Influences Of Pretreatment On Quality Of Carrot Slices Under Radio Frequency Combined With Hot Air Dehydration

Carrots(Daucus carota L.var.sativa DC.),also known as golden ginseng,golden bamboo shoots,and clove radish,are rich in carotenoids and other anti-oxidant and cancer-suppressing substances,and contain a variety of vitamins and minerals.However,due to the high moisture content of carrots(? 80%,w.b.),it is prone to damage and ulceration during seasonal harvest,which severely shorten the market shelf life of carrot products.The drying technology for carrot slices to reduce the moisture content to safe level is an effective solution to the shortcomings of carrots after harvest(? 10%,w.b.).The commonly used hot air-drying technology has the problems of low heating and mass transfer efficiency,long time heating,and poor quality of dried products.However,the radio frequency(RF)heating technology has the advantages of fast heating rate,large penetration depth,and better product quality after processing.The development of RF combined with hot air drying has significantly improved the drying efficiency of carrot slices and preserved a higher total carotenoid content.Based on the optimization of the RF combined with hot air-drying process of carrot slices and the quality analysis of the dried products,the sucrose osmotic pretreatment technology was used to improve the texture properties of the carrot slices.The carrot slices pre-treated by sucrose osmotic under combined drying process improved color,taste,and nutrient content with high efficiency for drying rate.The main research results are as follows:(1)The hot air assisted RF drying characteristics of three-layer carrot slices under different electrode gaps showed that as the electrode gap decreased,the drying rate of each layer of carrot slices increased,but the high-temperature treatment in final drying stage of carrot slices with high drying rate caused the color attributes of the slice to be oxidized and destroyed(P < 0.05).Therefore,to ensure that the carrot slices dried efficiently while reducing the loss of product color and quality,100 mm was selected as the optimal electrode gap.Based on the results of the former study,the rearranging layers method was used to improve the drying uniformity of carrot slices among each layer under the hot air assisted RF drying process.According to the study of rearranging layers for carrot slices at different intervals in the hot air assisted RF system,shortening the rearrange layers time interval of three-layer carrot slices during the drying process effectively improved the drying uniformity between the slices,but resulted in the degradation of the heating uniformity in slices.To ensure that the optimized hot air assisted RF drying process parameters had better temperature stability,30 min was selected as the optimal layer rearranging interval.(2)According to the comparative analysis of the drying kinetics and heating uniformity of carrot slices under different protocols of RF combined with hot air drying,the hot air assisted RF treatment was likely to cause the effect of heating concentration on the slices in the final stage of drying,so it should choose transferring the slices from the hot air assisted RF drying to the hot air drying before the final stage of drying to avoid the high temperature affecting the quality of the final product.According to the product quality comparison results under the optimal RF combined hot air drying,it is found that the dried slices had higher total carotenoid content and higher color quality with the shorter duration(P < 0.05).In summary,the optimized RF combined hot air drying protocol consisted of the electrode gap of 100 mm,rearranging layer time interval of 30 min,and after hot air assisted RF system drying for 270 min and then switched to hot air drying.(3)Based on the requirements for improving the texture quality of carrot slices,the sucrose osmotic pretreatment method was selected to improve the texture quality of dried carrot slices.By comparing the quality analysis of carrot slices with different concentrations and durations in pretreatment,it is found that at 50 oC,50%(kg/L,w/v)concentration of sucrose solution,the duration of 30 min,and the material-to-liquid ratio of 1:12 was the optimal sucrose osmotic protocol.Then,the drying characteristics and quality of dried carrot slices under the optimized combined drying protocol were studied.The results showed that1/2 divided carrot slices achieved more stable temperature control in combined drying,and reduced duration for drying the slices to reach the final moisture content.Carrot slices after sucrose osmotic pretreatment had a more uniform and delicate microscopic pore structure,a higher total carotenoid content and a better product color(P < 0.05).Therefore,the 1/2 divided carrot slices with pretreatment of sucrose osmotic can achieve efficient and stable temperature control drying of carrot slices under the optimized combined drying protocol.