Revealing the Impact of Starch–Pectin Interactions on the Textural Properties of Different Potato Varieties During Boiling

Abstract

This study aimed to investigate the changes in textural properties of two potato varieties (JZ-226 and XS-6) during boiling and to elucidate the interaction mechanisms within their starch–pectin composite systems, and their impacts on textural characteristics. The results showed that during the boiling process, both potato varieties exhibited decreased hardness and chewiness. As the boiling time lengthened, starch underwent gelatinization; the amylose content dropped; water solubility increased; and the swelling power, transparency, and iodine blue value reduced. Meanwhile, pectin degraded, with the degree of esterification increasing; the content of protopectin and other bound pectins decreased; and water-soluble pectin increased, along with molecular weight rising. In the early stages of gelatinization (15 min), the addition of pectin inhibited the short-range orderliness of starch, forming a relatively stable network structure. However, prolonged gelation disrupts the gel network structure of the starch–pectin complex, leading to further textural changes. Compared to XS-6, the pectin in JZ-226 demonstrated a stronger ability to inhibit starch short-range orderliness, forming a more stable network structure, thereby maintaining superior hardness. These findings provide a theoretical basis for understanding the molecular mechanisms underlying textural changes in potato processing and offer technical support for developing functional potato products.

1. Introduction

Potato (Solanum tuberosum L.), a prominent member of the Solanaceae family, is primarily consumed for its tubers. As the fourth most important food crop in the world, the potato primarily consists of starch (its main component), along with small amounts of proteins, pectins, cellulose, hemicellulose, etc. [1]. Moreover, it is rich in carbohydrates, dietary fiber, and micronutrients, thus possessing functional properties such as enhancing satiety, promoting intestinal health, and boosting immunity. Potatoes can be prepared using a variety of cooking techniques, including stir-frying, stewing, steaming, boiling, and baking. Among them, boiling is widely recognized as the most fundamental and commonly employed approach due to its superior ability to preserve the inherent flavor and nutritional integrity of potatoes [2]. The quality attributes of boiled potato, particularly its textural properties (e.g., hardness, chewiness and adhesiveness), serve as core indicators influencing consumer acceptability and adaptability to subsequent processing [3]. However, in actual production, significant differences in textural properties are observed among different potato varieties after boiling. Specifically, even when subjected to the same boiling process, some varieties exhibit higher hardness and brittleness, resulting in a crisp texture, while others demonstrate a soft and mealy texture [4]. Such variations may be closely related to the complex changes in macromolecular substances within potatoes, especially starch and pectin, during the thermal boiling process, and the potential intricate interactions between starch and pectin [5]. On one hand, the changes induced by boiling, such as starch gelatinization, granule swelling, crystal melting, and amylose leaching, can directly influence the hardness of the boiled products [6]. On the other hand, the loosening of cell junctions caused by boiling can directly facilitate the degradation and dissolution of pectin, a key component of the cell wall, thereby weakening the intercellular adhesion and ultimately leading to cell separation, which subsequently promotes tissue softening [7].

Existing research has predominantly focused on exploring the gelatinization characteristics of starch or the degradation behavior of pectin during the boiling process. However, during the actual boiling process, the amylose leached from starch granules may interact with the dissolved pectin molecules through hydrogen bonding and molecular entanglement, thereby forming an irregular and weakened mixed gel network. Such interactions can significantly alter the microscopic structure, crystalline structure, and other properties of the system, ultimately leading to variations in textural characteristics [8,9]. Currently, this hypothesis has not yet been investigated and validated through experimental research, which poses an obstacle to systematically elucidating the specific mechanisms underlying the significant textural differences observed in boiled potatoes of different varieties.

