Research On Microwave Vacuum-Hot Air Sectional Drying Process Of Panax Notoginseng

Panax notoginseng（Burk.）F.H.Chen,which is one of the precious traditional Chinese medicines in China,mainly produced in Wenshan Prefecture,Yunnan Province.At present,the harvested Panax notoginseng is mainly used for dried food or medicine consumption,except for a small amount of fresh food.However,the drying of it is still mainly natural drying,which not only takes a long time,but also is easily affected by weather,insects and microorganisms,resulting in the decrease of bioactive substances and affecting the quality of Panax notoginseng.In recent years,microwave vacuum drying technology has been more and more used in the processing and production of agricultural products and traditional Chinese medicine because of its high drying efficiency,short time,low temperature and so on.However,the drying rate of microwave heating is too fast,which is easy to produce"hot spots"and lead to local overheating or scorched paste,which is not conducive to the retention of heat-sensitive substances such as saponins,resulting in a decline in quality.Therefore,sectional drying can be used,and hot air can be used as the heat source in the later stage of drying to ensure its drying quality and uniformity.Therefore,this paper takes Panax notoginseng as the research object,uses microwave vacuum-hot air sectional drying technology to dry Panax notoginseng,and studies its drying characteristics and the effects of different microwave vacuum drying parameters on the drying time and quality of Panax notoginseng.And compared with hot air drying to determine the feasibility of the new drying process.Then the response surface method is used to optimize the microwave vacuum-hot air sectional drying process and determine the best drying process parameters.Finally,the law of water transfer and mechanical properties in the drying process of Panax notoginseng are studied.To provide theoretical reference and technical support for the industrial production of Panax notoginseng.The main contents and conclusions of this paper are as follows:1.The influences of different conversion moisture content（20%,30%,40%）,power density（0.5,0.75,1,1.5 W/g）,chamber pressure（1,3,5,10 k Pa）and upper temperature limit（45,50,55℃）on the drying process of the main root of it were studied,and the drying characteristics were analyzed,and the fitting model of sectional drying was established.The results show that the microwave vacuum-hot air staged drying of Panax notoginseng is mainly in the stage of decreasing and constant speed,and the constant speed stage occurs in the stage when the moisture content of dry base decreases from 1 g/g to 0.5 g/g.In the drying process,the D_eff of microwave vacuum section is between 7.84×10^-10 and 1.80×10^-9 m²/s,which increases with the increase of temperature upper,power density and the decrease of chamber pressure,and the D_eff of the second stage is between 1.45×10^-10 and 6.82×10^-10 m²/s,and its value is related to the change of drying conditions of the first stage.Through the sectional fitting of the drying process,it is found that the Weibull model is most suitable to describe the law of microwave vacuum-hot air sectional drying of Panax notoginseng.2.In the process of microwave vacuum-hot air staged drying of the main root of Panax notoginseng,the effects of different power density,temperature and chamber pressure on the content and drying time of total saponins of it were studied.Compared with the dried products of Panax notoginseng under the best hot air drying conditions in the literature,the feasibility of the new drying process was determined.The results showed that the total saponins content of Panax notoginseng dried products obtained by microwave vacuum-hot air staged drying process decreased with the increase of power density and drying temperature,and increased with the increase of chamber pressure.The content of total saponins is more than 5%and it is close to that of the dried products obtained by hot air,which meets the requirements of the Pharmacopoeia for the content of saponins.The drying time of the new drying process can be significantly shorter than that of hot air drying:44%～80.6%.3.Taking power density,temperature upper limit and chamber pressure as influencing factors,microwave vacuum drying time,total drying time,total saponins content and color difference as response values,using three factors and three levels of Box-Behnken experimental design,microwave vacuum-hot air sectional drying process was optimized and the best drying process parameters were determined.The results show that the optimum process parameters of microwave vacuum-hot air staged drying of Panax notoginseng are as follows:power density 0.5 W/g,temperature upper limit 50℃,chamber pressure 7 k Pa.Under these conditions,the relative errors between the predicted value and the verified value are less than10%,and the test values can be obtained as follows:microwave vacuum section drying time 6h,total drying time 56 h,total saponins content 9.8%,color difference value 32.64.4.In order to describe the change process of moisture and physical characteristics of Panax notoginseng during microwave vacuum-hot air sectional drying,the axial compression test,radial puncture test,internal and external moisture change test and volume change test with the decrease of moisture ratio from 1 to 0.1 were studied.The results showed that the volume of the main root of Panax notoginseng decreased linearly with the decrease of water ratio,and the linear fit was good;with the progress of drying,the difference of water ratio between the central xylem and outer skin of Panax notoginseng increased at first and then decreased,and the axial hardness and surface radial hardness decreased at first and then increased,which was related to the outward diffusion and evaporation of water in Panax notoginseng.Comprehensive analysis of these experimental results can describe the internal water transfer and diffusion evaporation process in the process of microwave vacuum-hot air drying of Panax notoginseng.That is,the acceleration period in the early stage of drying is that the water diffusion rate of the outer skin of Panax notoginseng increases with the increase of temperature and gradually forms the gradient difference of water concentration,and the internal water gradually spreads outward.In the middle stage of drying,the gradient difference of water concentration tends to be stable,the difference of internal and external moisture content tends to decrease,and the drying rate continues to decrease and stabilize.At this time,due to the increase of the overall hardness of the material by water evaporation,the surface hardness of the outer skin continues to increase,and the steam balance moves towards the interior of the material.In the later stage of drying,the difference of water concentration gradient and the difference of moisture content between outer skin and xylem gradually decreased,the gasified surface continued to move inward,the hardness of Panax notoginseng gradually increased,and the volume of the main root of Panax notoginseng continued to shrink and tended to be stable.