Effect of Field Curing Duration on Physical–Mechanical Properties and Impact Damage of Potato Tubers at Harvest Maturity

Abstract

Mechanical harvesting damage is a critical factor constraining potato quality and storage performance. Field curing is a commonly employed pre-treatment prior to mechanical picking of potatoes, which promotes skin suberization and reduces mechanical damage; however, the determination of optimal curing duration lacks a theoretical basis. This study investigated ‘Xisen No. 6’ potatoes at harvest maturity. Curing was performed by field sun-drying (open-air exposure) immediately after mechanical excavation, with five duration gradients (0, 1, 2, 3, and 4 h) established under the recorded meteorological conditions. Twenty-two physical–mechanical and damage parameters were measured, and principal component analysis (PCA) was employed for comprehensive evaluation. The results demonstrated that curing induced a transformation of tubers from “soft-elastic bodies” to “hard-brittle bodies”. This study first revealed the contradictory evolution pattern between skin abrasion damage and tissue impact damage, which exhibited a strong negative correlation (r = −0.89, p < 0.01). PCA indicated that a 3 h curing duration could effectively balance the control of both damage types. These findings provide a quantitative solution to the dilemma of reducing skin damage while controlling impact damage during mechanical potato harvesting, offering significant guidance for optimizing harvesting process parameters and reducing postharvest losses.

1. Introduction

Potato is the fourth largest food crop globally and serves as an important dual-purpose crop for both staple food and vegetable consumption in China, playing a significant role in ensuring food security and promoting rural revitalization [1,2,3,4]. According to the Food and Agriculture Organization of the United Nations (FAO), China ranks first in global potato production, with an output exceeding 93 million tons [5]. Fresh consumption dominates potato utilization in China, and staged harvesting with mechanized picking is widely adopted in production [6]. To enhance the damage resistance of potatoes during mechanized picking and reduce the occurrence of tissue impact damage and skin abrasion damage, excavated potato tubers require field curing treatment before mechanical picking operations [7,8].

Proper control of field curing duration is critical to potato harvest quality. If the curing period is too short, periderm suberization remains insufficient and the skin does not fully mature, resulting in weak adhesion between the skin and the underlying flesh. During mechanical picking operations, such immature tubers are prone to skinning and abrasion injuries, which provide entry points for pathogens, significantly increasing postharvest decay rates and shortening storage life [9,10,11]. Conversely, excessive curing duration leads to continued moisture loss, causing tuber dehydration and shriveling, reduced marketable yield, and deteriorated storage quality. Prolonged field exposure may also induce chlorophyll biosynthesis and skin greening, accompanied by the accumulation of toxic glycoalkaloids, rendering the tubers unfit for consumption [12,13]. According to potato production technical standards and the practical experience of growers in major producing regions, field curing is generally conducted at ambient temperatures above −1.7 °C, avoiding midday periods of high temperature and intense solar radiation, under moderately high relative humidity conditions. Mechanical picking following 2–4 h of field curing on the day of harvest is a widely adopted practice [14,15,16]. However, this time range is still primarily determined by empirical judgment. To date, systematic studies on the effects of different field curing durations on the physical and mechanical properties of potato tubers and the associated damage mechanisms remain largely absent, and there is a lack of scientific theoretical basis and quantitative guidance.

The objective of this study was to systematically investigate the effects of field curing duration on the physical and mechanical properties and impact damage of potato tubers at harvest maturity, and to determine the optimal curing duration that effectively minimizes both types of damage. This study hypothesized that moisture loss during field curing would induce systematic changes in the physical and mechanical properties of tubers, and that these changes would exert differential effects on skin frictional damage and tissue impact damage, with an optimal curing duration existing that balances the control of both damage types. To test this hypothesis, ‘Xisen No. 6’ potato cultivar was used as the experimental material, with five curing duration gradients (0–4 h) established. A total of 22 physical, mechanical, and damage-related parameters were systematically measured, and principal component analysis was employed for comprehensive evaluation. The findings of this study are expected to provide a theoretical basis for the scientific determination of field curing duration for harvest-mature potatoes and for mechanized low-damage harvesting operations.

