Study On Microwave Drying Process Of Carrot Based On Humidity Controlled Method

Humidity is an important parameter in the drying process and cannot be ignored.Reasonable regulation of the humidity in the drying process can effectively improve the quality of dried products.Although more and more studies have been done on humidity in recent years,but most of them were about hot air drying and were rarely able to monitor and control the humidity in real time.Therefore,this study used carrots which have a wide market demand as the material and applied the microwave drying method.Then built the experimental system to explore the impact of different humidity on the drying process and quality,improved the humidity control method,and finally established the drying kinetic model which considered the humidity to provide the theoretical basis for industrial production.The system was composed of microwave drying unit,humidification control unit,humidity detection unit and computer control unit.Microwave drying unit can achieve continuous linear control of power and precise control of drying temperature.Humidification control unit can achieve automatic humidification process.Humidity detection unit can achieve accurate measurement of humidity.The entire drying process was controlled by the program in the LabVIEW.The temperature,humidity and weight of the drying process can be measured in real time,and the system has been proved that can be used in the later experimental study.The wet air with relative humidity of 30%,50%,70% and 90% were introduced into the material tank for 0.5,1,1.5h during the drying process.It was found that longer high humidity maintenance could improve the quality of the dried carrots.If the humidification time is the same,the lower the humidification humidity is,the longer the humidity is maintained.If the humidification humidity is the same,the longer the humidification time is,the longer the humidity maintaining interval is.When humidificating,the environment humidity of the material tank is composed of the humidity of incoming wet air and the water vapor evaporated from the material,so the environment humidity restricts the drying rate of the carrot.But when non-humidificating,the drying rate of the carrot determines the environment humidity of the material tank.The results showed that the quality of carrot was better in color,rehydration rate,VC content and β-carotene retention rate under the conditions of humidification with 30%-1.5 h,50%-1.5 h and 70%-1.5 h.Based on the three humidification conditions with better quality,the best humidification condition was obtained by Central Composite response surface method,the target is the VC content and β-carotene retention rate are the highest.Then the material humidity change curve of the best humidification condition was obtained by subtracting the humidification humidity from the humidity inside the material tank.The material humidity change curve was taken as the reference curve.Using the two-stage control strategy and the PID control module,the humidity of the drying process was changed according to the reference curve.At the same time,the quality of the carrot was measured to certificate the feasibility of humidity control method.Four kinds of drying kinetics models were used to simulate the change of moisture content during the microwave drying process.The Page model was used in the humidification section and the Logarithmic model was used in the non-humidification section to obtain the best fitting result.The model of moisture ratio was calculated and the experimental results verified that the model can simulate the change of water content during carrot drying process very well.