Experimental Study On Drying Characteristics Of Microwave Vacuum Drying Jujube Slices

Jujubes has a long history of planting in China,and its annual output ranks first in the world,with unique nutritional and medicinal value.In this paper,the microwave vacuum technology was used to dry the jujube slices.We applyed the microwave vacuum technology to dry the jujube slices and designed microwave vacuum drying production line control system with PLC as controller.Firstly,based on the microwave vacuum drying characteristics of jujube slices,BP neural network model was carried out.The effects of drying parameters on the main quality of red jujube slices after drying were studied.Finally,the indexes of drying test were optimized and verified.Main research contents and conclusions are as follows:(1)This paper built electrical and software development of the microwave vacuum drying production line control system based on PLC.The control system adopted one siemens S7-200 series CPU as the main control unit.Through the touch screen in real-time monitoring parameters of microwave intensity and vacuum degree,these parameters used PID control,so it makes the microwave vacuum drying production line in a stable condition and improved the productivity and the drying quality.(2)Microwave vacuum drying of jujube slices was mainly divided into two stages: speed-up drying and speed-down drying.The constant-speed drying stage was not obvious.Different microwave intensity had significant effect on the drying rate.When the microwave intensity increased,the drying rate increased and the drying time were shortened.The higher the relative pressure,the shorter the drying time.In the initial drying stage,the drying rate increased as the relative pressure increased,but when the relative pressure was30 k Pa,which was significantly higher than the initial drying rate at a relative pressure of 40 k Pa to 60 k Pa.It may be because the relative pressure was low,which was beneficial to the heat conduction of the material,and improved the drying rate of the jujubes slice;when the material was thin,the microwave energy can quickly pass through the middle of the material,the drying time was obviously shortened,and the drying rate was remarkably improved.(3)Selecting microwave intensity,relative pressure,slice thickness,drying time as different drying parameters to establish BP neural network model between these parameters and moisture content.It can predict the moisture content of the jujube slices during the drying process.It can also predict moisture content of other fruits and vegetables.For the optimization of the microwave vacuum drying process of jujube slices,we can refer to the model.(4)The compression mechanical properties of red jujube slices treated with different drying parameters were tested by texture analyzer.The force-deformation curves were measured,and the mechanical properties and their changes were analyzed.The destructive force,failure energy,compressive strength,elastic modulus and crispness of jujube slices were obtained.The variance analysis was carried out by SPSS software.The results showed that different slice thickness had significant effect on destructive force,failure energy,compressive strength and crispness.Different microwave intensity had certain effect on destructive force and failure energy and crispness,different relative pressure had certain effect on failure energy and different slice thickness had certain effect on elastic modulus.The influence of microwave intensity on compressive strength and elastic modulus was not significant.The influence of relative pressure on destructive force,compressive strength,elastic modulus and crispness was not significant.The influence of different drying parameters on each mechanical property parameter was:slice thickness>microwave intensity>relative pressure.Finally,analysing the correlation among the parameters of mechanical properties.(5)After selecting the microwave intensity,relative pressure and slice thickness as different drying parameters for single factor test,set the color difference,rehydration ratio and crispness of jujube slices as the response value,and using Design-Expert analysis software to adopt Box-Behken design test.The research on the microwave vacuum drying process of jujube slices was carried out.The results showed that the regression degree and significance test of the regression equation showed that the regression variance of each index was extremely significant;By setting the index,the color difference was the minimum value,the rehydration ratio was the maximum value,and the crispness was between 4N and 6N.After taking the value and the simultaneous regression equation,the optimum conditions for drying the jujube slices were as follows:the microwave intensity was 4 W/g,the relative pressure was 52.3k Pa,and the slice thickness was2.15 mm.It had been verified that the experimental values of the indicators were close to the predicted values,The regression model can be used to predict the quality of the jujube slices.