2. Materials and Methods

2.1. Experimental Materials

The experiment was conducted on 3 October 2024, at 7:00 a.m. in Qitai Town, Shangdu County, Ulanqab City, Inner Mongolia Autonomous Region, China (41°34′ N, 113°35′ E, altitude approximately 1400 m). This region is characterized by a mid-temperate continental monsoon climate, with a mean annual temperature of 3.1 °C, a frost-free period of 105–120 days, an annual sunshine duration of 2970 h, large diurnal temperature variations, and abundant solar radiation. In 2024, the total precipitation across the city reached 623.1 mm, which was 72.9% above the long-term average and the highest on record since meteorological observations began. During the autumn harvest period (mid-late September to early October), the climatic conditions were characterized by average daytime highs of 12–18 °C, average nighttime lows ranging from −5 to 8 °C, and relative humidity fluctuating between 30% and 91% [17]. The predominant soil type for potato cultivation in this region is loamy sand. The experimental cultivar was ‘Xisen No. 6’, a potato variety well-suited for single-season cropping in northern China [18]. This cultivar is a medium–late maturing, dual-purpose variety for both fresh consumption and processing, with a growing period of approximately 91 days. The tubers are oblong in shape, with yellow skin and yellow flesh, and shallow eyes. The tuber dry matter content is 22.60%, starch content 15.10%, vitamin C content 14.80 mg/100 g fresh weight, crude protein content 1.78%, and reducing sugar content 0.14%. In this experiment, potatoes were planted in late April 2024 using a single-ridge double-row planting configuration, with a row spacing of 1000 mm and a plant spacing of 300 mm, resulting in a planting density of approximately 63,000 plants per hectare. Shallow-buried drip irrigation was employed without plastic film mulching. Due to unfavorable weather conditions, including persistent rainfall, the harvest was delayed until early October. On the day of the experiment, meteorological elements and environmental parameters were recorded in real time using an HM-QX16 Hengmei handheld intelligent agricultural meteorological environment monitoring instrument (Shandong Hengmei Electronic Technology Co., Ltd., Weifang, China) (

Table 1. Environmental parameters and meteorological elements.

Date	Time Interval	Temperature (°C)	Barometric Pressure (hPa)	Relative Humidity (%)	Extreme Wind (km/h)	Illumination (Lx)	Soil Moisture (%)
October 3rd	7:00	−1.4	857.9	88%	3.9	167.9	27.6
	8:00	1.4	858.1	79%	6.7	61.1	24.3
	9:00	5.3	860.2	58%	9.4	76.4	18.7
	10:00	7.5	860.4	47%	11.9	119.9	14.8
	11:00	8.6	860.3	42%	12.2	132.5	8.7

Note. Measurements were taken hourly beginning at 7:00 a.m. on 3 October 2024.

), serving as the ambient environmental conditions for the field curing treatments.

2.2. Experimental Design

Two to three days prior to harvest, mechanical vine killing was performed on the potato plants using a Hongzhu 1JH-180 vine killer (Qingdao Hongzhu Agricultural Machinery Co., Ltd., Qingdao, China), leaving an average stubble height of less than 8 cm to minimize interference of stems and foliage with subsequent digging operations and to promote periderm maturation of the tubers. On the day of the experiment, a Hongzhu 4U-170B potato harvester (Qingdao Hongzhu Agricultural Machinery Co., Ltd., Qingdao, China) was used for digging operations (Figure 1), after which the potato tubers were deposited in windrows on the soil surface. A single-factor experimental design was adopted, with field curing duration as the experimental factor. Five treatment levels were established, 0 h (sampled immediately after digging), 1 h, 2 h, 3 h, and 4 h. The tubers deposited in windrows on the soil surface were subjected to field curing under natural ambient conditions. The entire experiment was completed within a single day, with samples collected sequentially according to the designated time intervals. Potato tubers in good condition, with similar shape and weight close to the population mean, were selected as experimental units [19]. Five replicates were set for each treatment. Since most measurement methods were destructive in nature, independent tuber samples were used for each test item under each treatment. Specifically, for each of the five curing duration treatments, separate sets of five tubers were allocated to: moisture content determination (5), density measurement (5), uniaxial compression testing (5), static and dynamic friction coefficient measurement against four contact materials (5 × 4 = 20), coefficient of restitution measurement against four contact materials (5 × 4 = 20), tissue impact damage testing (5), and skin abrasion damage testing (5). This resulted in 65 tubers per treatment and a total of 325 tubers (65 × 5 treatments) used throughout the entire experiment. All experimental measurements were completed in the laboratory within 1 h of sampling.

2.3. Experimental Methods

2.3.1. Determination of Physical and Mechanical Properties

The moisture content of tubers was determined using the oven-drying method [20]. Potato tuber slices were placed in an electrothermal constant-temperature drying oven and dried at 120 °C until a constant weight was achieved (the difference between two consecutive weighings was less than 0.02 g). The mass before and after drying was recorded, and the moisture content was calculated using Equation (1) as follows:

$$w={\displaystyle \frac{100(m_g-m_f)}{m_g}}$$

(1)

where w is the moisture content, %; m_f is the mass of the potato tuber after drying, g; and m_g is the mass of the potato tuber before drying, g.

Tuber density was measured using a DahoMeter DH-300X densimeter (Shenzhen Dahometer Technology Co., Ltd., Shenzhen, China). Critical pressure was obtained using a Model 2500 texture analyzer manufactured (Food Technology Corporation, Sterling, VA, USA) (Figure 2). Cylindrical specimens (diameter 17.35 ± 0.05 mm, height 20.00 ± 0.05 mm) were prepared from potato tubers using a custom-made sampler. Uniaxial compression tests were performed on the specimens using a 25.4 mm diameter metal cylindrical probe at a loading rate of 10 mm/min, with the deformation ratio set at 40%. Compression was terminated upon specimen failure, and the force–deformation curve was recorded to obtain the critical pressure. The elastic modulus, Poisson’s ratio, and shear modulus were calculated using Equations (2)–(4) as follows [21,22,23,24,25]:

$$E={\displaystyle \frac{\delta }{\epsilon }}={\displaystyle \frac{FL}{\mathsf{\Delta }LA}}$$

