Study On The Dielectric Properties Of Berry Under Microwave Conditions

Select the blue honeysuckle berry and raspberry of typical berry as materials studied, thechanges of dielectric properties in microwave processing were investigated. The microwaveassisted foam-mat drying （MFD） method is suitable for dehydration processing of berry with thehigh viscosity properties and heat-sensitive components. The microwave assisted extractionmethod was applied to extract the active ingredients of berries such as anthocyanin, which ischaracterized by high extraction efficiency and application of green solvents for example ethanol.During the microwave heating processing, dielectric properties of material play a key role in thebehavior of material in high frequency electromagnetic field, which influence the microwaveenergy absorption and conversion within material. Dielectric properties include dielectric constantand dielectric loss factor.In the condition of MFD the changes of temperatures of blue honeysuckle puree followed theexponential equations. The changes of dielectric properties of foamed berry puree may bedescribed using the functions of temperature and moisture content considering the variablephysical properties of material. These results contributed to reveal the mechanism of microwaveenergy absorption and conversion within the material during microwave processing.In this research, the changes of dielectric properties within raspberry powder in themicrowave-assisted foam drying process were studied. The effects of factors in terms ofmicrowave power, wind amount, material mass and drying time on the degradation ratio ofanthocyanin and vitamin C of raspberries were studied by using responding surface methodology.The effects of temperature and moisture of material on the dielectric properties were studied. Theconclusions were developed as follows:（1） The effects of each factor at descend trend ondielectric constant and dielectric loss factor were microwave materials mass, drying time,microwave power and wind amount.（2） The effects of each factor at descend trend on degradationratio of anthocyanin were microwave power, drying time, wind amount and material mass.Degradation ratio of anthocyanin was positive correlation with microwave power and drying time.（3） The effect of each factor at descend trend on degradation ratio of vitamin C were material mass,microwave power, wind amount and drying time.In the study of microwave assisted extraction of anthocyanin from raspberry powder theextraction time, extraction temperature, ethanol concentration, material-liquid ratio were selectedas variables. The yield of anthocyanin extracted from berry and the variation of dielectricproperties were studied. The results showed that:（1） the effects of each factor at descend trend onthe yield of anthocyanin extracted from berry were material-liquid ratio, ethanol concentration, extraction time and extraction temperature.（2） The effects of each factor at descend trend onextraction dielectric constant were material-liquid ratio, extraction temperature, extraction timeand ethanol concentration.（3） The effects of each factor at descend trend on dielectric loss factorin extract liquor were material-liquid ratio, ethanol concentration, extraction time and extractiontemperature. With material-liquid ratio increased, dielectric loss factor of extract liquor decreased.Heat and mass transfer process within foamed raspberry under microwave assisted foameddrying conditions was simulated by using Matlab software, which described the variation ofmaterial temperature and moisture content with the thickness of material. When initial materialmass was400g, microwave power was7kW, wind amount was70m³·h^-1and drying time was240s, material temperature and moisture variation with drying time and material thickness wasdetermined. Under the same drying time, material temperature was increased then decreased withthickness and moisture content. Under the same material thickness, temperature tended toincreased trend and moisture content was decreased with increasing drying time. Thethree-dimensional surface of the material temperature and moisture changed with the thickness andthe drying time was drawn under different power. When microwave power and initial materialmass were changed simultaneously, the variation of the material of internal temperature wassimulated. The material moisture content variation was obtained under different initial materialmass. The initial thickness of pulp influenced significantly moisture content under differentmaterial thickness. When initial thickness of material was at low level, moisture of materialsurface quickly evaporated, and the moisture evaporation decreased from top to bottom layer alongmaterial thickness. While initial material thickness was still in high level, the surface evaporationof material was slow. From material top to the middle, moisture quickly evaporated; while fromthe middle to the bottom, there were no significant changes of moisture evaporation.The simulation results agreed well with the experimental data. The simulation results ofmicrowave foamed drying process were reliable, which guide the experiment implement andpractical production. The simulation results also provided theoretical basis for applying microwaveassisted foamed drying method efficiently and producing high quality pulp.