Study On Microwave Drying Control And Quality Based On Dielectric Property Detection

Potato contains rich carbohydrates,proteins and various trace elements which is necessary for human body.It can also be used to treat stomachache,eczema and scald,having high medicinal value.With the increasing demand for fruits and vegetables,dried fruits and vegetables such as mango,strawberry and potato have appeared in the market.The advantage of dried fruits and vegetables is easy to carry and preserve.For potato drying technology,microwave drying has been widely used in major food processing factories because of its special heating mechanism to ensure higher drying efficiency and better product quality.It is noteworthy that temperature and microwave power as two key process parameters still exist a lot of room for optimization and improvement.In this paper,potato is used as raw material.Based on a microwave drying system with on-line measurement of material temperature and moisture content and real-time adjustable output of microwave power,the key parameter "dielectric properties" which affects the microwave absorption ability of materials is introduced into the host computer control program to guide the microwave power output and temperature control of the system.By comparing the energy consumption,product quality and drying efficiency of various drying schemes,the excellent microwave drying process suitable for industrial fruits and vegetables was found.Major research contents as follows:(1)Binary equation of dielectric properties(dielectric constant and dielectric loss factor)with temperature and moisture content was established by using data analysis software MATLAB,and the goodness of fit and feasibility of the equation were evaluated.The time-dependent dielectric properties of potato chips during microwave drying at preset temperature were analyzed,and the microwave penetration depth of potato chips at 2450 MHz was calculated to provide a reference for heating uniformity.The results show that dielectric constant and loss factor of potato chips during microwave drying are not the same as those of temperature and moisture content.The effect of free water content on dielectric constant is particularly significant in the early and middle drying stages,and the effect of drying stage is significantly decreased.The penetration depth is stable in the early and middle stages of microwave drying and the significant decrease of dielectric constant in the middle and late stages of microwave drying will result in a large increase of penetration depth.(2)The microwave drying characteristics and product quality of potato chips with or without temperature control were studied.Temperature control deviation,drying rate and product quality differences at different preset temperatures during different drying periods were compared.The relationship between material absorbable power and moisture content(time)is established by deferred average method.The results show that the temperature insidethe material increases rapidly in the late stage of drying,which leads to carbonization and product quality damage.Adjusting the output power of microwave system in microwave drying process by using the method of delay averaging is helpful to the stability and control of material temperature.Because of its reduction of power level,the energy consumption of scheme 3 in the whole drying process can be reduced by about 35% compared to scheme 1.(3)Based on the dielectric properties of potato chips and the regression equation of dielectric loss factor with moisture content and temperature,the advantages and application prospects of three different power-temperature controlled drying schemes and microwave output power proportional variation with dielectric loss factor(variable on/off frequency)were studied.The results show that the three-stage variable power temperature control drying has a higher power level in the initial stage of drying,and a lower power level tends to be stable in the middle and late stage of drying,which can ensure the homogeneity of heating materials.The drying scheme of microwave power output with loss factor feedback is helpful to the stabilization and control of material temperature in the middle drying stage.For potato chips,when the moisture content drops to about 17% of the wet base,the feedback power of the loss factor and the positive correlation with the temperature change can reduce the temperature of the material,and the final product will show better quality.The optimal initial microwave drying power for 5 mm thick potato chips with loss factor feedback is around 180W(4 W/g).The drying process can be further optimized by changing the on/off ratio of different time intervals.Since there is no need to add temperature feedback control in the drying process,it has a certain guiding role and practical significance for large-scale industrial microwave processing of agricultural products.