*3.3. Shrinkage*

The shrinkage curves of potato slices under different pretreatment conditions during drying process are shown in Figure 4I–III, and the results of the analysis of variance of the dried potato slices are shown in Figure 4IV. It can be seen that the shrinkage of the potato slices mainly took place at the early drying stage, and gradually slowed down in the later drying stage. It has been reported that the shrinkage at the initial stage of drying is approximately equal to the volume of moisture lost, while in the middle and late drying stages, with the fixation of the "skeleton", the shrinkage becomes slow [45].

**Figure 4.** Shrinkage of potato slices during drying under different pretreatments, such as (**I**) blanching, (**II**) saline immersion, and (**III**) ultrasound pretreatment. (**IV**) The shrinkage 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).

As shown in Figure 4I, the blanching time had a grea<sup>t</sup> influence on the shrinkage of the potato slices. The shrinkage of the dried potato slices at 30, 60, and 90 s were 53.97%, 44.67%, and 42.27%, respectively, which decreased by 2.83%, 19.57%, and 23.89% compared with the untreated samples (55.54%). This was because the blanching caused the cell walls to collapse [46], which reduced the effect of surface stress. Mahiuddin et al. [47] also reported that the destruction of the cell structure has an effect on the shrinkage properties of materials.

Figure 4II indicates the shrinkage of the potato slices by saline immersion under different solution concentrations. It can be seen that the saline immersion pretreatment had a grea<sup>t</sup> influence on the shrinkage of the dried potato slices. The shrinkage of the potato slices decreased with the increase in the salt solution concentration. The potato slices had minimal shrinkage when the salt solution concentration reached 20%, which caused a decrease of 42.69% compared to the untreated sample. Fante et al. [48] found that an increase in sucrose solution concentration led to a decrease in the shrinkage of dried plum slices in the drying process. This may be due to the fact that salt or sucrose particles can fill the space left by the removal of moisture in the material, which would support the skeleton structure of the material to a certain extent.

From Figure 4III, it was found that the ultrasound pretreatment slightly reduced the shrinkage, but the pretreatment time had no significant effect on the shrinkage of the dried potato slices. Liu et al. [49] observed large microchannels and pores in ultrasoundpretreated samples, while the structure of the untreated material was relatively compact. In addition, ultrasound waves may have extended the intercellular spaces by the cavitation effect [50], which may have partially offset the volume reduction caused by moisture removal.

#### *3.4. Height Standard Deviation (HSD)*

The curling degree was evaluated by the HSD of the material surface. The HSD curves of the potato slices during drying under different pretreatments are shown in Figure 5. At the early stage of drying, the HSD of the material changed little or showed a downward trend, which was mainly due to the softening of the material structure by hot-air heating. The HSD increased rapidly in the middle and late drying stages, indicating that the material had an obvious curling phenomenon. The shape changes in the materials in the drying process may be due to the uneven stress caused by the shrinkage of the cells and pores [51].

**Figure 5.** Height standard deviation of potato slices during drying under different pretreatments, such as (**I**) blanching, (**II**) saline immersion, and (**III**) ultrasound pretreatment. (**IV**) The height standard deviation 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 HSD of the dried potato slices after blanching for 30, 60, and 90 s were 2.64 mm, 2.31 mm, and 2.16 mm, respectively. However, there was no significant difference between the blanching pretreatment and the untreated samples, indicating that the blanching pretreatment could not reduce the curling phenomenon during drying. Although the structure of the material would have been damaged by the blanching process, the starch

gelatinization caused by the high temperature may have played a certain role in supporting the structure.

As shown in Figure 5IV, the HSD of the dried potato slices after saline immersion pretreatment under solution concentrations of 5%, 10%, and 20% were 2.25 mm, 1.06 mm, and 0.47 mm, respectively, which decreased by 5.88%, 55.46%, and 80.25% compared with the untreated samples. The results demonstrated that saline immersion could inhibit the curling of the potato slices in the drying process, showing a relatively flat shape. This may be because osmotic ions entered the tissue and blocked the transmission of internal stress [52]. In addition, it also may have been due to the structure of "hard outside and soft inside" after processing by osmotic dehydration [53].

For the ultrasound pretreatment, the HSD after ultrasound pretreatment for 10, 30, and 60 min were 1.67 mm, 2.41 mm, and 4.30 mm, respectively. It was found that ultrasound pretreatment for 10 min could reduce the HSD of the potato slices, which indicated that a shorter time of the ultrasound pretreatment could reduce the curling degree. When the ultrasound time was extended to 60 min, the HSD (4.30 mm) increased by 80.67% compared with the untreated samples, which indicated that very serious curling of the slices occurred. This may be because the short-time ultrasound pretreatment made the potato tissue more uniform, thereby resulting in a more uniform transfer of internal stress. However, with the increase in the ultrasound time, the cavitation effect of micro-jets and micro-agitation at the bubble inter-face led to the destruction of the cell structure and formed cracks and pores [54]. The non-continuous and non-uniform structure increased the effect of stress and showed the appearance of curling from a macroscopic perspective.