Against the aforementioned backdrop, this study aims to: (i) compare the differences in textural properties between two potato varieties after boiling; (ii) conduct an in-depth analysis, based on the observed textural disparities, of the effects of boiling treatment on the physicochemical properties of key macromolecular substances (starch and pectin) in potatoes, encompassing parameters such as amylose content and swelling power of starch, and monosaccharide composition, molecular weight, and degree of esterification of pectin; and (iii) investigate the potential interactions between starch and pectin within the dynamic system of boiling, and their impacts on structural characteristics, thereby elucidating the specific mechanisms by which these interactions regulate the textural properties of boiled potatoes. The research findings will provide a solid theoretical basis for the targeted screening and optimization of potato varieties, and offer scientific and technical guidance for the refinement and upgrading of boiling processes that can meet specific textural requirements.

2. Materials and Methods

2.1. Materials

Heat-resistant α-amylase (4000 U/g), sodium bisulfite, and citric acid were purchased from Macklin (Shanghai, China). Iodine standard solution (0.02 mol/L) and anhydrous ethanol (analytical grade) were obtained from Dingsheng Chemical (Tianjin, China). The KIRbio kits were sourced from Jinzhiyan Biotechnology (Beijing, China). Amylose standard and sodium acetate (analytical grade) were acquired from Sigma Aldrich Chemical Company (Darmstadt, Germany). Analytical-grade 95% ethanol, sodium hydroxide, iodine, potassium iodide, acetic acid, and sodium nitrate were procured from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Trifluoroacetic acid (chromatographic grade), methanol (analytical grade), and sodium hydroxide (chromatographic grade) were bought from ANPEL (Shanghai, China).

2.2. Sample Preparation

Two undamaged potato varieties with uniform sizes, namely Jizhang 226 (JZ-226) and Xisen No. 6 (XS-6), were sourced from Longxi County, Gansu Province, China. The processing procedures were as follows: First, the cleaned and peeled potatoes were cut into equally sized cubes (1.5 cm × 1.5 cm × 1.5 cm). A portion of the fresh cubes was directly set aside as the control group (named CK). Next, another portion of the cubes was boiled in water for 30 min, with samples being collected every 5 min. Subsequently, a portion of the obtained potato samples was used for textural property measurement, and the remaining portion was freeze-dried into powder using a freeze dryer (LABCONCO, Yongshengjiahe Technology Co., Ltd., Beijing, China) for the subsequent extraction of starch and pectin.

2.3. Extraction of Potato Starch and Pectin

2.3.1. Extraction of Potato Starch

The extraction of potato starch was conducted based on the method reported by Liu et al. [10] with slight modifications. The specific steps are as follows: First, mash fresh potato cubes (CK) into a paste and dry at 40 °C for 24 h. Then, vacuum freeze-dry the cooked potato cubes. Next, crush the two types of potato powders from the above drying and freeze-drying processes and sieve them through an 80-mesh sieve. Subsequently, take 100 g of each powder, add distilled water at a solid-to-liquid ratio of 1:40 (w/v), and stir to prepare the mixtures. Each mixture was thoroughly stirred and then left to stand for 2–3 h. Following that, each mixture was slowly poured into 70% ethanol, followed by an 18 h sedimentation step. After sedimentation, the precipitate from each mixture was collected and centrifuged at 4000× g at 25 °C for 10 min. This centrifugation process was repeated multiple times until the precipitate turned white. Finally, the white precipitate was subjected to vacuum freeze-drying and then sieved through a 100-mesh sieve to obtain the final potato starch. All the samples were named in the format of “variety-starch-cooking time”, such as JZ-226-S-15 and XS-6-S-25, where S represents starch.

