Morphological Changes Of Apple Slices During Drying Based On Three-dimensional Vision Technology

Drying is an important way of apple processing.However,various morphological changes such as curling and shrinking are very likely to occur during drying of apple slices,which will affect the quality and fine processing of dried products.In this study,three-dimensional(3D)vision technology was applied to the detection of morphological changes during drying of apple slices.Firstly,the effects of different drying conditions and material states on the surface area shrinkage,height standard deviation and mean curvature of apple slices during drying were studied.Secondly,the effects of different pretreatment methods on the three-dimensional morphology,color,flavor,total polyphenol content and antioxidant properties of apple slices during drying were studied.On this basis,the pretreated apple slices were selected to study the changes in pectin content and nanostructure of their cell wall,which were combined with the threedimensional morphological changes of apple slices during drying to explore the correlation.The main research results of this paper are as follows:(1)The point cloud images of apple slices during drying were obtained by 3D vision technology,and the surface area shrinkage,height standard deviation and mean curvature were selected through point cloud processing to evaluate the 3D morphological change of apple slice.This method could accurately describe the shrinkage,surface curl and local smoothness of apple slices during drying.(2)The higher the drying temperature,the greater the drying wind speed(except11 m/s)or the relative humidity of the medium,the faster the shrinkage change of apple slices during drying or the greater the shrinkage at the drying end,and the smaller the local smoothing degree.The greater the slice thickness,the smaller the shrinkage of apple slices during drying,and the worse the local smoothness.The surface curling degree of apple slices during drying decreased with the increase of drying temperature,drying air speed and slice thickness,and increased with the increase of relative humidity of drying medium.The drying wind direction had no significant effect on the shrinkage and local smoothness of the samples during drying,but the surface curling degree in the normal direction was 64.5% lower than that in the tangential direction.The slicing angle had no significant influence on the three-dimensional morphological changes of the samples during drying.(3)The blanching pretreatment and pectinase pretreatment increased the shrinkage of apple slices during drying,reduced the degree of surface curling,and reduced the degree of local smoothing.The shrinkage rate of the apple slices after ultrasonic treatment increased slightly during drying.The ultrasonic time was short,the degree of surface curling and local smoothing did not change significantly,while the degree of surface curling decreased when the ultrasonic time was 60 min,and the local smoothness degree also decreased.Osmotic dehydration treatment with sugar increased the shrinkage of the apple slices,decreased the surface curl,and changed the local smoothness.In addition,the drying kinetics,color,flavor and other quality characteristics of apple slices changed to varying degrees after different pretreatments.(4)The cell structure of apple slices treated with blanching and pectinase was obviously destroyed,but the characteristic groups of pectin basically remained unchanged.After pretreatment,the content of water-soluble pectin(WSP)in apple slices increased,and the content of alkali-soluble pectin(NSP)decreased.The chain length and chain width of WSP samples blanched for 60 s were analyzed,and the chain lengths were mainly distributed between 300-600 nm,600-900 nm and 900-1200 nm,the proportion was 26.67%,33.33 % and 16.67%,respectively.The chain widths were mainly distributed between ≤50 nm,50-60 nm and 60-70 nm,accounting for 27.50%,40.00% and 17.50%,respectively.The chain length and chain width of the pectin macromolecules were significantly reduced when ultrasonically treated for 60 min,and the three-dimensional morphological changes of the samples were also significantly reduced.This research clarified the effects of different factors on the three-dimensional morphological changes of apple slices during drying,which is of great significance to the improvement of the appearance quality of dry products.The relationship between three-dimensional morphological changes and pectin characteristics was revealed,which has an important influence on elucidate the mechanism of three-dimensional morphological changes and morphological regulation.