(2)

$$\upsilon =-{\displaystyle \frac{\epsilon T}{\epsilon L}}$$

(3)

$$G={\displaystyle \frac{E}{2(1+\upsilon )}}$$

(4)

where E is the elastic modulus of the test sample, MPa; δ is the stress of the test sample, MPa; ε is the longitudinal strain of the test sample; F is the critical pressure of the test sample, N; L is the height of the test sample before compression, mm; ΔL is the height change of the test sample after compression, mm; A is the cross-sectional area of the test sample, mm²; υ is the Poisson’s ratio of the test sample; εT is the transverse strain of the test sample; εL is the longitudinal strain of the test sample; and G is the shear modulus of the test sample, MPa.

The static and dynamic friction coefficients between potato tubers and tubers, soil, rubber, and 65Mn steel were all measured using an MXD-02 friction coefficient tester (Shanghai Le’ao Testing Instrument Co., Ltd., Shanghai, China) [26]. The coefficient of restitution was determined using the free-fall impact method [27,28,29]. A coordinate system was established using graph paper. Potato tubers (400 ± 5 g) were released from a vertical height of 350 mm above the base plate. The entire collision process was captured using a HiSpec 5 high-speed imaging acquisition system (Fastec Imaging Corporation, San Diego, CA, USA). The recorded videos were subsequently imported into TEMA 3.4-005 motion analysis software to analyze the complete free-fall collision process and determine the rebound height. The coefficient of restitution was calculated using Equation (5) as follows:

$$e=\sqrt{{\displaystyle \frac{H}{H_O}}}$$

(5)

where e is the coefficient of restitution; H is the maximum rebound height after collision, mm; and H_O is the drop height before collision, mm.

2.3.2. Determination of Damage Characteristics

Tissue collision and epidermal friction are among the primary causes of potato tuber damage during mechanized picking operations [30,31,32]. Both the tissue collision damage acceleration and epidermal friction damage acceleration of potato tubers were measured using a custom-built pendulum-type collision test apparatus. A potato embedded with an acceleration collision sensor was released from a specified height [33,34,35] and allowed to collide with a vertically fixed rod. The collision data were recorded by a data acquisition analyzer (Figure 3a). Subsequently, the rod in the apparatus was repositioned horizontally, the potato was replaced, and the aforementioned procedure was repeated to collect epidermal friction damage acceleration data (Figure 3b).

In accordance with the Quality Standards for Fresh Potato Grading (Trial), the extent of damage was quantitatively assessed using a binary image segmentation method [36,37]. The damaged area was photographed against a standard grid paper background (120 mm × 120 mm). Following image threshold segmentation processing, the damage index was characterized by the proportion of red pixel area (Figure 4 and Figure 5). The damage proportion was calculated using Equation (6) as follows:

$$\eta ={\displaystyle \frac{100S}{S_O}}$$

(6)

where η is the damage ratio, %; S is the number of pixels in the red area; and S_O is the number of pixels in the frame.

2.3.3. Statistical Analysis Methods

To comprehensively investigate the effects of different field drying durations on the physical–mechanical properties and collision damage of harvest-ready potatoes, 22 indicators were selected for examination, including moisture content, density, critical pressure, elastic modulus, Poisson’s ratio, shear modulus, static friction coefficient, dynamic friction coefficient, coefficient of restitution, damage acceleration, and damage proportion. Principal component analysis (PCA) was employed for dimensionality reduction to obtain a comprehensive experimental index and establish an optimized model [38].

2.4. Data Processing

Data were organized using Excel 2024 (Microsoft Corporation, USA). One-way analysis of variance (ANOVA), correlation analysis, regression analysis, and principal component analysis (PCA) were performed using SPSS 27.0 (SPSS Institute Inc., USA). Regression fitting and graphical plotting were conducted using Origin 2024 (OriginLab Corporation, USA) software.

3. Results and Discussion

3.1. Effects of Different Field Drying Durations on the Physical–Mechanical Properties of Potato Tubers

Field curing of potato tubers deposited in windrows on the soil surface after digging essentially utilizes natural environmental conditions to promote moisture loss from the tuber surface and periderm suberization, with the primary objective of enhancing skin mechanical strength and reducing the risk of pathogen invasion. The curing effectiveness is governed by the combined influence of multiple environmental factors, including temperature, relative humidity, and wind speed. The freezing onset temperature of potato tubers is approximately −1.7 °C, below which intercellular water crystallization occurs, causing irreversible frost damage. In the present experiment, the measured temperature ranged from −1.4 to 8.6 °C (Table 1), and the minimum temperature at 7:00 a.m. (−1.4 °C) remained above the tuber freezing point, indicating that the tubers had not experienced sub-freezing exposure. As the temperature continued to rise thereafter, the risk of frost damage was considered negligible. Relative humidity determines the water vapor pressure deficit between the tuber surface and the surrounding environment, serving as a key driving factor for surface moisture evaporation [39]. During the experiment, relative humidity decreased progressively from 88% in the early morning to 42% (Table 1), exhibiting a sustained declining trend that was conducive to rapid evaporative moisture loss from the tuber surface. Meanwhile, wind speed increased gradually from 3.9 km/h to 12.2 km/h during the experimental period, further accelerating convective heat transfer and moisture evaporation from the tuber surface. Therefore, the prevailing conditions of temperature, relative humidity, and wind speed on the day of the experiment were suitable for postharvest field curing treatment.