2.3.2. Extraction of Potato Pectin

The extraction of potato pectin was carried out with minor modifications to Yang’s method [11]. Specifically, for the CK samples, the fresh potatoes were cut into cubes and then mashed after mixing with water at a 1:1 (w/v) ratio. Subsequently, 0.05% (w/v) sodium bisulfite was added to prevent oxidation. The resulting wet pulp was then dried at 60 °C overnight. After that, the pulp was ground into a fine powder and passed through an 80-mesh sieve to obtain a homogeneous potato pulp powder. For the boiled potato samples, the freeze-dried powder was used directly. Subsequently, water (1:30 w/v) was added to the 100 g pulp powder, followed by the addition of 5% (w/v) heat-resistant α-amylase, and then the pH value was adjusted to 6.25. The mixture was heated at 95 °C for 30 min, cooled to room temperature, and then centrifuged at 7000× g at 25 °C for 10 min to collect the precipitate. Then, the precipitate was rinsed once with 85% ethanol, re-centrifuged at 7000× g at 25 °C for 10 min, and dried at 50 °C overnight. The finally dried potato pulp was ground and passed through a 40-mesh sieve to obtain potato residue. Finally, distilled water was added to the residue in a ratio of 1:15 (w/v) to form a mixture. The pH of the mixture was adjusted to 2.04 ± 0.02 using 10% citric acid. The mixture was then heated at 90 °C for 60 min, followed by centrifugation at 7000× g at 25 °C for 30 min. Then, the supernatant was collected and treated with three volumes of anhydrous ethanol at 4 °C overnight. After centrifugation, the precipitate was sequentially washed twice with 70%, 80%, and 90% ethanol solutions, respectively. Subsequently, it was dispersed in distilled water and freeze-dried to obtain pectin. All the samples were named in the format of “variety-pectin-cooking time”, such as JZ-226-P-15 and XS-6-P-25, where P represents pectin.

2.4. Analysis of the Textural Properties of Potatoes

The hardness (N), chewiness (N) and adhesiveness (N·sec) of the potato samples were measured with a texture analyzer (TAXT-Plus, Stable Micro Systems, Surrey, UK) fitted with a P2 probe. The test parameters were as follows: the testing mode was TA; the pre-test, test, and post-test speeds were set at 30.00 mm/min, 30.0 mm/min, and 600.00 mm/min, respectively; the test distance was 8.0 mm; and the trigger force was 10.0 g. Each sample group was measured nine times (n = 9).

2.5. Determination of Physicochemical Properties of Potato Starch

2.5.1. Determination of Moisture Content in Potato Starch

The starch sample (0.5 g) was placed in a rapid moisture analyzer (HR83-P, METTLER TOLEDO, Columbus, OH, USA) to measure moisture content, with the results expressed in %.

2.5.2. Determination of Amylose Content

Approximately 10 mg of the starch sample was weighed and transferred into a 15 mL centrifuge tube. Then, 100 μL of 95% ethanol and 900 μL of NaOH solution were added. After thorough vortex mixing, the tube was immersed in a boiling water bath for 13 min. Once cooled to room temperature, the solution was diluted to a final volume of 10 mL, mixed well by shaking, and allowed to stand for 10 min. Subsequently, 0.5 mL of the supernatant was aspirated into another centrifuge tube. Then, 0.1 mL of acetic acid and 0.2 mL of potassium iodide solution were added. The volume was again adjusted to 10 mL, and the tube was kept in the dark for 10 min. Finally, the absorbance of the solution was measured at a wavelength of 720 nm [12].

2.5.3. Determination of Water Solubility Index and Swelling Power of Starch

The determination was performed as described in a previous report [13], with minor modifications. The starch sample (0.1 g) was precisely weighed and subsequently added to 10 mL of deionized water. The mixture was then oscillated for 10 s to ensure the starch was well suspended. Afterwards, the mixture was incubated in a 95 °C water bath for 60 min, with oscillation occurring every 10 min. Following the incubation, the mixture was cooled to room temperature in an ice-water bath and then centrifuged at 8000× g at 25 °C for 20 min. The supernatant was carefully poured into a pre-dried and constantly weighed aluminum bucket, and then both the aluminum bucket containing the supernatant liquid and the test tube with the precipitate were placed in an 80 °C oven to dry until a constant weight was achieved. The calculation method is as follows:

$$\mathrm{W}\mathrm{S}\mathrm{I},\%={\displaystyle \frac{\mathrm{M}4-\mathrm{M}3}{\mathrm{M}}}\times \%$$

$$\mathrm{S}\mathrm{P},\mathrm{g}/\mathrm{g}={\displaystyle \frac{\mathrm{M}2-\mathrm{M}1}{\mathrm{M}(1-\mathrm{W}\mathrm{S}\mathrm{I})}}$$

