Effects of different pretreatment methods on drying kinetics, three-dimensional deformation, quality characteristics and microstructure of dried apple slices

The purpose of this study was to evaluate the effects of different pretreatment methods including blanching (30, 60 and 90 s), ultrasound (10, 30 and 60 min), pectinase treatment (15, 30 and 60 min), and osmotic dehydration treatment (0.5, 1.0 and 4.5 h) on the drying characteristics, three-dimensional (3D) deformation, color, total polyphenol content (TPC), antioxidant activity and microstructure of apple slices during drying. The shrinkage and height standard deviation (HSD) were calculated to evaluate the 3D deformation such as surface curl and flatness of the apple slices. The results showed that blanching, pectinase and osmotic dehydration treatment could increase the shrinkage and reduce the surface curl of the samples. Blanching, ultrasound and osmotic dehydration pretreatment could inhibit the color changes of dried apple slices, while pectinase treatment could aggravate the color deterioration of the samples. The apple slices pretreated with blanching for 60 s contained the highest TPC of 4.86 mg/g, and the samples also had the highest antioxidant activity. In addition, microstructure of the pretreated samples was significantly different from those of untreated samples. Therefore, pretreatment technology before drying is of great significance to improve the appearance and quality of products.