Study On Microwave Drying And Quality Change Mechanism Of Germinated Brown Rice

Germinated brown rice(GBR)is a novel functional rice product with high nutritional and health value.However,fresh GBR is easily suffered from microbial invasion due to high moisture content,leading to the quality spoilage and the loss of nutrition and commodity value.The rapid and timely drying of GBR to the safe storage moisture content of 13.0%?14.5%(w.b.)is not only beneficial to extend the storage period,but also provide strong guarantee for the further processing of follow-up products.Microwave drying has remarkable advantages of fast drying rate,high energy efficiency,and easy controllability,which can meet the demand of rapid and timely drying of fresh GBR.Nevertheless,the drying process of GBR subjected to microwave field is complex and the change of final product quality is unstable,which limits the wide application of microwave technology in GBR industry.To improve the applicability of microwave drying of GBR,the bench tests and numerical simulation methods were employed to study the microwave drying and change mechanism of quality attributes including color,fissure and gamma aminobutyric acid(GABA)content,and the quality control mode of microwave drying of GBR was purposed.Firstly,the drying characteristics and kinetics of GBR were examined by the bench tests in the microwave oven,and the simulation of drying process of GBR rotating on a turntable was achieved by developing the coupled multi-physical field model including microwave field,heat transfer and mass transfer and the corresponding simulation strategy to achieve the rotation of materials.Next,the simulated spatial distribution of electric field,temperature and moisture content in the material layer were analyzed to better understand the microwave drying process of GBR.Then,the critical temperature and appropriate microwave intensity were proposed to ensure the final drying quality based on the mechanism of microwave drying on quality indexes.Thirdly,the drying process and quality changes of GBR in a continuous microwave dryer were investigated,and the simulation strategy to achieve the material movement on a conveyor belt was developed to imitate the moving drying process of GBR.The change mechanism of quality(color and fissure)was revealed by combining bench tests and simulation results.Finally,the processing conditions suitable for continuous microwave drying of GBR were developed to control final product quality.The main research contents and conclusions are as follows:(1)The drying characteristics and kinetics of GBR under microwave intensities of 1?5 W/g were investigated.The drying duration can be shortened remarkably by accelerating the drying process via increasing microwave intensity.The drying rate increased with the increase of microwave intensity,and no obvious constant rate period was found in the drying process,where it was mainly dominated by the falling rate period.Microwave drying resulted in microstructure changes inside GBR,and the formation of microscopic fissures or pores were conducive to moisture diffusion and evaporation to increase the drying rate.In the later drying period,the increased temperature of GBR caused by gradual accumulation of volumetric heat,resulted in gelatinization or melting of starch inside GBR.The increased moisture transfer resistance caused the decrease of drying rate.The effective moisture diffusion coefficient(D_eff)of GBR in the range of 1.55×10^-8 to 1.17×10^-7 m²/s,and the calculated value of activation energy(E_a)was 2.48 W/g,and the diffusion factor(D₀)was 1.76×10^-7 m²/s.The Midilli et al.model was the most suitable to describe the microwave drying of GBR via comparing six commonly used thin layer drying models.(2)Based on the coupled multi-physical field model involving electromagnetic,heat and mass transfer and the proposed simulation strategy for achieving material rotation,the simulation of rotational drying process of GBR was achieved.The simulated temperature and moisture were in good agreement with the measured ones,which proved that the model and simulation strategy developed can describe successfully the rotational microwave drying process of GBR.The spatial distribution of electric field and absorption of microwave energy by GBR layer during drying process were clarified based on the simulation results.The temperature distribution tended to extend from the center to the edge of GBR layer and increased gradually with the temperature gradient,while it was opposite for the moisture distribution.(3)The effects of different microwave intensities of 1?5 W/g on the fissures of GBR were explored,and the effects of fissures on its cooking quality were also investigated.Microwave intensity had significant effect on the fissure ratio of GBR after drying(p<0.05).As the microwave intensity was higher than 4 W/g,the fissure degree of GBR in the later drying period was mainly in range of 4?5 fissures.Considering the drying efficiency and suitable fissure degree(3?4 fissures),the microwave intensity of 3?4 W/g was suitable for the microwave drying of GBR.The mixed fissures between 3 and 4 in GBR kernel were prone to improve cooking properties(namely the lowest optimum cooking time and total solids loss,and highest water uptake ratio and volume expansion ratio)and increase the cooking quality(namely the lowest hardness,and highest adhesiveness,springness,cohesiveness,stickness and chewiness),which is the best suitable fissure amounts for microwave drying of GBR.The optimum cooking time was negatively correlated with water uptake ratio,volume expansion ratio and total solids loss(p<0.01).Hardness was positively correlated with adhesiveness and stickness(p<0.01),while it was negatively correlated with springiness and chewiness as well as cohesiveness(p<0.01).The observation of scanning electron microscope(SEM)confirmed that microwave drying destroyed the compact arrangement structure of starch granules inside GBR,and induced the generation of microscopic cracks and pores.The fissure amount of 3?4 provide suitable penetration routes of water into GBR kernel,and further improve the cooking quality.(4)The effects of microwave intensities of 1?5 W/g on the quality attributes(color and GABA content)of GBR were examined,and the correlations between drying properties and quality formation were analyzed.Increasing microwave intensity accelerated the color changes of GBR,and the critical temperature to avoid serious browning and charring kernels inside material layer was 132 ^oC and 170 ^oC,respectively.Fractional conversion model was most suitable for describing the color change kinetics of GBR.The GABA content of GBR overall tended to decrease,and suitable average temperature to retain high GABA content may be governed in the range of 64?67^oC,which can provide parameter basis for retaining high GABA content of GBR.Considering the drying efficiency,color and nutritional quality,microwave intensity of 3?4 W/g may be suitable for microwave drying of GBR.(5)The continuous microwave drying process of GBR was achieved by building the coupled multi-physical field model and the simulation strategy to achieve GBR layer moving on a conveyor belt.Meanwhile,the technology conditions suitable for the golden appearance and the formation of suitable fissure(3?4 fissures)for GBR after drying were optimized.The formation mechanism of fissures in kernels was analyzed by simulating the moisture and stress in a single rice kernel.The movement of the material in microwave cavity can improve the drying uniformity of final product.The stress along the short axis of rice kernel increased with the increase of microwave intensity,which aggravated the evolution of fissures in kernel.As the microwave intensity was lower than 5 W/g,the moisture distribution in the rice kernel was relatively uniform,which resulted in appropriate fissure degree after drying.Increasing air velocity can strengthen the evaporation and diffusion of moisture inside rice kernel,and further increased the moisture gradient inside and outside the rice kernel,resulting in the fissure propagation and evolution.The temperature range suitable for the formation of golden appearance color quality of GBR during microwave drying was 90?132 ^oC.The suitable technology parameters to achieve final quality for continuous microwave drying of GBR were determined as microwave intensity of 4 W/g,air velocity of 1.0 m/s,drying time of 10 min for per drying pass and tempering ratio of 1:2.Research results may provide the theorical and technology basis for microwave drying GBR with high efficiency and desirable quality.