In the formula, WSI represents the water solubility index; SP stands for the swelling power; M denotes the mass of the sample; M1 indicates the mass of an empty test tube; M2 represents the mass of the test tube and the sample; M3 signifies the mass of the aluminum container; and M4 refers to the mass of the aluminum container and sample.

2.5.4. Determination of Starch Transparency

Approximately 50 mg of the starch sample was added to 5 mL of distilled water and thoroughly mixed. Subsequently, the mixture was heated in a 95 °C water bath for 30 min. During this heating process, vortexing was carried out every 5 min to ensure the formation of a uniform starch paste. After the mixture had cooled to room temperature, its transmittance at 620 nm was measured using a UV-Visible spectrophotometer (UV-1800, Shimadzu, Kyoto, Japan) to determine the transparency [14].

2.5.5. Determination of Starch Iodine Blue Value (IBV)

Approximately 0.125 g of the starch sample was weighed and then transferred into a 25 mL volumetric flask, and diluted with distilled water to volume. Subsequently, the solution was heated and stirred for 5 min. After that, 0.5 mL of the resulting solution was pipetted into another 25 mL volumetric flask. Then, 0.5 mL of the iodine standard solution (0.02 mol/L) was added to this flask, and the solution was also diluted to volume. Finally, the absorbance of the solution was measured at a wavelength of 650 nm, and the IBV was calculated using the following formula:

$$\mathrm{I}\mathrm{B}\mathrm{V}=\mathrm{E}\times 54.2+5$$

In this formula, E stands for the absorbance value at 650 nm.

2.6. Determination of Physicochemical Properties of Potato Pectin

2.6.1. Determination of Esterification Degree (DE) of Pectin

First, 200 mg of the pectin sample was moistened with 1 mL of ethanol. Then, 20 mL of deionized water was added, and the mixture was stirred for dissolution in a 40 °C constant-temperature water bath. An equal volume of deionized water served as the blank control. Subsequently, 5 drops of phenolphthalein indicator (1%) were added to both the sample and blank solutions respectively. Titration was performed using a 0.1 mol/L NaOH standard solution until the solution turned pale pink and the color remained unchanged for 30 s, indicating the titration endpoint. The volumes of NaOH consumed by the sample group and the blank group were recorded as V1 and V0, respectively. Next, 10 mL of a 0.1 mol/L NaOH solution was added to the pectin solution, which was then stirred at room temperature for 2 h to carry out the saponification reaction. After the reaction was completed, 10 mL of a 0.1 mol/L HCl solution was added for neutralization. Finally, titration was conducted again using a 0.1 mol/L NaOH solution until the endpoint was reached, and the volume of NaOH consumed was recorded as V2 [15]. The degree of esterification was calculated according to the following formula:

$$\mathrm{D}\mathrm{E}(\%)={\displaystyle \frac{100\mathrm{V}2}{\mathrm{V}1+\mathrm{V}2-\mathrm{V}0}}$$

