*3.8. Microstructure*

The scanning electron microscopy (SEM) images of the dried potato slices under different pretreatments are shown in Figure 6. The microscopic results of the different pretreated samples and untreated samples differed greatly. As shown in Figure 6a, the untreated samples had both dense and porous structures, which may be caused by the nonuniform shrinkage of the material structure. From Figure 6b, we also found intact starch granules, indicating that the starch did not swell and gelatinize during the drying process.

**Figure 6.** Microstructures of dried potato slices under different pretreatments in different magnifications. (**<sup>a</sup>**,**b**) Untreated potato samples; (**<sup>c</sup>**,**d**) blanching pretreatment for 90 s; (**<sup>e</sup>**,**f**) saline immersion under solution concentration of 20%; and (**g**,**h**) ultrasound pretreatment for 60 min.

The microstructure of the dried samples after blanching pretreatment are shown in Figure 6c,d. The tissue structure of the blanched dried potato slices was uniform and dense, and no obvious pore structure was found. This may be caused by the collapse of the cellular structure after the blanching and drying process. In addition, starch granules were not found in the micrograph field, indicating that the blanching treatment resulted in starch breakage and gelatinization [46]. This was similar to the results of a study on sweet potato bars [57].

The samples from the saline immersion pretreatment had a relatively loose and porous structure (Figure 6e,f). The cytoskeletal structure became coarse as compared to the untreated samples, and starch granules were no longer visible in the samples. This may be due to the internal modification of the starch particles by the components of osmotic solution during processing [67]. After the ultrasound pretreatment, the boundaries of the cells were fuzzy, while the starch granules could be also clearly seen (Figure 6g,h). This was because

the ultrasound pretreatment caused changes in the cell structure and formed microchannels on the surface of the potato samples, and the microchannels were combined with the original pore structure, which may be due to the cavitation and sponge effects of the ultrasound waves [68]. The observation of the microstructure of the material was helpful in understanding the effects of pretreatment on the drying rate and appearance deformation.