Field curing significantly altered the moisture status and tissue structure of potato tubers. As curing duration increased, tuber moisture content decreased from 48.09–51.94% to 22.73–28.48%, representing a reduction of approximately 49%. Concurrently, density increased from 0.96–1.07 g·cm⁻³ to 1.09–1.18 g·cm⁻³, an increase of approximately 12% (p < 0.01, Figure 6). One-way analysis of variance (ANOVA) results (

Table 2. Experimental data of the effects of different outdoor sun-drying time periods on the moisture content and density of potato tubers.

Analysis Object	Source	Sum of Squares	Degrees of Freedom	Mean Square	F	p	Significance
w (%)	Between Groups	1838.4906	4	459.6227	127.3616	<0.01	**
	Within Groups	72.1760	20	3.6088
ρ (g·cm−3)	Between Groups	0.0433	4	0.0108	5.7503	<0.01	**
	Within Groups	0.0377	20	0.0019

Note. ** p < 0.01; w denotes moisture content, %; ρ denotes density, g·cm⁻³; “Between groups” refers to the variation among different field curing duration treatments (0, 1, 2, 3, and 4 h); and “Within groups” refers to the variation among replicate samples within the same curing duration treatment.

) indicated that the effects of curing duration on both moisture content and density were highly significant. The inverse relationship between moisture content and density reflects a structural remodeling process within the tuber tissue. At harvest maturity, the periderm has not yet fully suberized, resulting in relatively low transpiration resistance [7,8]. Moisture is continuously lost through periderm transpiration, leading to a progressive decline in cell turgor pressure, reduction in intercellular spaces, and an increasingly compact tissue structure. These significant changes in physical properties provide the foundation for the subsequent alterations in mechanical properties and damage behavior.

The tissue densification caused by moisture content reduction inevitably leads to alterations in mechanical properties, which was confirmed by the compression test results. As curing duration increased, the compressive mechanical behavior of tubers changed significantly (p < 0.01, Figure 7). ANOVA results (

Table 3. Analysis of variance (ANOVA) for the effects of different outdoor sun-drying time periods on compression characteristics of potato tubers at harvest maturity.

Analysis Object	Source	Sum of Squares	Degrees of Freedom	Mean Square	F	p	Significance
F (N)	Between Groups	12,919.93	4	3229.98	21.56	<0.01	**
	Within Groups	2995.79	20	149.79
E (MPa)	Between Groups	2.16	4	0.54	29.17	<0.01	**
	Within Groups	0.37	20	0.02
υ	Between Groups	0.06	4	0.01	17.95	<0.01	**
	Within Groups	0.02	20	0.001
G (MPa)	Between Groups	0.50	4	0.12	18.23	<0.01	**
	Within Groups	0.14	20	0.01

Note. ** p < 0.01; F is the critical stress, N; E is the elastic modulus, MPa; v is the Poisson’s ratio; G is the shear modulus, MPa; “Between groups” refers to the variation among different field curing duration treatments (0, 1, 2, 3, and 4 h); and “Within groups” refers to the variation among replicate samples within the same curing duration treatment.

) revealed highly significant differences in critical pressure, elastic modulus, Poisson’s ratio, and shear modulus among the different curing duration treatments. Critical pressure increased from 197.54–223.53 N to 271.15–288.10 N, representing an increase of approximately 33%; elastic modulus increased from 2.12–2.59 MPa to 3.00–3.44 MPa, an increase of approximately 37%; and shear modulus increased from 0.77–0.82 MPa to 1.11–1.44 MPa, an increase of approximately 60%. The elevation of these parameters is likely associated with the reduction in cell turgor pressure resulting from moisture loss. Previous studies have demonstrated that as turgor pressure declines, the cell wall progressively becomes the primary load-bearing structure, leading to an increase in tissue stiffness [37]. Furthermore, during the curing process, periderm cell walls undergo progressive deposition of suberin and waxes, forming a hydrophobic barrier layer. This suberization process not only enhances the mechanical strength of the skin and its resistance to pathogen invasion but also alters the mechanical response characteristics of the surface tissue. Concurrently, the middle lamella between cells may contract due to dehydration, resulting in enhanced intercellular bonding forces, which macroscopically manifests as increases in elastic modulus and shear modulus [40]. The decrease in Poisson’s ratio from 0.40–0.48 to 0.29–0.33 indicates a reduced capacity for lateral deformation. Regression analysis of the experimental results showed that the fitting equations between the four parameters and curing duration all yielded R² values greater than 0.95, indicating good fitting accuracy. Collectively, field curing induced a transition in tuber mechanical behavior from a “soft elastic body” to a “hard brittle body”. Although this transition enhances the load-bearing capacity under quasi-static compression conditions, the increased stiffness may intensify localized stress concentration during impact, rendering parenchyma cells more susceptible to brittle fracture [41], which may constitute one of the intrinsic causes for the subsequent aggravation of impact damage.

