2.5.1. Shrinkage

The surface model was composed of tens of thousands of triangles. First, the distance between two points was calculated by Euclid's formula, and the three side lengths of each triangle could be obtained. For example, the distance between points p1 (x1, y1, z1) and p2 (x2, y2, z2) can be calculated by Equation (3). Then, the area of each triangle was calculated through Helen's formula, as in Equation (4), and the sum of the area of all triangles was calculated, which was the surface area. The shrinkage of the potato slices

during drying could be calculated by the change in the surface area at different drying time points (Equation (5)). The specific equations are as follows:

$$\mathbf{d}\_{\mathbb{P}1\mathbb{P}2} = \sqrt{(\mathbf{x}\_1 - \mathbf{x}\_2)^2 + (\mathbf{y}\_1 - \mathbf{y}\_2)^2 + (\mathbf{z}\_1 - \mathbf{z}\_2)^2} \tag{3}$$

$$\mathbf{S\_{ABC}} = \sqrt{\mathbf{p}(\mathbf{p} - \mathbf{d\_{AB}})(\mathbf{p} - \mathbf{d\_{AC}})(\mathbf{p} - \mathbf{d\_{BC}})} \tag{4}$$

$$\text{Shrinkage} = \frac{\text{S}\_0 - \text{S}\_t}{\text{S}\_0} \tag{5}$$

where dp1p2 is the distance between the two points of p1 and p2; SABC is the area of the triangle ABC; and p is half of the circumference of the triangle ABC. S0 is the surface area of the sample before drying, and St is the surface area of the sample during drying.

#### 2.5.2. Height Standard Deviation

The appearance of the material changed from flat to curled during drying, which caused a change in the surface height value. The HSD could reflect the degree of dispersion of the surface height among individuals in a group. Therefore, the HSD was used to characterize the degree of curling of the material. The larger the value, the more uneven the surface of the material and the more severe the curling. The height value of the processed point cloud was extracted by the software, and the standard deviation of the height was calculated by Equation (6).

$$\text{Height standard deviation} = \sqrt{\frac{\sum\_{i=1}^{n} \left(\mathbf{h}\_{i} - \mathbf{h}\_{\text{AV}}\right)^{2}}{n-1}} \tag{6}$$

Among them, n is the number of point clouds; hi is the height of the i-th point, mm; and hav is the average height of n points, mm.

#### *2.6. Color Measurement*

The color of fresh and dried potato slices was determined using colorimeter (SC-10; Shenzhen 3nh technology Co., Ltd., Shenzhen, China). The color was represented by coordinates *L*\* (lightness), *a*\* (redness/greenness), and *b*\* (yellowness/blueness). For each condition, the collection of color parameters was repeated 9 times and averaged. In addition, the total color difference (ΔE) was calculated by Equation (7).

$$
\Delta \mathcal{E} = \sqrt{(L\_0^\* - L^\*)^2 + (a\_0^\* - a^\*)^2 + \left(b\_0^\* - b^\*\right)^2} \tag{7}
$$

where, *L*0<sup>∗</sup>, *<sup>a</sup>*0<sup>∗</sup>, and *b*0<sup>∗</sup> are the color parameters of the untreated dried potato slices, and *L*<sup>∗</sup>, *<sup>a</sup>*<sup>∗</sup>, and *b*∗ are the color parameters of the pretreated dried potato slices.

#### *2.7. Determination of Total Polyphenol Content (TPC)*

Polyphenol extract was prepared by the following method: A total of 1 g of potato slice powder was extracted with 70% ethanol solvent. The potato powder and 50 mL solvent were mixed evenly at room temperature and then treated by ultrasound for 1 h at 40 ◦C, followed by centrifugation at 4000 rpm for 20 min to obtain the supernatant. The supernatant was the final polyphenol extract, and it was stored at 4 ◦C for further analysis.

The total polyphenol content (TPC) of the potato slices was determined by an improved Folin–Ciocalteu method [36]. Five hundred microliters of polyphenol extract were mixed with 1 mL Folin–Ciocalteu's reagent. After 2 min incubation at room temperature, 2 mL Na2CO3 (7.5%, *w*/*v*) was added and then fixed to 10 mL with distilled water. The resulting mixture was incubated for 60 min at room temperature. At the end of the incubation, the absorbance was measured at 775 nm using a UV–Vis spectrophotometer (754, Shanghai Jinghua Technology Instrument Co., Ltd., Shanghai, China). The results of the TPC were expressed as mg gallic acid equivalents (GAE) per gram of dried potato slices.

#### *2.8. Determination of DPPH Radical Scavenging Assay*

The DPPH radical scavenging assay was analyzed according to the method of Zhu et al. [37] and modified appropriately. DPPH solution (2 mL) solution was mixed with a certain volume of sample polyphenol extracts and then fixed to 5 mL with 70% ethanol solution. The reaction mixture was shaken well by a vortex blender (VORTEX-2, Shanghai Hutong Industrial Co., Ltd., Shanghai, China) and left standing for 30 min in a dark environment at room temperature. In the control group, 70% ethanol solution was used to replace the extract, and the preparation method was similar to that of the experimental group. The absorbance of the experimental group and the control group at 517 nm was measured by UV–Vis spectrophotometer (754, Shanghai Jinghua Technology Instrument Co., Ltd., Shanghai, China). The results were presented as percentage of DPPH radical scavenging activity utilizing the Equation (8).

$$\text{DPPH scavenging activity } (\%) = \frac{A\_0 - A}{A\_0} \times 100\% \tag{8}$$

where *A*0 is the absorbance of the control group, and *A* is the absorbance of the sample group.

