Study On The Drying Mechanism And Dryer Design Of Germinated Brown Rice Under Microwave Drying Conditions

In order to resolve the problems in the variation of drying quality and the shortage of dryer of germination brown rice(GBR),some key research issues are studied systemically and deeply to determine the drying properties and optimize the technology process parameters,and to reveal the mechanism of microwave drying conditions on the GBR quality,and to design a continuous and three-layer microwave dryer for GBR drying by means of theoretical analysis,simulation and stand test.Some conclusions were developed as follows.1.The effects of microwave drying parameters on the drying properties of GBR were analyzed that both microwave intensity and surface air velocity have obviously positive effects on the drying rate and grain temperature.The reasonable drying process and parameters were optimized as microwave heating then tempering with heat insulation followed by microwave heating again under microwave intensity of 4-5W/g and 2.5-4W/g,drying duration in each stage of 9-12 min,and with one third as much as the tempering duration,surface air velocity of3.5-4.5m/s.2.The effect mechanisms of microwave drying conditions on the GBR quality were revealed in fissure generation,γ-aminobutyricacid(GABA)content and yellow chroma.Low fissure generation of GBR results from the full wetting in germination stage,volumetric heating and stable volume of grain in microwave drying.No significant change of GABA in GBR was found due to the balance effects between the increase of GABA content from the synthesis reaction caused by the glutamic acid decarboxylase(GAD)and its decrease from the degradation caused by the catalytic action of pyruvate aminotransferase in microwave drying.In the microwave drying GBR,both grain temperature in range of 33.93-69.71 o C and moisture content from 27.95% →24.00% may excite GAD activity and contribute to the movement of activity molecule to improve the GABA synthesis.However,in later stage of microwave drying GBR,GAD inactivation caused by relatively high temperature and stagnant movement of GABA molecule caused by the low moisture content restrict GABA synthesis when grian temperature rise from 41.98 to 81.36℃ and moisture content decline from decline17.89% to 14.92%.Higher temperature causes more obvious brown appearance of GBR to improve its yellow chroma with the increase of microwave intensity and surface air velocity.However,exceed drying temperature may result in the heavy brown GBR to decline the yellow chroma value.Heavy golden yellow emerge in whole grain of GBR in microwave intensity of 2.75 W/g and surface air velocity of 2.50m/s.Multi-factors regression model were fitted to calculate the key components in GABA,protein and moisture of GBR by means of nondestructive detection of near infrared spectroscopy,which provide these valuable models to measure and evaluate the GBR quality with speediness,exactness,impersonality,and non – invasion.3.The microwave energy absorption and mass and heat transfer process were investigated in microwave drying of GBR.The dielectric properties models of GBR were proposed as the function of temperature and moisture content with dielectric constant of 4-15 and dielectric loss factor of0.8-4.0.The distribution and value of electric intensity in drying chamber of microwave dryer were simulated by using ANSOFT HFSS software.Both vertical and horizontal alignment of ports of magnetron wave guidance and straight edges of peripheral surfaces of drying chamber have uniform distribution of electric field intensity and high utilization of microwave energy.Heat and mass transfer process of GBR include the non-isothermal flow transfer,water evaporation at surface and mass convection diffusion process in microwave drying.4.A microwave dryer system was designed specially for GBR including a dryer and control system.The microwave dryer designed with roundabout movement conveyer belt to form three drying layers well adapted for the microwave drying technology with two drying stages.And a tempering bin and feeding apparatus were designed to match the microwave dryer.The distribution and value of temperature and moisture content in GBR layer may be detected and controlled by using control system of microwave dryer designed to achieve the multi-input microwave power with two drying stages of microwave heating + tempering+ microwave heating again.The research results may improve the drying quality of GBR,and provide the continuous microwave dryer for the GBR drying with expected advantages of high drying rate and fine drying quality to resolve contradictions between the drying efficiency and final quality of GBR.