The alterations in mechanical properties also influenced the surface frictional behavior of tubers. The changes in friction coefficients exhibited significant material dependence (p < 0.01, Figure 8). ANOVA results (

Table 4. Analysis of variance (ANOVA) for the effects of different outdoor sun-drying time periods on friction coefficients of potato tubers at harvest maturity.

Analysis Object	Source	Sum of Squares	Degrees of Freedom	Mean Square	F	p	Significance
μs, pp	Between Groups	0.0118	4	0.0029	4.49	<0.01	**
	Within Groups	0.0131	20	0.0007
μs, ps	Between Groups	0.00013	4	0.00003	15.30	<0.01	**
	Within Groups	0.00004	20	0.000002
μs, pr	Between Groups	0.0120	4	0.0030	4.57	<0.01	**
	Within Groups	0.0131	20	0.0007
μs, pm	Between Groups	0.00045	4	0.00011	120.57	<0.01	**
	Within Groups	0.00002	20	0.000001
μd, pp	Between Groups	0.0187	4	0.0047	6.14	<0.01	**
	Within Groups	0.0152	20	0.0008
μd, ps	Between Groups	0.0320	4	0.0080	20.26	<0.01	**
	Within Groups	0.0079	20	0.0004
μd, pr	Between Groups	0.0278	4	0.0069	8.19	<0.01	**
	Within Groups	0.0169	20	0.0008
μd, pm	Between Groups	0.0327	4	0.0082	24.35	<0.01	**
	Within Groups	0.0067	20	0.0003

Note. ** p < 0.01; μs, pp is the coefficient of static friction between potato and potato; μs, ps is the coefficient of static friction between potato and soil; μs, pr is the coefficient of static friction between potato and rubber; μs, pm is the coefficient of static friction between potato and 65Mn steel; μd, pp is the coefficient of dynamic friction between potato and potato; μd, ps is the coefficient of dynamic friction between potato and soil; μd, pr is the coefficient of dynamic friction between potato and rubber; μd, pm is the coefficient of dynamic friction between potato and 65Mn steel; “Between groups” refers to the variation among different field curing duration treatments (0, 1, 2, 3, and 4 h); and “Within groups” refers to the variation among replicate samples within the same curing duration treatment.

) indicated that all eight friction coefficient parameters showed highly significant differences among the different curing duration treatments. Dynamic friction coefficients generally decreased with increasing curing duration. Specifically, the tuber–soil dynamic friction coefficient decreased from 0.020–0.023 to 0.009–0.011, a reduction of approximately 52%; the tuber–65Mn steel dynamic friction coefficient decreased from 0.337–0.393 to 0.231–0.283, a reduction of approximately 30%; while the tuber–tuber and tuber–rubber dynamic friction coefficients showed no pronounced declining trends. This change is likely attributable to the weakening of capillary adhesion forces resulting from surface moisture loss during curing. Previous research has demonstrated that as the moisture content of agricultural materials decreases, the number of capillary bridges formed by surface water films diminishes, leading to a corresponding reduction in interfacial adhesion forces [42]. Concurrently, skin hardening may reduce microscopic deformation during the frictional process and decrease the real contact area. This pattern is consistent with the observations reported by Nyendu et al. [43], who found that the dynamic friction coefficient of DDGS against various materials decreased with declining moisture content. However, the variation patterns of static friction coefficients were more complex. An increasing trend was observed when in contact with rough porous materials (tubers and rubber), whereas a decreasing trend was noted when in contact with smooth hard materials (soil particles and 65Mn steel). This differentiated behavior may be related to surface morphological changes caused by skin shrinkage of tubers following curing. It is hypothesized that the mechanical interlocking effect between the shriveled skin and rough porous materials is enhanced, while the number of real contact points with smooth hard materials is reduced. However, this mechanism requires further verification through microscopic surface topography analysis. Regression analysis of the experimental results showed that the fitting equations for all eight parameters yielded R² values greater than 0.90. This finding has practical implications for the design of mechanized harvesting equipment, suggesting that conveying components should preferably employ rough materials such as rubber to increase frictional forces and reduce skin abrasion caused by tuber slippage.

Friction characteristics reflect quasi-static contact behavior, whereas the coefficient of restitution characterizes the dynamic impact response. The experimental results demonstrated that the coefficient of restitution increased significantly with increasing curing duration (p < 0.01, Figure 9). ANOVA results (

Table 5. ANOVA of the coefficient of restitution of potato tubers at harvest maturity under different curing duration gradients.