2.6.2. Determination of Pectin Molecular Weight

The molecular weight of pectin was measured using a combined system of gel permeation chromatography-differential refractive index detection-multi-angle laser light scattering (GPC-DRI-MALLS, Agilent Technologies, Santa Clara, CA, USA). Specifically, the liquid chromatography system employed was the U3000 (Thermo, Newark, DE, USA), the differential refractive index detector used was the Optilab T-rEX (Wyatt Technology, Goleta, CA, USA), and the laser light scattering detector utilized was the DAWN HELEOS ‖ (Wyatt Technology, CA, USA). During the experiment, the sample was dissolved in a 0.1 M NaNO₃ aqueous solution containing 0.02% (w/w) NaN₃ to achieve a final concentration of 1 mg/mL. Subsequently, the solution was filtered through a 0.45 μm filter and then subjected to online detection. The experimental conditions were as follows: the column temperature was set at 45 °C, the injection volume was 100 μL, the mobile phase A was a 0.1 M NaNO₃ solution with 0.02% NaN₃, the flow rate was 0.6 mL/min, and isocratic elution was carried out for 75 min [16]. Finally, the number-average molecular weight (Mn), weight-average molecular weight (Mw), and polydispersity index (Mw/Mn) were determined.

2.6.3. Determination of Monosaccharide Composition of Pectin

The individual neutral sugars of potato pectin were quantified by ion chromatography (ICS 5000+, Thermo Fisher Scientific, USA). Specifically, clean chromatography vials were taken, an appropriate amount of the sample was weighed, and 1 mL of a 2 mol/L TFA acid solution was added. The mixture was heated at 121 °C for 2 h. Then, nitrogen gas was introduced to purge and dry the sample. Subsequently, 99.99% methanol was added for rinsing, followed by another drying step. This methanol rinsing procedure was repeated 2–3 times. The sample was dissolved in sterile water and transferred to a chromatography vial for testing. A Dionex™ CarboPac™ PA20 (150 × 3.0 mm, 10 μm) liquid chromatography column was employed, with an injection volume of 5 μL [17]. Different concentrations of neutral sugar mixtures, including fucose (Fuc), rhamnose (Rha), arabinose (Ara), galactose (Gal), glucose (Glc), xylose (Xyl), mannose (Man), fructose (Fru), ribose (Rib), and galacturonic acid (GalA-UA), were used as standards.

2.6.4. Determination of Pectin Content in Different Fractions

An amount of 0.1 g of pectin sample was weighed for the determination of protopectin (AIR), water-soluble pectin (WSP), ionic-soluble pectin (ISP) and covalently bonded pectin (CSP). All the steps were carried out in accordance with the instructions provided in the corresponding reagent kits. The determination principle is based on the fact that pectin binds to other components in the plant cell wall through different chemical forces. By using solvents with incrementally increasing selectivity, these forces are gradually disrupted, allowing for the stepwise extraction of pectin in different binding states.

2.7. Preparation of Starch–Pectin Complex

During the boiling of potatoes, the internal macromolecular substances (starch and pectin) may undergo the following changes: First, starch gelatinizes and pectin degrades. Second, the leached-out amylose combines with the degraded pectin to form a starch–pectin complex. Third, the already-degraded starch and pectin further bind to the starch–pectin complex. Thus, to better explain the changes and explore potential mechanisms during boiling, simulations of these scenarios are conducted.

Given that extracted potato starch and pectin account for 15% and 1% of potato dry matter respectively, 15 g starch and 1 g pectin were added to 100 g of water. The mixture then underwent gelatinization at 95 °C for 15 min and 25 min to obtain starch–pectin composites with different gelatinization times, named JZ-226-S-P-15, JZ-226-S-P-25, XS-6-S-P-15, and XS-6-S-P-25 (where “S” = starch, “P” = pectin). Next, the composites after 15 min gelatinization were freeze-dried. Based on these, starch and pectin extracted from 15-minute-boiled potatoes were added according to their potato proportions (15% starch, 1% pectin). A further 10 min gelatinization at 95 °C was carried out to get samples with degraded starch and pectin, combining with the starch–pectin composite. The final composites were named JZ-226-S-P-25-S, JZ-226-S-P-25-P, XS-6-S-P-25-S, and XS-6-S-P-25-P, representing groups with pre-gelatinized starch and pectin addition.

