Experimental Research On Microwave Drying Mechanism And Drying Parameters Optimization For Japonica Sorghum

As an important coarse cereals crop in China,japonica sorghum has been widely used in fields such as brewing,starch production,ethanol production,and making feed.Therefore,developing vigorously the sorghum industry is of great practical significance for improving the competitiveness of agricultural products and increasing farmers’ income.With the continuous increase of concentrated planting scale of sorghum,mechanized drying technology has become an inevitable choice for the drying treatment of sorghum after harvest.In response to the current problems in mechanized drying of sorghum,such as single drying method,high carbon emissions,insufficient drying quality,inability to resist pests and diseases during storage for sorghum after drying and so on,microwave technology is applied to the drying treatment of japonica sorghum in this study.Japonica sorghum in the northern region as the research object,adopting various methods such as development of test bench,theoretical model establishment,simulation,experiment study,microwave drying mechanism and parameters experiment research of japonica sorghum are conducted.The study will provide theoretical and data support for the industrial application of microwave drying of japonica sorghum.The main research contents are as follows:(1)Northern japonica sorghum varieties such as Longza 10 and Fengza 42 were selected as research object,the weighing method to be used to determine the thousand grain weight of sorghum grains,the measurement method is used to measure the triaxial size of grains,the food national standard method is used to determine the total protein content,total starch and amylose content,and tannin content of japonica sorghum.The results are as follows:the average weight of a thousand grains is 33.11 g and 31.19 g,the volume average size range of a single grain is respectively 4.02-4.30 mm and 3.95-4.25 mm,the range of amylose content is 20.24% and18.99%,the range of total starch content is 67.60% and 61.03%,the range of tannin content is1.207% and 0.905%,the range of total protein content is 9.2% and 8.3%.These data will provide necessary support for the design of microwave drying test bench,model simulation,and drying quality research.(2)Based on microwave field and mass and heat transfer theory,combining the experiment process plan and material characteristics,a microwave drying test bench is developed.Using theoretical design and empirical methods,the dimensions of the drying chamber and waveguide,rotating bracket device,moisture removal system,control system are designed.Using simulation methods,the waveguide layout and microwave radiation distance are optimized and designed.The results are as follows:Drying chamber size is 630mm×610mm×650mm,microwave radiation distance is 500 mm,rotating bracket device with a speed of 5r/min,axial flow fan dehumidification system driven by an independent motor,PLC control system with automatic execution of drying parameters.The waveguide port size is 80mm×40mm,three waveguides are arranged in a “one vertical and two horizontal” Pinzi pattern.(3)On the basis of completing the pretest in the test bench,using single factor thin layer drying test,the effects of parameters such as drying power per unit mass,single microwave action time,wind speed for dehumidification,and intermittent ratio on the drying characteristics of sorghum such as moisture content,drying rate,and material temperature were researched.Based on experimental data and classical dynamic equations,the Page kinetic model equation for thin layer microwave drying of sorghum has been established,and the feasibility of the model has been verified.The results are as follows:The influence of drying power per unit mass and single microwave action time on drying characteristics is greater than wind speed for dehumidification and intermittent ratio.The decreasing process of moisture content includes three stages: pre drying,constant rate drying,and reduced rate drying.The maximum error between the predicted values of the Page dynamics model and the experimental values shall not exceed 3%.(4)Based on the conservation laws of energy and components,FICK’s laws,and Lambert’s theorem,sorghum grains are discretized by using micro element volume control unit,the spherical coordinate heat and mass transfer equation of a single sorghum grain during microwave drying is established.Using Matlab software,the changes of moisture content and temperature at different positions within a single sorghum grain were simulated and analyzed.The results are as follows:As the drying time increases,the temperature of any point from the surface layer of a grain to the center area of a grain,first increases rapidly,then slowly increases,finally stabilizes.Except for the surface layer of a grain,the moisture content decrease process of any point inside a grain includes three drying stages: pre drying,constant rate drying,and reduced rate drying.(5)The main quality indicators of the samples obtained from the single factor thin layer drying test are determined by the food national standard method,including total protein content,tannin content,total starch and amylose content,starch particle morphology,starch functional group,starch viscosity,breakdown value,and setback value,starch phase transition temperature and gelatinization enthalpy;performance indicators such as unit mass energy consumption,average drying rate,and final moisture content are analyzed.Using analysis of variance,quality and performance indicators were screened out,that are significantly influenced by drying parameters such as drying power per unit mass,single microwave action time,wind speed for dehumidification,and intermittent ratio.The results are as follows:Four drying parameters have a significant impact on the quality and performance indicators of japonica sorghum,such as amylose content,starch setback value,starch breakdown value,unit mass energy consumption,and average drying rate.Through comprehensive evaluation,unit mass energy consumption,average drying rate,and starch setback value are optional indicators for subsequent optimization experiments.(6)Using drying power per unit mass,single microwave action time,wind speed for dehumidification,and intermittent ratio as experimental factors,using unit mass energy consumption,average drying rate,and starch setback value as evaluation indicators,response surface drying experiments are conducted on japonica sorghum.The regression model of each evaluation indicator and factor combination influence law are determined,the optimal combination of drying process parameters are obtained.The results are as follows:The single microwave action time,drying power per unit mass,and intermittent ratio are important factors that affect the quality and process performance of japonica sorghum.Under the optimal conditions of drying power per unit mass of 3W/g,single microwave action time of 40 s,wind speed for dehumidification of 1.0m/s,and intermittent ratio of 1:4,the unit mass energy consumption for microwave drying of japonica sorghum is 20090.42 k J/kg,with average drying rate of 0.782%/min and starch setback value of 734.25 m Pa·s.