Analysis Object	Source	Sum of Squares	Degrees of Freedom	Mean Square	F	p	Significance
epp	Between Groups	0.0555	4	0.0139	12.86	<0.01	**
	Within Groups	0.0216	20	0.0011
eps	Between Groups	0.0632	4	0.0158	19.19	<0.01	**
	Within Groups	0.0165	20	0.0008
epr	Between Groups	0.0655	4	0.0164	11.72	<0.01	**
	Within Groups	0.0280	20	0.0014
epm	Between Groups	0.0589	4	0.0147	9.04	<0.01	**
	Within Groups	0.0326	20	0.0016

Note. ** p < 0.01; epp is the coefficient of restitution between tuber and potato; eps is the coefficient of restitution between tuber and soil; epr is the coefficient of restitution between tuber and rubber; epm is the coefficient of restitution between tuber and 65Mn steel; “Between groups” refers to the variation among different field curing duration treatments (0, 1, 2, 3, and 4 h); and “Within groups” refers to the variation among replicate samples within the same curing duration treatment.

) revealed highly significant differences in the coefficients of restitution for all four contact materials among the different curing duration treatments. The tuber–65Mn steel coefficient of restitution increased from 0.654–0.749 to 0.777–0.884, an increase of approximately 19%; the tuber–tuber coefficient of restitution increased from 0.543–0.606 to 0.667–0.740, an increase of approximately 22%; and the tuber–rubber and tuber–soil coefficients of restitution exhibited similar upward trends. Regression analysis indicated that all four parameters exhibited good linear relationships with curing duration (R² > 0.95). The increase in the coefficient of restitution implies reduced energy dissipation and enhanced elastic recovery capacity during impact. In conjunction with the compressive property analysis presented in Section 3.2, it can be inferred that field curing increases tuber stiffness, resulting in reduced deformation during impact and decreased energy consumption through plastic deformation, thereby leading to a higher coefficient of restitution. However, this does not indicate an enhancement of the tuber’s damage resistance. On the contrary, the increased stiffness leads to more severe stress concentration during impact; although macroscopic deformation is reduced, localized tissue is more susceptible to microscopic fracture. Feng et al. [29] observed a similar trend in their study on the coefficient of restitution of potatoes at harvest maturity, and the present study further elucidates the intrinsic relationship between this phenomenon and damage behavior.

3.2. Effects of Different Open-Air Curing Durations on Tissue Impact Damage and Skin Abrasion Damage Characteristics of Potato Tubers

The systematic alterations in physical and mechanical properties described above are ultimately reflected in the damage behavior of tubers. The damage tests revealed opposing trends in the two types of damage with increasing curing duration, which constitutes the core finding of this study (p < 0.01, Figure 10). ANOVA results (

Table 6. ANOVA of tissue impact damage and skin abrasion damage of potato tubers at harvest maturity under different curing duration gradients.

Analysis Object	Source	Sum of Squares	Degrees of Freedom	Mean Square	F	p	Significance
an, pr (m·s−2)	Between Groups	845,296.845	4	211,324.211	46.005	<0.001	**
	Within Groups	91,869.556	20	4593.478
at, pr (m·s−2)	Between Groups	1,717,505.467	4	429,376.367	216.433	<0.001	**
	Within Groups	39,677.454	20	1983.873
Rc, pr (%)	Between Groups	22.423	4	5.606	167.064	<0.001	**
	Within Groups	0.671	20	0.034
Rf, pr (%)	Between Groups	19.260	4	4.815	98.875	<0.001	**
	Within Groups	0.974	20	0.049

Note. ** p < 0.01; an, pr is the collision-induced damage acceleration of tuber tissue, m·s⁻²; at, pr is the friction-induced damage acceleration of tuber epidermis, m·s⁻²; Rc, pr is the proportion of collision-induced damage of tuber tissue, %; Rf, pr is the proportion of friction-induced damage of tuber epidermis, %; “Between groups” refers to the variation among different field curing duration treatments (0, 1, 2, 3, and 4 h); and “Within groups” refers to the variation among replicate samples within the same curing duration treatment.

) indicated that all four indicators of tissue impact damage and skin frictional damage showed highly significant differences among the different curing duration treatments. Regarding tissue impact damage, the impact acceleration increased from 1136.80–1218.98 m·s⁻² to 1637.02–1768.91 m·s⁻², an increase of approximately 45%; correspondingly, the proportion of tissue impact damage increased from 2.24–2.47% to 4.65–5.69%, an increase of 119%. Regarding skin frictional damage, the frictional acceleration decreased from 1001.74–1249.52 m·s⁻² to 420.40–494.20 m·s⁻², a reduction of approximately 59%; and the proportion of skin frictional damage decreased from 5.17–5.99% to 3.15–3.39%, a reduction of 41%. Regression analysis demonstrated that the fitting equations for all four parameters yielded R² values greater than 0.96, indicating good fitting accuracy. The contradictory evolution of the two damage types originates from the differential modification effects of field curing on tuber tissues. For skin frictional damage, curing promotes periderm suberization, leading to thickened skin cell walls, enhanced intercellular bonding forces, and improved shear resistance, thereby reducing frictional damage. For tissue impact damage, curing causes moisture loss from tubers and a reduction in parenchyma cell turgor pressure; the middle lamella between cells undergoes dehydration-induced contraction, resulting in increased overall stiffness and enhanced brittleness. During impact, the cell walls lack hydraulic buffering capacity, internal peak stress increases, and parenchyma cells become more susceptible to brittle fracture, leading to aggravated impact damage [40]. Xie [31] and Zhao [32] investigated potato impact damage and skin frictional damage, respectively, but treated them as independent problems. The present study is the first to reveal the trade-off relationship between these two damage types during the field curing process, providing a new perspective for understanding the mechanisms of potato harvest damage. This contradictory relationship carries significant practical implications. The conventional view holds that extending curing duration enhances tuber damage resistance; however, the present study demonstrates that this understanding is incomplete. While curing indeed reduces skin damage, it does so at the cost of aggravating internal tissue damage. Therefore, the determination of optimal curing duration requires a balance between the two types of damage.

