| Drying of Chinese herbal extract is a key preparation step for the manufacture of traditional Chinese medicine,which is characterized with long drying time and high energy consumption.It is a practical problem to be solved urgently to explore the appropriateness of using green new drying technology for the drying of extract.As a new type of green energy-saving technology,ultrasound-assisted drying effectively improves drying quality while raising drying efficiency.However,there is a lack of clear characteristics and strengthening mechanism of the ultrasound-assisted drying of traditional Chinese herbal extracts,restricting this technology from being applied in traditional Chinese medicine pharmaceuticals.The simulation study on the heat and mass transfer mechanism and process lays a solid foundation for this technology to be promoted.In this study,the Schisandra chinensis extract,which is rich in polysaccharides and of high viscosity,and the licorice extract used widely and easy to foam with saponins,were taken as the objects of study.The drying temperature,ultrasound power,ultrasound action time,vacuum degree and other factors were investigated for drying kinetics influence on the extracts.The low-field nuclear magnetic resonance(LF-NMR)technique was used to study the water migration law during the drying process of the extract.The effects of vacuum drying and ultrasound-assisted vacuum drying on the physical and chemical properties of the extract powder were compared and analyzed to explore if the ultrasound-assisted vacuum drying technology fits the extract.The near-infrared spectroscopy(NIR)and hyperspectral image(HSI)technologies were used to establish a prediction model for the moisture content of the extract drying process,providing technical support for judging the production end point and saving energy and reducing consumption.Based on the simulation software of COMSOL Multiphysics,a multi-physical field heat and mass transfer model of traditional Chinese medicine extract dried through ultrasound-assisted vacuum drying was established to reveal the mechanism of ultrasound-enhanced drying through visualized path.The results showed that ultrasound-assisted drying effectively improved the average drying rate during the drying process,thereby reducing the drying time.At a temperature of80℃,the conventional vacuum drying time of Schisandra chinensis extract and licorice extract were 960 min and 660 min respectively,reduced by 540 and 360 min,and 43.75%and45.45%,respectively with the ultrasound-assisted drying.Because of the shortening of drying time,the unit energy consumption of ultrasound-assisted vacuum drying of schisandra chinensis extract and licorice extract was reduced by 25.26%and 27.52%,respectively at the temperature of 80℃.Both Weibull function and Dincer function better simulate the drying kinetic curves of Schisandra chinensis extract and licorice extract,and calculate the effective water diffusion coefficient,activation energy and water mass transfer coefficient in the drying process,providing key parameters for subsequent process simulation.There is mainly bound water in the Schisandra chinensis extract(98.72%),and semi-bound water in the licorice extract(96.25%).Ultrasound-assisted vacuum drying transform the difficult-to-flow water into free water,thereby accelerating drying.Ultrasound-assisted vacuum drying has a high retention rate for the index components of the extract,however,too long ultrasound time and too high ultrasound power affect the stability of chemical components.The physical properties of samples after ultrasound-assisted vacuum drying were not much different from those of the vacuum-dried samples.Ultrasound-assisted drying promote the formation of more and denser pores in the drying process which is more obvious as the ultrasound power increases,which is beneficial to the migration of drying moisture.NIR moisture prediction model for the licorice extract drying process was used to predict the drying moisture of Schisandra chinensis extract and the PLS test set Rp and RMSEP were0.9589 and 0.1592,respectively,indicating that the NIR moisture prediction model has good cultivar suitability.HSI moisture prediction model of licorice extract at selected hyperspectral bands(610.9-639.7 nm,853.4-879.1 nm and 946.6-972.8 nm)was established to predict the drying moisture of Schisandra chinensis extract,getting results of good accuracy,and the Rp and RMSEP of the prediction set were 0.8278 and 0.1636,respectively.Clarified characteristic spectral bands can provide data support for the subsequent detection equipment to realize low-cost and portability.The results of the model of ultrasound-assisted vacuum drying heat and mass transfer process showed that:when at the ultrasound power of 200 W,the local sound pressure amplitude in the extract exceeded the cavitation sound pressure threshold of 0.7 MPa,possibly generating cavitation effect in the extract.Besides,it is not advisable to use a high-frequency ultrasound generator for ultrasound-assisted drying of the extract.The ultrasound thermal effect increases with the increase in ultrasound power.At a ultrasound power of 40-200 W,the temperature rose about 2-10℃after 60 min of ultrasound treatment.Due to the ultrasound thermal effect,ultrasound-assisted vacuum drying contributes to the uniform distribution of moisture in the drying process,forming multiple"dry-wet"boundary layers,which is more conducive to the migration and evaporation of moisture.According to comparison between the predicted value of the model and the experimental results,there is a maximum difference of 1.3℃between the experimental value of the heat transfer model and the theoretical value;the determination coefficient R~2 of the water and mass transfer model were all greater than 0.9531,and the RMSE were all less than 0.0752,indicating that the drying process model was established to be reasonable and reliable,providing a reference for other extract materials in the process design and the development of drying equipment. |
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