**1. Introduction**

Potato (*Solanum tuberosum* L.) is known as one of the world's five major crops along with corn, rice, wheat, and sorghum [1]. Potato is rich in nutrition, including starch, protein, vitamins, polyphenols, and trace elements, so it is used as a favorite composition of functional food [2,3]. Therefore, potato is getting higher and higher in the position of agricultural and sideline products, and the demand is also growing. However, potato, like other vegetables, has a high moisture content, so it is easy for it to rot and sprout during storage [4]. This has a grea<sup>t</sup> effect on the quality of potatoes [5]. Drying is an effective way to prolong the shelf life of fruits and vegetables.

There are many drying methods used in the processing of fruits and vegetables, including hot-air drying, infrared drying, freeze drying, microwave drying, and hybrid drying technology [6,7]. Each drying technique has its own advantages and disadvantages. However, the most commonly used drying method in potatoes is still hot-air drying [8]. Drying can effectively prevent the growth of microorganisms, reduce enzyme activity, and slow down some water-mediated chemical reactions [9,10]. However, the drying process always consumes a lot of energy and will have a significant impact on the shape, color, flavor, and nutrition of dried products [10]. Therefore, it is necessary to develop operations to minimize the adverse effects of the drying process, reduce the time and energy requirements, and maximize the retention of the original characteristics of the product [11].

Fruits and vegetables are usually subjected to physical or chemical pretreatment before drying to shorten the drying time, reduce the energy consumption, and preserve

**Citation:** Bai, J.-W.; Dai, Y.; Wang, Y.-C.; Cai, J.-R.; Zhang, L.; Tian, X.-Y. Potato Slices Drying: Pretreatment Affects the Three-Dimensional Appearance and Quality Attributes. *Agriculture* **2022**, *12*, 1841. https:// doi.org/10.3390/agriculture12111841

Academic Editor: Hongbin Pu

Received: 30 September 2022 Accepted: 1 November 2022 Published: 3 November 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

the quality of products [12]. It was found that blanching pretreatment can damage the structure of cell membranes and thus shorten the drying time [13]. Mehta et al. [14] reported that dried vegetables coupled with blanching as a pretreatment showed less degradation in terms of polyphenols and flavonoids. Liu et al. [15] observed that blanching pretreatment could not only shorten the drying time but also inhibit browning and maintain the anthocyanin level in purple-flesh sweet potato drying. It has also been reported that vacuum-dried potato chips pretreated with blanching have a better texture and a lower glycemic index [16]. Osmotic solution immersion pretreatment, such as sucrose or salt solution, has been widely used in drying pretreatments because of its ability to ensure the quality of dried products [17]. Zou et al. [18] reported that sucrose solution immersion pretreatment can improve the color and sensory quality of dried products. It was reported that osmotic solution pretreatment shortens the drying time and reduces the specific energy consumption in potato drying [19]. Moreover, Chinenye et al. [20] found that the volume of potato chips treated by saline immersion was higher by 6% than non-treated samples.

Ultrasound as a pretreatment method has attracted considerable interest in drying processes, since it can form microscopic channels in the tissue due to cavitation and sponge effects, which can promote the migration of water and accelerate the drying process [21,22]. For potato slices drying processes, it has been reported that ultrasound pretreatment can effectively shorten the drying time and reduce the specific energy consumption [23]. Zhang et al. found that ultrasound pretreatment can increased hardness of potato chips and reduce the destruction of the cellular structure [24]. The results of Xu et al. [25] showed that ultrasound pretreatment could improve the content of flavonoids and polyphenols in dried products. Rashid et al. [26] also reported that appropriate ultrasound pretreatment can well maintain phytochemical compounds. Generally speaking, suitable pretreatment before a drying process can improve the drying efficiency and enhance the product quality, but few people have paid attention to the influence of pretreatment on the appearance changes in dried products.

Appearance (especially for 3D appearance) is one of the most important indicators for people when evaluating dried products, and it has a grea<sup>t</sup> impact on subsequent further processing, packaging, and transportation [27]. For consumers, products with a uniform and regular appearance generally have a better degree of acceptability. At present, the main method for studying the appearance changes in dried samples is through two-dimensional images. For example, Khazaei et al. [28] applied an analog camera collect images to monitor shrinkage during dehydration in grape drying. However, a single camera can only obtain the data of a projected area of a sample's surface, and the thickness change in the material cannot be measured effectively. Therefore, Sampson et al. [29] used top and side cameras to obtain the thickness and projected area of materials so as to measure the volume changes in apple slices during the drying process. However, a side camera cannot fully reflect the thickness change during the drying process of the material. In addition, a two-dimensional image cannot perfectly simulate the morphological change in the drying process that occurs in a 3D space. Therefore, it is necessary to use 3D image technology to evaluate the shape change in materials during drying. Cai et al. [30] used a Kinect V2 sensor to build an image acquisition platform, and the morphological changes in potato slices under different drying temperatures were studied. However, the detection accuracy of a Kinect sensor is relatively low [31], which makes the quantification and analysis of 3D information rough. Therefore, there has been less information about the 3D appearance changes in fruits and vegetables during drying by pretreatment methods.