3.3. Evaluation Modeling of Treatment Effects Under Different Open-Air Curing Duration Gradients Based on Principal Component Analysis (PCA)

3.3.1. Correlation Analysis and Principal Component Analysis

To comprehensively evaluate the integrated effects of different curing duration treatments, correlation analysis and principal component analysis were performed on 22 indicators. The Pearson correlation analysis results (Figure 11) revealed complex interrelationships among the indicators. Notably, the proportion of tissue impact damage exhibited a strong positive correlation with impact acceleration (r = 0.942), while the proportion of skin abrasion damage showed significant positive correlations with friction acceleration, moisture content, and tuber–soil dynamic friction coefficient. Importantly, the proportion of tissue impact damage demonstrated a strong negative correlation with the proportion of skin abrasion damage (r = −0.89, p < 0.01), statistically validating the antagonistic evolution pattern between these two damage types.

Given the significant correlations among indicators, principal component analysis was employed for dimensionality reduction (

Table 7. Eigenvalues and cumulative variance contribution of principal components.

Principal Components	Eigenvalus	Contribution Rates/%	Cumulative Contribution Rates/%
1	15.705	71.386	71.386
2	1.198	5.446	76.832

and

Table 8. Factor loading matrix of experimental indicators on principal components.

Test Indicators	Principal Component
	1	2
w	−0.925	0.073
ρ	0.739	0.228
F	0.883	0.049
E	0.914	−0.05
υ	−0.863	−0.068
G	0.884	−0.041
μs, pp	0.65	0.536
μs, ps	−0.527	0.593
μs, pr	−0.69	−0.278
μs, pm	−0.878	0.072
μd, pp	0.749	−0.004
μd, ps	−0.856	−0.216
μd, pr	−0.79	0.13
μd, pm	−0.913	0.038
epp	0.862	−0.27
eps	0.874	−0.149
epr	0.806	0.342
epm	0.816	−0.332
an, pr	0.941	0.059
at, pr	−0.971	−0.004
Rc, pr	0.945	0.078
Rf, pr	−0.958	0.11

). Following the criterion of eigenvalues greater than 1, two principal components were extracted, accounting for a cumulative variance contribution of 76.83%. The first principal component captured 71.39% of the original information, with its loading distribution showing high positive loadings for tissue impact damage proportion and impact acceleration, and high negative loadings for skin abrasion damage proportion, friction acceleration, and moisture content. This indicates that the first principal component primarily reflects the trade-off relationship between the two damage types during the curing process: higher scores on the first principal component correspond to intensified tissue impact damage but reduced skin abrasion damage.

3.3.2. Comprehensive Evaluation

The experimental indicators were standardized, and the principal component coefficients corresponding to each indicator were obtained. Using these coefficients as weights, the calculation formulas for the two principal component scores were established as Equations (7) and (8):

$$\begin{array}{l}{F}_1=-0.233X_1+0.186X_2+0.222X_3+0.231X_4-0.217X_5\\ +0.223X_6+0.164X_7-0.132X_8-0.174X_9-0.221X_{10}\\ +0.189X_{11}-0.216X_{12}-0.199X_{13}-0.230X_{14}+0.217X_{15}\\ +0.220X_{16}+0.203X_{17}+0.205X_{18}+0.237X_{19}-0.245X_{20}\\ +0.238X_{21}-0.242X_{22}\end{array}$$

(7)

$$\begin{array}{l}{F}_2=0.066X_1+0.208X_2+0.044X_3-0.045X_4-0.062X_5\\ -0.037X_6+0.489X_7+0.541X_8-0.253X_9+0.065X_{10}\\ -0.004X_{11}-0.197X_{12}+0.118X_{13}+0.034X_{14}-0.246X_{15}\\ -0.136X_{16}+0.312X_{17}-0.303X_{18}+0.053X_{19}\\ -0.004X_{20}+0.071X_{21}+0.101X_{22}\end{array}$$

