Experimental Study On Microstructure Changes Of Fruits And Vegetables During Drying

China has a large scale of fruit and vegetable cultivation and a long history,it is the world's largest country for the production and processing of fruits and vegetables.Drying as an important form of post-production deep-processing of fruits and vegetables,occupies a particularly important link in today's fruit and vegetable industry chain.During the drying process,due to the complex porous structure of fruits and vegetables,the moisture and temperature distribution in the material will be affected by the changes of cell tissue and its structure.In order to further improve the quality and efficiency of drying,studying the drying process at the cellular structure level becomes even more important for the fruit and vegetable tissue structure impact.This subject will study the microstructure changes of fruit and vegetable tissues during the drying process,paraffin section and microscopy are used to obtain the tissue and cell structure image of materials.At the same time,the index system for measuring the microstructure parameters of materials is proposed based on the tissue and cell image.The relevant microstructure parameters are obtained by image processing software such as MATLAB and Photoshop,and then the data statistics software such as SPSS and Origin are used to analyze the microstructure parameters,and the drying curves are made with different moisture content ratio to further obtain the relationship between the micro parameters and macro parameters of materials in the drying process.In this paper,peaches and carrots are selected as dry materials,and hot air drying experiments are performed at 60?,70?,and 80?.The results show that the changes in the microstructure parameters of the materials are significantly affected by temperature during the drying process.When the water content ratio is between 1 to 0.6,the higher the drying temperature,the cell cross-sectional area,cell equivalent diameter,and cell roundness of the material are smaller.In addition,during the early stage and middle stage of hot air drying,the microstructural parameters of the material changed greatly,the cell cross-sectional area,cell equivalent diameter,and cell roundness of the material decrease rapidly as drying proceeds,and decrease smoothly at the end of drying process.At the same drying temperature,with the drying proceeds,the roundness of carrot cells decreased more slowly than peach cells,which is due to the smaller cell structure of carrot materials,resulting in smaller changes of cell roundness,porosity,pore coordination number and other microstructure parameters during the drying process.The changes in the microstructure of the material under five different drying methods including hot air drying,vacuum drying,freeze drying,hot air-vacuum combined drying,and freeze-hot air combined drying are compared.It can be seen that compared with the average value of the initial micro-parameters of the cells,in a single drying method,the changes in the average value of the micro-parameters such as the cell cross-sectional area,the cell cross-section perimeter,and the cell equivalent diameter are:hot air drying<vacuum drying<freeze drying;At the end of drying,under different drying methods,the order of the increasing degree of roughness factor of peach and carrot is:freeze drying<vacuum drying<freeze-hot air combined drying<hot air-vacuum combined drying<hot air drying,which indicates that compared with the single drying method,the degree of micro-parameters such as the cell wall surface roughness factor and cell roundness becomes slower under the corresponding combined drying.The variation trends of micro parameters of peach and carrot during different drying processes are inconsistent.The macro-micro parameter fitting relationships(mathematical models)are all non-linear correlation,and the determination coefficient R2 is mostly above 0.9,the macro parameters(water content ratio,shrinkage ratio)are fitted with corresponding micro parameter mean ratios.Under different drying conditions,with the decrease of water content ratio,the average cell roundness ratio becomes smaller and the average wall roughness factor ratio becomes larger,but the main stages of change are different.Peach cells have a larger change in the water content ratio of 1 to 0.6,and the changes of hot air drying and vacuum drying are mainly concentrated in the water content ratio of 1 to 0.8,while the changes of carrot cells are mainly in the water content ratio of 0.8 to 0.4.Based on the porous characteristics of fruit and vegetable tissue structure,a comprehensive parameter measurement index system is proposed for the change of material microstructure in the drying process.The research results can provide technical guidance for the drying quality control of fruit and vegetable under different conditions,as well as the establishment of mathematical and physical models in the process of fruit and vegetable drying,and provide reference for the development of new drying process.