## *3.5. Color*

Color is a significant quality parameter of dried potato slices, which influences the customer's perception and purchasing power [55]. The color values of all the samples are presented in Table 1. Blanching, saline immersion, ultrasound, and drying had significant effects on the color parameters of the dried potato slices. As seen in Table 1, the *L*\*, *a*\*, and *b*\* values of the untreated potato slices were 72.62, 8.24, and 26.13, respectively. It was found that the untreated samples had the largest value of ΔE, which was due to browning caused by the drying process [56]. The *L*\* value of the potato slices pretreated with blanching was lower, which may be related to the gelatinization of starch by blanching pretreatment. Xiao et al. [57] reported that the clarity of gelatinized starch could reduce the lightness of starch products. Compared with the untreated samples, the values of *a*\* and *b*\* were significantly reduced. The value of ΔE of the dried samples after blanching pretreatment was also significantly lower than that of the untreated samples. In particular, when the blanching time was 30 s, the color change was the least, and the ΔE value was 3.10. This indicated that blanching pretreatment could better retain the original color, which may be because blanching inactivates polyphenol oxidase. It has also been reported that this phenomenon is due to the leaching of reducing sugars by blanching pretreatment, which is the substrate of the Maillard reaction [58]. Thus, this minimized the non-enzymatic browning reaction and reduce the color variation in the slices.

The effect of the saline immersion pretreatment on the color is shown in Table 1. The values of *L*\*, *a*\*, and *b*\* were all smaller than those of the untreated samples. With the increase in the salt concentration, the value of the sample color parameters decreased continuously, which indicated that a high concentration salt solution could achieve a better retention effect in terms of color. This may be due to the loss of polyphenol oxidase, which is due to the leakage effect of a high-concentration salt solution.

For the ultrasound pretreatment, the color parameters of the potato slices were slightly less than those of the untreated samples. With the extension of the ultrasound time, the ΔE value gradually decreased, which indicated that long-time ultrasound pretreatment was in favor of maintaining the color of the samples. This may be because the ultrasound pretreatment reduced the oxygen content of the sample and inhibited the browning reaction [38,59].


**Table 1.** Changes in color, total polyphenol content, and antioxidant capacity of potato slices after drying under different pretreatments.

Note: Data are expressed as the average ± standard deviation for three replicates. Values in the same column with different letters for each parameter are significantly different (*p* < 0.05).

#### *3.6. Total Polyphenol Content (TPC)*

The effects of the different pretreatment methods on the TPC of the dried potato slices are shown in Table 1. Compared with the untreated samples, the blanching and ultrasound pretreatment had a better retention of polyphenols, while the saline immersion pretreatment was not conducive to the retention of polyphenols.

Compared with the untreated samples, the total polyphenol content in the blanchingpretreated samples was generally increased. However, with the extension of the blanching time, the total polyphenol content gradually decreased. This indicated that short-time blanching pretreatment was beneficial to the retention of polyphenols. This may be due to the loss of polyphenol oxidase activity by blanching pretreatment, which resulted in a better retention of more polyphenols [59]. However, a prolonged blanching time made the cellular structure vulnerable to damage during drying, which led to the oxidation of polyphenols [40].

For the samples treated with saline immersion, the content of polyphenols was lower than that of the untreated samples. When the solution concentration reached 20%, the polyphenol content was the lowest. This was the loss of polyphenols due to leakage of the salt solution [60,61].

Similar to the blanching pretreatment, a shorter ultrasound treatment was more beneficial for polyphenol retention. This may be due to the fact that ultrasound pretreatment can produce stomata in plant tissues, thus improving the extraction of polyphenols during the preparation of sample [62]. However, when the ultrasound time was too long, the total phenol content decreased slightly, which was due to the loss of food ingredients caused by the enlargement of pores [63]. This was consistent with the study of polyphenol content in dried onions slices by Ren et al. [64].

#### *3.7. DPPH Radical Scavenging Assay*

The DPPH free radical activity values of the dried potato slices under different pretreatments are shown in Table 1. It was observed that the trend of DPPH was similar to that of TPC retention. The high positive correlation between phenolic compounds and antioxidant activity was also reported in another study [65]. In this study, the free radical scavenging activity of the blanched samples was the best, followed by the samples pretreated with ultrasound and saline immersion. When the blanching time was 30 s, the sample showed the highest activity (56.45%), which was similar to the results of Feng et al. [66].
