhile polysaccharide content slightly decreased from 9.77% to 9.23%. ©, 2015, Chinese Society of Agricultural Engineering. All right reserved.