Study On The Mechanism & Quality Of Microwave Vacuum And Drying & Puffing Apple Slices And Machine Desigen

Microwave vacuum drying combines the advantages of both high efficiency of microwave heating and low temperature vacuum drying. Applying microwave to dry materials under the condition of low temperature can realize materials low temperature fast drying thus it has the potential to improve energy efficiency and product quality. Treating apple slices with microwave vacuum drying can not only dry but also have better puff effect and apple slices can be both dried and puffed by microwave vacuum within few minutes. Here drying characteristics and expansion properties of microwave vacuum drying & puffing apple slices were analyzed, the operating conditions was optimized, microwave vacuum drying & puffing apple slices were comprehensively analyzed and evaluated, and the microwave vacuum drying & puffing equipment has been designed such as the structures of drying chamber, microwave power supply and other key parts. The main research content and results in this dissertation were summarized as the following:1. Drying characteristics of microwave vacuum drying & puffing apple slices were studied. Inner water of apple slices is vaporized by the following three different transferring mechanisms: immigration under moisture gradient, vapor diffusion and direct removal of liquid water forced by pressure & temperature gradient so the drying time is shorter and the drying speed is higher. The pump removal by vapor diffusion can expand the structure of apple slices to obtain a puffy porous texture.2. Effects of microwave power, vacuum level, apple slices thicknesses and initial moisture content of pretreated apple slices on the drying & puffing process, time and speed were analyzed in detail.3. Expansion properties of microwave vacuum drying & puffing apple slices were studied. Effects of microwave power, vacuum level, apple slices thickness and initial moisture content of pretreated apple slices on the expansion ratios were analyzed in detail. Results showed that under the condition of microwave power was 12.0W/g with the vacuum level of 0.085MPa, the apple slices thickness was 8mm and initial moisture content of 60%, the expansion ratio reached its maximum and remained 321%.4. At the apple slices thickness of 8mm, in terms of microwave power, vacuum level and initial moisture content, the predicted second-order regression model of the drying time, sensory quality and porosity values of apple slice were obtained according to the experimental data by using orthogonal rotatable central composite design, and effects of these variables and their interaction on the drying time and product quality were analyzed by response surface methodology.5. The predicted second-order regression equation of synthetical performance parameters assessed for the drying time, sensory quality and porosity values of apple slices was established. Considering the higher drying efficiency and product quality, the optimum data were found: when thickness of apple slices is 8mm, microwave power is 12.0W/g, vacuum level is 0.089MPa and initial moisture content is 69.2% . 6. Comparison analysis on energy consumption and product quality of apple slices with microwave vacuum drying and convective air drying has been done. Drying capacity of microwave vacuum increased 48.46%, and energy consumption decreased 32.32% per kilogram water compared with convective air drying. The apple slices had better texture and flavor, more honeycomb network structure and larger pores, and the reservation rate of vitamin C content was 15.8% higher than those of air dried.7. Design method of the drying chamber was analyzed. Firstly, the total microwave power was calculated base on the productivity, and the maximum power dissipation density was figured out according to the maximum breakdown electric field density referred to the chart of the manual, then the minimum volume was calculated, at last the drying chamber diameter of 600mm and the length of 1200mm considering material coefficient, power density, microwave field uniform and etc were determined. Moreover, it should be high character factor of the drying chamber.8. The structure and optimum parameters of the microwave system and the material inlet and outlet system, the composing of vacuum system and the control system were studied and confirmed. The results of the equipment performance test proved its advantages of reasonable structure, sample manipulation, steady performance and reliable security, and microwave power could reach 6.59kW, the productivity of apple slices is up to 8.38 kg/ h, vacuum level could be adjusted from 0.080MPa to 0.096 MPa.