Study On Microwave Vacuum Drying Characteristics And Process Optimization Of Dendrobium Officinale

Dendrobium officinale is a perennial herb of the genus Dendrobium of Orchi daceae.It is a rare traditional Chinese medicine and has the functions of nourishing the stomach,nourishing Yin and clearing heat,enhancing immunity,etc.Fresh Dendrobium officinale has a high moisture content.It browns easily and is often infected by microorganisms after being placed at room temperature for a week or so,thus reducing its edible quality and affecting its commercial value.Dendrobium officinale dried by traditional hot air has high shrinkage rate,uneven structure and poor water reactivity.Microwave vacuum drying is a good combination of the advantages from microwave drying and vacuum drying,which is suitable for drying heat-sensitive materials and improving product quality.In this paper,Taking fresh Dendrobium officinale as the research object.The influence of parameters of microwave vacuum drying on the quality characteristics of Dendrobium officinale was studied,and the optimum drying process parameters were determined.The effects of different drying methods on the quality characteristics of Dendrobium officinale were also studied.This study provides scientific basis for the optimization of microwave vacuum drying and the quality control of Dendrobium officinale.The main research contents are as follows:1.The technology of microwave vacuum drying was used to dry the Dendrobium officinale,and the effects of three factors,including microwave intensity,vacuum degree and material length,on the drying characteristics of Dendrobium officinale were studied.The results suggested that the process of microwave vacuum drying could be divided into three stages:acceleration,constant speed and descending speed according to the water loss rate.Increasing the microwave power,increasing the vacuum degree and reducing the loading amount increase the rate of water loss,as well as shorten the drying time.In the microwave vacuum drying of Dendrobium officinale,five drying models of Henderson and Pabis,Wang and Singh,Page,Tian,and Lewis were used to fit the drying process and establish the drying kinetic model of Dendrobium officinale.According to the fitting coefficient and the fitting curve,under the conditions of different microwave intensities,the fitting degree of the equation of the Wang and Singh model was the highest,of which the R²value was above 0.99,while its RMSE?0.012?and?²?0.0218?values were the smallest.In addition,the Page model has a good fitting effect on the drying curve of Dendrobium officinale under different vacuum degrees and material lengths,with its R²value being above 0.99,RMSE and?²values being the smallest.2.The effects of microwave power,vacuum degree and material length on polysaccharide content,color,polyphenol oxidase,water distribution status,total free amino acid and DPPH·free radical and·OH radical content of polyphenols in Dendrobium officinale were studied.Compared with the fresh samples,when the microwave intensity reached9 W/g,the dried samples had a maximum polysaccharide content of52.21%.The results of amino acid analyzer showed that the contents of total free amino acids and flavor amino acids of Dendrobium officinale were significantly increased after microwave vacuum drying.Under the microwave intensity of 9 W/g,12 W/g and 15 W/g,the PPO activity of Dendrobium officinale dried by microwave vacuum did not exist after the16th,12th and 6th minute,respectively,indicating that microwave vacuum drying could passivate the PPO activity of Dendrobium officinale in a short time,and a higher the microwave intensity leads to a shorter time required for passiviation.Maillard reaction is the main reason for non-enzymatic browning in the drying process.The results of low field NMR showed that the free water relaxation time(T₂₃)and its peak area A₂₃were significantly reduced.The results showed that Dendrobium officinale polyphenols still had strong hydroxyl radical scavenging ability.By observing the microstructure of the sample after drying,the pore-like structure of the sample was seriously damaged with the increase of microwave power density.The higher the microwave intensity,the better the rehydration characteristics of Dendrobium officinale.3.With the polysaccharide content,DPPH·radical scavenging rate,and total polyphenol content of Dendrobium officinale as evaluation indicators,the response surface method was used to optimize the parameters of microwave vacuum drying,incluiding microwave intensity,vacuum degree,and material length.The optimal conditions for microwave vacuum drying of Dendrobium officinale are:microwave intensity is 12 W/g,vacuum degree is-75 k Pa,and material length is 5cm.4.The effects of four drying methods,including hot air drying,vacuum drying,microwave vacuum drying and freeze drying,on the content of polysaccharides,color,polyphenol oxidase,water distribution,and free amino acids were studied.The scavenging effects of polyphenols from Dendrobium officiale on DPPH·free radicals and·OH radicals were also studied.GC-IMS analysis of volatile substances showed that a total of 42 kinds of volatile substances were detected,with the most volatile substances detected under hot air drying,and the peak area of volatile substances was 35132.19 a.u.The total amino acid content from each drying methods was ranked as:microwave vacuum drying group>vacuum drying group>freeze drying group>hot air drying group.Freeze-drying is superior to other drying methods in terms of polysaccharide content,polyphenol content,and antioxidant capacity.Comprehensive comparison of four different drying methods suggest that hot air drying and vacuum drying are not as good as microwave vacuum drying or freeze drying.Further considering the energy consumption for drying,microwave vacuum drying can better retain the nutrients and antioxidant capacity of Dendrobium officinale...