## *2.9. Microstructure*

Microstructure images of the dried potato slices were obtained using a scanning electron microscope (SEM) (S-3400 N, Hitachi Ltd., Tokyo, Japan) according to the method described by Chu et al. [38]. Dried potato slices were cut into 5 mm × 5 mm with a blade and coated with gold in an ion sputter. The samples were observed in the high vacuum mode at an accelerating voltage of 15.0 kV. Samples were observed at a magnification of 100× and 500×.

#### *2.10. Statistical Analysis*

All statistical analyses were performed using three sets of parallel experimental data, and the experimental results were expressed as mean ± SD. Statistical analysis was performed using SPSS software (version 25.0, SPSS Inc., Chicago, IL, USA). The one-way analysis of variance and Duncan's test (*p* < 0.05) were used to determine whether there were significant differences between the groups.

#### **3. Results and Discussion**

#### *3.1. Moisture Ratio (MR)*

Figure 2 shows the MR curves and drying time of the potato slices under different pretreatments during hot-air drying. Compared with the untreated potato samples, blanching, saline immersion, and ultrasound pretreatment had obvious effects on the drying curves and drying time. The drying curve of the potato slices under different blanching times is shown in Figure 2I. The drying time was decreased by about 14.29% when the blanching time increased to 90 s. This phenomenon may be due to the fact that blanching can expel the intercellular air retention in sample tissues and weaken the resistance of cell membranes and cell walls to water diffusion through structure softening [39]. Similar results were found in studies on the drying process of apricots [40] and carrots [41].

For the saline immersion pretreatment in Figure 2II, when the salt solution concentration increased to 20%, the drying time of the potato slices decreased by about 35.71% compared with the untreated samples. The reason for this result may be that saline immersion can remove part of the free water in the material [18], which obviously led to a reduction in the drying time. In addition, it was reported that accumulation of solute (sucrose or salt) occurred in the space between the wall and plasmalemma, which plasmolyzed the cytoplasm and the vacuoles [42].

**Figure 2.** Drying curves and drying time of potato slices under different pretreatment conditions such as (**I**) blanching, (**II**) saline immersion, and (**III**) ultrasound pretreatment. (**IV**) The drying time for (A) untreated potato samples, (B–D) with blanching pretreatment for 30, 60, and 90 s, (E–G) with saline immersion under solution concentration of 5%, 10%, and 20%, and (H–J) with ultrasound pretreatment for 10, 30, and 60 min. Means denoted by a different lowercase letter indicate significant difference between treatments (*p* < 0.05).

The effect of ultrasound time on the drying time is shown in Figure 2III. It was found that the drying times were about 65, 60, and 50 min for the potato samples treated for 10, 30, and 60 min, respectively. This may be due to cell disruption and microscopic channels being formed after ultrasound pretreatment, which led to a reduction in the resistance against moisture migration [43].

Figure 2IV shows the drying time and variance analysis results of the potato slices under different pretreatment conditions. All three pretreatments enhanced the drying kinetics relative to the untreated samples The saline immersion pretreatment had the greatest influence on the drying time, followed by the ultrasound and blanching pretreatments. In general, the different pretreatments had different effects on the structure of the materials and further affected the process of heat and mass transfer during the drying.

#### *3.2. Three-Dimensional Appearance Characterization*

The 3D appearance images of the dried potato slices under different pretreatments are shown in Figure 3. The three images from top to bottom in each column represent the physical, 3D reconstruction, and height distribution diagrams of the potato slices. It was found that the appearance of the potato slices had significant curling, shrinkage, and browning after the drying process. Moreover, the appearance of the dried potato slices varied greatly with different pretreatments.

**Figure 3.** Three-dimensional appearance images of dried potato slices under different pretreatments. (**a**) Untreated potato samples. (**b**–**d**) Blanching pretreatment for 30, 60, and 90 s. (**<sup>e</sup>**–**g**) Saline immersion under solution concentration of 5%, 10%, and 20%. (**h**–**j**) Ultrasound pretreatment for 10, 30, and 60 min. The three images from top to bottom in each column represent the physical, three-dimensional reconstruction, and height distribution diagrams of the potato slices.

Figure 3b–d shows the appearance of the potato slices after pretreatment by blanching for 30, 60, and 90 s, respectively. When the blanching time was 30 s, the dried potato slices curled obviously. However, when the blanching time was extended to 60 s or 90 s, the potato slices became relatively flat. It has been reported that blanching can destroy the cellular structure and alter the moisture distribution of materials, which leads to a more uniform moisture distribution in materials [44]. The uniform distribution of moisture in the material could have reduced the stress caused by shrinkage in the drying process.

The appearance of the potato slices after pretreatment by saline immersion under solution concentrations of 5%, 10%, and 20% is shown in Figure 3e–g. It can be seen from the figures that, as the salt solution concentration increased to 10% and 20%, the saline immersion pretreatment obviously inhibited the shrinkage and curling of the potato slices during drying. The reason for this phenomenon may be that salt particles could fill the spaces reduced by moisture removal during the drying process. In contrast, for the samples pretreated by ultrasound pretreatment, especially for a long time (60 min), the appearance of the material was seriously curled. This may be attributed to the destruction of the material structure by the "cavitation effect" of ultrasound.

In summary, saline immersion and blanching pretreatment could effectively inhibit the shrinkage and curling of the potato slices, while ultrasound pretreatment aggravated the deformation during the drying process.