(8)

where F₁ and F₂ denote the scores of the first and second principal components, respectively; X₁–X₂₂ are the dimensionless values of the 22 experimental indicators after standardization. Specifically, X₁ is the moisture content w, %; X₂ is the density ρ, g·cm⁻³; X₃ is the critical pressure F, N; X₄ is the elastic modulus E, MPa; X₅ is the Poisson’s ratio υ; X₆ is the shear modulus G, MPa; X₇ is the static friction coefficient between tuber and tuber μs, pp; X₈ is the static friction coefficient between tuber and soil μs, ps; X₉ is the static friction coefficient between tuber and rubber μs, pr; X₁₀ is the static friction coefficient between tuber and 65Mn steel μs, pm; X₁₁ is the dynamic friction coefficient between tuber and tuber μd, pp; X₁₂ is the dynamic friction coefficient between tuber and soil μd, ps; X₁₃ is the dynamic friction coefficient between tuber and rubber μd, pr; X₁₄ is the dynamic friction coefficient between tuber and 65Mn steel μd, pm; X₁₅ is the coefficient of restitution between tuber and tuber epp; X₁₆ is the coefficient of restitution between tuber and soil eps; X₁₇ is the coefficient of restitution between tuber and rubber epr; X₁₈ is the coefficient of restitution between tuber and 65Mn steel epm; X₁₉ is the tuber tissue impact damage acceleration an, pr, m·s⁻²; X₂₀ is the skin frictional damage acceleration at, pr, m·s⁻²; X₂₁ is the proportion of tuber tissue impact damage Rc, pr, %; and X₂₂ is the proportion of skin frictional damage Rf, pr, %. Since all indicators were standardized, the above regression models are empirical equations with inconsistent dimensions.

Using the variance contribution rates of each principal component as weights, the comprehensive evaluation formula for curing experimental indicators was established as Equation (9):

$$F=0.71386F_1+0.05446F_2$$

(9)

where F denotes the comprehensive evaluation score; F₁ and F₂ represent the scores of the first and second principal components, respectively; and 0.71386 and 0.05446 are the variance contribution rate weights corresponding to each principal component.

From the ranking of comprehensive scores (

Table 9. Principal component scores and comprehensive evaluation rankings for different curing duration treatments.

Text Groups	Principal Component Score (PCS)		Comprehensive Core	Ranking
	F1	F2
0	−1.352238	0.031908	−0.964	5
1	−0.722038	0.078908	−0.511	4
2	0.020634	−0.174526	0.005	3
3	0.65103	−0.13489	0.457	2
4	1.402608	0.1986	1.012	1

), longer curing durations corresponded to higher comprehensive scores. However, this ranking primarily reflects the unidirectional variation trend of physical–mechanical properties rather than the balanced control effect between the two damage types. Therefore, analysis from the perspective of damage control balance is more practically meaningful. As shown in Figure 11, a curing duration of 3 h achieved an optimal position in terms of comprehensive evaluation (comprehensive score: 0.457, ranked 2nd), effectively balancing the control effects of both damage types. This conclusion aligns with the 2–4 h curing duration range recommended by current technical standards, while further refining the determination of the optimal time point, thereby providing a more operationally practical reference for production practices. It should be noted that this study was conducted on a specific cultivar (Xisen No. 6), production region (Shangdu County, Inner Mongolia, China), and harvest season (October); thus, the generalizability of these conclusions requires validation across multiple cultivars and production regions. Additionally, the single-pendulum impact apparatus simulates single-impact events, whereas the cumulative effects of multiple compound impacts experienced by tubers during actual mechanized harvesting warrant further investigation.

4. Conclusions

This study utilized ‘Xisen No. 6’ potato at harvest maturity as the experimental material and established a comprehensive evaluation system encompassing 22 physical, mechanical, and damage-related parameters to systematically investigate the effects of 0–4 h field curing duration on tuber physical and mechanical properties and impact damage. The following main conclusions were drawn:

(1)

During field curing, moisture loss from tubers induced systematic changes in physical and mechanical properties. Moisture content and Poisson’s ratio decreased, while density, critical pressure, elastic modulus, shear modulus, and coefficient of restitution increased, indicating an overall mechanical behavior transition of tubers from a “soft elastic body” to a “hard brittle body”. The changes in frictional properties exhibited significant material dependence: dynamic friction coefficients generally decreased with curing duration, whereas static friction coefficients increased when in contact with rough porous materials and decreased when in contact with smooth hard materials. This finding can serve as a reference for the material selection of contact components in harvesting machinery.

(2)

The aforementioned mechanical behavior transition exerted opposing effects on the two types of mechanical damage. With increasing curing duration, the proportion of skin frictional damage decreased, while the proportion of tissue impact damage increased, with a strong negative correlation between the two (r = −0.89, p < 0.01). This result demonstrates that the conventional understanding that “extending curing duration enhances tuber damage resistance” is incomplete.

(3)

The optimal curing duration that balances the control of both damage types was determined to be 3 h. The two damage curves intersected at approximately 2–3 h of curing, and the comprehensive evaluation based on principal component analysis further confirmed this optimal point. It is recommended that in practical production, potato tubers deposited in windrows after digging undergo approximately 3 h of field curing before mechanical picking operations, with the specific duration subject to appropriate adjustment based on cultivar characteristics and seasonal climatic conditions. Future research will build upon the physical and mechanical analysis presented in this study by incorporating the determination of physiological and biochemical indicators such as starch and reducing sugar contents, as well as storage quality monitoring, to establish a more comprehensive evaluation system for postharvest field curing treatment.