Research On The Effects Of Drying Conditions On Chemical Components Of Citrus Peel

The paper carried out a comparative study of drying kinetics, color and structural properties, mainly functional components and antioxidant activities of citrus peel that dried by the sun drying(SD), freeze drying(FD), hot-air drying(HAD), vacuum drying(VD) and med- and short-wave infrared radiation drying(IRD), respectively. It through exploring the better drying methods for citrus peel and searching for changing mechanisms of mainly bioactive compounds and antioxidant activities, so that to improve drying efficiencies and qualities of citrus peel, increase the yield of bioactive compounds and provided an important theoretical foundation for industrial production of dried citrus peel. The main contents and conclusions of this experiment are as follows:(1) The drying kinetics of SD, FD, HAD, VD and IRD were researched, respectively. Results showed that different drying conditions had a significant influence on drying kinetics of citrus peel(p<0.05). The drying efficiencies of citrus peel in different drying methods were ranked high to low in the order: IRD, HAD, VD, FD and SD. The drying rate was significantly accelerated and the moisture effective diffusion coefficient( Deff) was significantly increased with the rised drying temperatures(60℃~90℃) and infrared powers(675W~2025W), while the influence of drying temperatures on them was more obviously than infrared powers. Falling-rate stage was the main drying period of HAD, VD and IRD. And Page model was established as the best fitting model of citrus peels. In IRD, the multiple linear regression analysis was used to solve the the equation between model parameters of Page model and drying parameters, as well as the equation between moisture effective diffusion coefficients and drying parameters. According to the Arrhenius formula to calculate the activation energy in HAD, VD and IRD was 10.3925 kJ/mol ~28.1013 kJ/mol, respectively.(2) The influence of different drying conditions on color and structural properties of citrus peel was studied. Results showed that different drying conditions had a significant effect on color and structural properties of citrus peel(p<0.05). The color of citrus peel in FD was most similar to that of the fresh, and the sample in FD had the least △E value and browning index value, and the highest total carotenoid content. Then was the VD and IRD(The both of them had no significant difference), HAD, while the sample in SD had the highest △E value and browning index value, and the lowest total carotenoid content. The L value, a value, b value and total carotenoid content of dried citrus peel were decreased with the increasing drying temperatures(60℃~90℃) and infrared powers(675W~2025W), while △E value and browning index value were risen, and the influence of drying temperatures on them was more obviously than infrared powers. Moreover, The browning reaction caused the total color differences of citrus peel, while the degradation of carotenoid was the contributing factor for the decreasing of b value. The variation of microstructure provided a theoretical basis for further study of crude fiber, textural parameters, drying efficiency,soluble solid and rehydration of citrus peel. The dried citrus peel had high crude fiber content and soluble solid content, suitable hardness, crispness and rehydration if its microstructure was loose, undamaged and regular, as well as with big cavities, otherwise the dried citrus peel had poor structural quality. The IRD at relatively low drying temperatures(60℃~70℃) gave the citrus peel better color and structural quality compared with SD, FD, HAD and VD.(3) The effect of different drying conditions on color and structural properties of citrus peel was investigated. Results showed that the total phenolic content, total flavonoids content, hesperidin content and syphrine content in dried citrus peel were significantly increased compared with the fresh one, while the Vc content was markedly decreased. Except the SD, the antioxidant activities of citrus peel in other drying methods were also increased compared with the fresh one. In 60℃~90℃, the total phenolic content and antioxidant activities were rised with the increased drying temperatures, total flavonoids content was rised and decreased with the elevated drying temperatures, and hesperidin content and Vc content were decreased with the increased drying temperatures. In brief, different drying methods(conditions) were fit for the retention of different bioactive conpounds. The FD, IRD that at higher drying temperatures and HAD that at high drying temperatures were benefit to improve the yield of total phenolic. The FD, HAD that at 80℃ and IRD that at 60℃ were good for improving the yield of total flavonoids. The FD and IRD that at lower drying temperatures(60℃~70℃) could preserve the Vc content best. The IRD that at higher drying temperatures was suitable for improving the DPPH and ABTS free radical scavenging capacities, while the VD that at higher drying temperatures was suitable for improving the FRAP power.