Study On Solar Low Temperature Drying Characteristics And Heat And Mass Transfer Mechanism Of Astragalus Slices

Astragalus membranaceus is a kind of medicinal and edible Chinese herbal medicine.It has rich medicinal value and has been paid more and more attention.However,the initial processing technology of fresh Astragalus membranaceus is backward and imperfect.Especially in the drying of fresh Astragalus membranaceus,traditional drying methods such as sun drying and shade drying are still used.There are problems such as long drying time and serious loss of medicinal components.Aiming at a series of problems such as long drying time and serious loss of medicinal ingredients in the drying process of fresh Astragalus membranaceus,the fresh Astragalus membranaceus in Hohhot,Inner Mongolia Autonomous Region was taken as the research object,and the solar low-temperature drying characteristic test bench was used to study the effects of different drying conditions on the quality of dried products of Astragalus membranaceus slices.Through the optimization of process parameters and the establishment of heat and mass transfer theoretical model,COMSOL Multiphysics software was used for numerical simulation verification,and the heat and mass transfer drying mechanism in the drying process of Astragalus membranaceus slices was comprehensively expounded.The results of this study provide a suitable drying process for the initial processing of Astragalus membranaceus and provide a theoretical basis for actual production.The main research is as follows:(1)Free water content is the main limiting factor in the solar low temperature drying process of Astragalus membranaceus.Taking drying temperature,slice thickness and hot air velocity as experimental factors,moisture ratio and drying rate as evaluation indexes,the kinetic characteristics of solar low temperature drying of Astragalus membranaceus slices were studied.Through single factor experiment,it was found that with the increase of drying temperature and hot air velocity and the decrease of slice thickness of Astragalus membranaceus,the decrease rate of moisture ratio of Astragalus membranaceus slices increased,and the drying rate increased.Among them,40-50°C was a suitable drying temperature range,4-6 mm was a suitable slice thickness range,and0.75-1.0 m/s was a suitable hot air velocity range.(2)Based on the single factor experiment,through the establishment of Wang and Singh,Newton,Page,Weibull and Logarithmic five kinds of drying model of Astragalus solar low temperature drying factor fitting test,determine the Astragalus slice solar low temperature drying characteristics,found that the Page model of the evaluation parameter determination coefficient R~2in 0.99824-0.99981,residual sum of squares SSE in0.00002-0.00211,the highest fitting accuracy,is the most suitable Astragalus solar low temperature drying kinetic model.The moisture diffusion coefficients under different drying conditions were calculated.It was found that the slice thickness and drying temperature of Astragalus membranaceus were significantly higher than the effective moisture diffusion coefficient,while the hot air velocity had little effect on the effective moisture diffusion coefficient.(3)The 2020 edition of the Pharmacopoeia of the People’s Republic of China has strict requirements on the content of calycosin and astragaloside IV.By detecting the content of main index components(astragaloside IV,calycosin)and functional components(formononetin)and characterizing sensory quality(color)of dried astragalus slices,it was found that the significance of different drying conditions on the quality of astragalus was:slice thickness>drying temperature>hot air speed.The color of dried Astragalus products will change with the change of experimental factors.The significance of color affected by different drying conditions is:hot air speed>slice thickness>drying temperature.Comprehensive analysis showed that the total color difference was the smallest when the drying temperature was 45°C,the slice thickness was 5 mm,and the hot air speed was 0.8 m/s.The color of Astragalus slices under this drying condition met the appearance requirements of Astragalus medicinal materials.(4)On the basis of single factor experiment,the content of astragaloside IV,rehydration ratio,color difference and drying rate in dried Astragalus slices were used as response surface evaluation indexes,and the slice thickness,drying temperature and hot air speed of Astragalus slices were used as the three factors of the experiment.The response surface optimization test of three factors and three levels was carried out.The Box-Benhnken central composite experimental design and multiple regression model were used to optimize the process parameters.The optimal combination of process parameters for solar low temperature drying of Astragalus slices was obtained as follows:drying temperature 45°C,slice thickness 5 mm,hot air speed 0.8 m/s,and the content of astragaloside IV was 0.12%.The maximum values of color difference and drying rate were 5.36 and 0.0704%·min-1,respectively,and the optimal value of rehydration ratio was 3.07.(5)The COMSOL Multiphysics simulation software was used to model the astragalus slices,and the water concentration distribution and temperature distribution during the drying process were numerically simulated.The main conclusions are as follows:During the drying process,the external temperature of the astragalus slices increased preferentially,forming a temperature gradient.The overall temperature distribution showed a diffusion trend from the outside to the inside,which was consistent with the low temperature drying characteristics of solar energy.The moisture distribution inside the slices of Astragalus showed a trend from inside to outside.With the increase of external temperature,the surface moisture began to evaporate,forming a moisture gradient.As the drying progressed,the internal temperature increased,the absorbed energy increased,and the moisture diffused outward,thereby reducing the internal and external moisture gradient.The change law of moisture in astragalus slices basically conforms to the change trend of moisture ratio in drying curve.Therefore,solar low temperature drying technology can provide a high efficiency and high quality drying technology for the initial processing of Astragalus membranaceus.