High-Voltage Electric Field Combined With Hot Air Drying Of Apple Slices

China’s apple production is huge,but the production season is strong,which can easily lead to problems such as overcapacity and storage rot.Deep processing of apple chips is an effective way to solve the above problems,and drying is an important part.However,among the commonly used drying methods,the product quality of hot air drying is poor,microwave drying has high energy consumption and uneven drying,vacuum drying and freeze drying equipment is expensive and high production cost.High-voltage electric field drying has advantages of low energy consumption,simple equipment,and less nutritional loss of products.However,there are few researches on the high-voltage electric field drying model of apple slices with voltage and distance between needles,and the optimal conditions of high-voltage electric field and hot air combined drying.In this paper,a high-voltage electric field and hot air combined drying system was designed to study the effects of voltage,distance between needles,temperature and wind speed on the drying characteristics of apple slices and the drying model.The combined drying conditions of high-voltage electric field and hot air were explored and optimized.It provides a reference for the practical application of high-voltage electric field drying technology.The main conclusions were as follows:(1)Completed the design of high-voltage electric field and hot air combined drying system.Mainly included the design of drying room structure,data acquisition control circuit and program,and the selection of the high-voltage power supply and hot air blower.The highvoltage electric field and hot air combined drying chamber was composed of a hot air channel and a high-voltage electric field room.The high-voltage electric field adopted a "needle-plate" electrode structure,and the discharge spacing was adjusted by a stepper motor lead screw moving pair.Apple slices were suspended under a weight sensor to detect weight,and fluorescent optical fiber sensors were used to measure the temperature of samples.The data acquisition control system was used to adjust the discharge spacing,as well as to realized the reading,display and storage of weight and temperature.(2)The volt-ampere characteristics of the "needle-plate" electrode and the thermal effect of the high-voltage electric field were studied.The more the number of needle electrodes and the smaller the discharge spacing,the lower the initial discharge voltage and breakdown voltage of the "needle-plate" electrode.During the drying process of the high-voltage electric field,the breakdown voltage first decreased,then increased,and the lowest was 53 k V.Increasing the temperature or decreasing the wind speed would reduce the breakdown voltage.In the drying process of the high-voltage electric field and hot air combination,the breakdown voltage also shown a law of first decreasing and then increasing,with a minimum of 46 k V.The high-voltage electric field had no obvious thermal effect,but in the combined drying of the high-voltage electric field and hot air,the high-voltage electric field would promote the increase of the temperature of the apple slices,and the higher the voltage,the faster the temperature of the apple slices rose.(3)The effects of voltage and distance between needles on drying characteristics of apple slices and drying model were studied.The drying rate of apple slices increased with the increasing voltage.During low-voltage drying,there was a constant rate period at the initial drying.The higher the voltage,the greater the effective water diffusion coefficient and rehydration ratio of apple slices,and the higher the drying specific energy consumption.Under the same voltage,the drying rate,effective water diffusion coefficient and drying specific energy consumption of apple slices were higher when distance between needles was 6 cm than those when distance between needles were 5 cm and 7 cm.The prediction functions of moisture ratio of apple slices based on voltage and distance between needles were established,and the prediction function based on the Logarithmic model could more accurately predict the change of moisture ratio of apple slices during high-voltage electric field drying.(4)The combined drying conditions of high-voltage electric field and hot air for apple slices were optimized,and compared with the drying of hot air.When wind speed was less than 0.4 m/s,the drying process of apple slices in high-voltage electric field was promoted.When wind speed was greater than this value,the drying process of apple slices in the highvoltage electric field was inhibited,and the higher the wind speed,the stronger the inhibition.The higher the temperature,the faster the drying of apple slices,but the more serious the browning and the lower the rehydration rate.The optimum parameters for combined drying of apple slices were optimized by the comprehensive evaluation method,and voltage was 40 k V,wind speed was 0.45 m/s,and temperature was 37 ℃.Under the combined drying conditions,the drying of apple slices was slower than that of hot air drying,but the rehydration ratio and browning were better than those of hot air drying.