The objective of this study was to investigate the effects of blanching, saline immersion, and ultrasound pretreatments on the drying time, internal quality, and external quality characteristics of dried potato slices, including the 3D appearance, color, total polyphenol content, antioxidant properties, and microstructure.

#### **2. Materials and Methods**

## *2.1. Material*

Fresh potatoes of the same variety "Holland fifteen" were purchased from a supermarket near Jiangsu University (Zhenjiang, China). All the potato samples were transported to the laboratory and stored at room temperature (about 20 ◦C) before experimentation. The average initial moisture content of potatoes was 84.23 ± 2.36% (wet basis). Before drying, the potatoes were washed, peeled, and sliced to a thickness of 2 mm using an electric slicer (MS-305C, Foshan Komle Electric Appliance Co., Ltd., Foshan, China). Then, the samples were subjected to pretreatment.

#### *2.2. Pretreatment Methods*

In this study, potato slices were subjected to three kinds of pretreatments. (1) For steam-blanching pretreatment, potato slices were processed by steam cooker (total volume 4 L) at atmospheric pressure. The power of the steam cooker was 1000 W to ensure the continuous boiling of the water. The blanching times were 30, 60, and 90 s, respectively. (2) Saline immersion pretreatment was referred to as the method of Chinenye et al. [20] with some modifications. Potato slices were soaked in a salt solution for 60 min. The concentrations of the salt solutions were 5%, 10%, and 20%, respectively. (3) For ultrasound pretreatment, the potato slices were immersed in distilled water and then subjected to an ultrasound bath. The parameters set to 240 W and 40 ◦C according to the relevant studies. The treatment times were 10 min, 30 min, and 60 min, respectively.

#### *2.3. Hot-Air Drying Experiment*

The potato slice samples were dried in hot-air drier, which was described in previous study [30]. The drying process was carried out at 65 ◦C with an air velocity of 3 m/s and a relative humidity of 10% (RH). A quantity of 100 ± 5 g samples was used for all drying runs in the experiment. The weight loss was periodically recorded by taking out the rotating glass and weighing it on an electronic balance within an accuracy of ±0.01 g during drying. Drying was stopped when the moisture content of the samples reached the desired final moisture content of 6.00% (wet basis). All the drying experiments were conducted in triplicate.

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

The moisture ratio was calculated using Equations (1) and (2).

$$\text{MR} = \frac{\text{M}\_{\text{f}} - \text{M}\_{\text{e}}}{\text{M}\_{\text{0}} - \text{M}\_{\text{e}}} \tag{1}$$

where M0 is the initial dry basis moisture content; Mt is the dry basis moisture content at the drying time t; MR is the moisture ratio; and Me is the equilibrium moisture content. The equilibrium moisture content, Me, was much smaller than M0 and Mt and could generally be ignored [32]. Therefore, the calculation of MR can be simplified as:

$$\text{MR} = \frac{\text{M}\_{\text{t}}}{\text{M}\_{\text{0}}} \tag{2}$$

#### *2.5. Three-Dimensional Appearance Evaluation Index*

The 3D image acquisition platform used in this experiment was independently built by the team [33]. Using binocular snapshot sensor (Gocator3210, LMI technologies Inc., Vancouver, BC, Canada), the measurement range was −50~50 mm in the horizontal direction, −77~77 mm in the vertical direction, −55~55 mm in the depth direction, and the detection accuracy was ±0.035 mm. The 3D point cloud images were periodically collected at an interval of 10 min during drying. The collected images were processed by the software Cloud Compare (version 2.1), including background removal, noise removal, point cloud filtering, and surface reconstruction.

The time-varying appearance images of one potato slice during drying is shown in Figure 1. The three images from top to bottom in each column represent a color physical image, 3D reconstructed image and height distributed image, respectively. The 3D reconstructed image obtained from the point cloud data was fairly close to the physical image of the potato slice, which benefitted from good measurement accuracy due to laser scanning [34,35]. Therefore, the reconstructed 3D images could well reproduce the appearance changes in the potato slices during drying. The height distribution of the potato slice in Figure 1 is represented by pseudo-color images, and the color from blue to red indicates that the height value of the pixels on the material changed from small to large. It was found that potato slice obviously curled with the process of drying, especially after a drying time of 40 min.

**Figure 1.** Time-varying appearance images of one potato slice during drying. (**<sup>a</sup>**–**h**) represent potato slices dried at 65 ◦C, 10% RH, 3 m/s for 0, 10, 20, 30, 40, 50, 60, and 70 min, respectively. The three images from top to bottom in each column represent color physical image, 3D reconstructed image, and height distributed image, respectively.
