Study On The Microwave Vacuum Drying Characteristics And Drying Technology Optimization Of Scallion

China had large cultivation and output of scallion, and Dried-scallion whichwas the mainly exports holds quite big market share in the international market. Themicrowave vacuum drying （MVD） technology was applied to scallion drying in thistopic. The MVD characteristic of scallion was studied, the drying dynamics modelwas established. and the relations of different drying parameters, drying time andmoisture content were quantified; Moreover, the topic studied the main quality effecton MVD scallion, the key point of quality changes and the best processing parametersof MVD scallion on the basis. The topic compared the consumption, the efficiencyand the main quality of MVD scallion with the traditional hot-air drying and thevacuum freeze-drying of scallion and analyze the feasibility of. MVD scallion.1、The microwave vacuum drying （MVD） technology was applied to scallion dryingin this topic. MVD characteristics of scallion was studied by the effects of microwavepower, vacuum degree, loadage and stack thickness on the dehydration rate. Theresults showed: MVD process of scallion was divided into three stages: speed-up,constant-speed and speed-down phase. The dehydration rate was quicker when themicrowave power and vacuum degree were higher and the loadage was lower. Thedrying time was shortest when the stack thickness of scallion was50mm. Dryingtime would be extended and the dehydration rate would be decreased as the stackthickness was increased or decreased. It was the significance level of the drying rateby the different drying parameters: microwave power> loadage> stack thickness>vacuum degree.2、The drying procedure of scallion could accurately be described by the Page modelNand satisfied the Page equation MR=e^-nN, of whichr=e⁽-3.2429-0.0029x₁+0.0054x₂+0.3491x₃+0.0043x₄andN=2.3827-0.0005x₁+0.0183x₂+0.2148x₃+0.0005x₄，R²=0.9209. There was thetest of goodness for fit, the model by which the law of moisture variation during thedrying procedure can be exactly estimated and could be used for describing MVDcharacteristics of scallion．3、The allinase activity of scallion was an important indicator to weight intrinsicquality of scallion. The quality of scallion would be higher as the activity of allinase was stronger. The pyruvate was intermediary product from the reaction of allinase. Toanalysis the changing regularity of allinase activity characteristics, the pyruvatecontent was used as the index to study the effect of different MVDparameters--microwave power, vacuum degree, stack thickness, loadage on thematerial temperature and drying time during the MVD process. The results indicatedthat the allinase activity would be decreased with the vacuum degree decrease and themicrowave power increase, and the allinase activity grew in their early stage anddropped later with loadage and stack thickness increase; The allinase activity wasstrong under the low temperature and short-time dry condition. The allinase activitywas influenced more by drying temperature than the drying time.4、The various qualities index of scallion, chlorophyll content, the sensory quality andthe rehydration, grew in their early stage and dropped later with higher vacuumdegree. It presented increased at first and dropped later with increasing microwavepower and the loadage, but the effect of microwave power is not remarkable.5、The chlorophyll content, the pyruvate content, the rehydration ratio as evaluationindexs were taken, L9（34） orthogonal design was adopted to optimize processingparameters: the vacuum degree, the microwave power and the loadage. The besttechnological conditions of MVD scallion were A2B2C1, namely vacuum degree-80kPa, microwave power3kW, stack thickness25mm.6、Effects of three drying methods MVD, hot-air drying, and vacuum freeze-drying onthe consumption, efficiency and the main quality of dried-scallion were studied withthe microstructure, allinase activity, chlorophyll content, rehydration ratio, sensoryquality and consumption as indexs. The results showed that the MVD has the shortestdrying time and lowest drying consumption. The chlorophyll content of the MVDscallion was higher than FD and AD in turn; While the rehydration ratio and theallinase activity of the MVD scallion were inferior to FD but higher than AD; theMVD scallion was emerald green and flat shape with the strong scallion fragrant, itstexture was pliable, but not easy to break and embrittle in front of the MVD samplerehydrating. After MVD sample rehydrating, the water which was used by rehydrationhad the strong scallion fragrant and sharp taste, but its color was breen. The fuzzymathematics method was used to synthetical evaluate the sensory quality, the result show that: MVD> FD> AD, namely the sensory quality of the MVD scallion was best;the scallion microstructure was destroyed most seriously, organization and fiber wasbroke, collapsed, overlapped, but the FD and MVD samples organizational structuremaintained well. The FD scallion retained the loose and porous reticulate structure,the structure was flat with the contraction slightly; The contraction level of the MVDscallion was higher than AD and FD obviously, the stem was not smooth, theorganizational structure was compact, but the MVD scallion reticulate structure wasstill clearly discerned.