2.8. Scanning Electron Microscopy (SEM) Observation

The morphology of the samples of the control and experimental groups was observed using a SEM (SU8020, Hitachi Ltd., Tokyo, Japan) at an accelerating voltage of 10 kV. The samples were mounted on circular aluminum stubs, and the stubs were immobilized on the stage with double-sided tape. Then, they were sprayed with gold in a vacuum environment. The images were captured at a magnification of 200× [18].

2.9. X-Ray Diffraction (XRD) Analysis

The crystal characteristics of the samples of the control and experimental groups were determined with an X-ray diffractometer (Bruker D8 Advance X, Bruker, Bremen, Germany). The specific operational parameters were as follows: The voltage was set at 40 kV, and the current at 40 mA. The diffraction patterns were recorded within the range of 5–40° (2θ), with a scanning speed of 6°/min and a step size of 0.02° [19]. Subsequently, the relative crystallinity was calculated using Jade software (version 6, Material Data Inc., Livermore, CA, USA), with the specific formula as follows:

$$\mathrm{R}\mathrm{e}\mathrm{l}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{v}\mathrm{e} \mathrm{c}\mathrm{r}\mathrm{y}\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{l}\mathrm{l}\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{y} (\%)=\mathrm{A}\mathrm{c}/(\mathrm{A}\mathrm{c}+\mathrm{A}\mathrm{a})\times 100$$

In this formula, Ac represents areas of crystalline, and Aa represents areas of non-crystalline.

2.10. Fourier Transform Infrared Spectroscopy (FT-IR) Spectral Analysis

The samples of the control and experimental groups were crushed, mixed with KBr in a mass ratio of 3/100, and then pressed into pellets. FT-IR spectral data were collected from 4000 to 400 cm⁻¹ using a Nicolet 6700 FT-IR spectrometer (Nicolet, Mountain, WI, USA). The spectrometer had a resolution of 4 cm⁻¹, a signal-to-noise ratio of 50,000:1, and performed 32 scans [20]. Finally, the spectra were deconvoluted using the OMNIC 8.0 software (Thermo Fisher Scientific, Waltham, MA, USA), and the band intensity ratios of 1047/1022 cm⁻¹ and 1022/995 cm⁻¹ were calculated.

2.11. Confocal Laser Scanning Microscopy (CLSM) Observation of Starch–Pectin Complex

A volume of 1 mL of the complex sample was taken and subjected to a 5 min staining process using a mixed fluorescent dye solution composed of Nile Blue (stains pectin) and FITC (stains starch). Subsequently, the sample was placed on a microscope slide, covered with a coverslip, and observed under a laser scanning confocal microscope (LSM 800, Zeiss, Jena, Germany) at excitation wavelengths of 528 nm and 488 nm.

2.12. Statistical Analysis

Bar charts were created using Prism (version 9.5.0 (730), GraphPad Software, Santiago, CA, USA), and line charts were generated with Origin 2021 (Origin Lab, Northampton, MA, USA). Statistical differences between the two sample groups were analyzed via an independent samples t-test. Meanwhile, for comparing differences among three or more groups, a one-way ANOVA followed by Duncan’s test was employed with a significance level of p < 0.05. All the analyses were conducted using the SPSS 24.0 software (IBM Corp., Chicago, IL, USA). Experimental data are presented as the mean ± standard deviation (SD) of three independent repeated experiments.

3. Results

3.1. The Textural Properties of Potatoes

Since both JZ-226 and XS-6 exhibited severe tissue damage when boiled for 30 min, the boiling time in our experiment was set at 25 min. The results of hardness, chewiness, and adhesiveness for the two potato varieties under fresh conditions (CK) and after boiling treatment are presented in Figure 1A–C. The average hardness and chewiness of JZ-226-CK were 7.8 N and 49 N, respectively, significantly higher than those of XS-6 (6.65 N and 44 N) (p < 0.01). In contrast, the average adhesiveness of XS-6-CK (0.21 N·s) is significantly higher than that of JZ-226-CK (0.17 N·s) (p < 0.05). As the boiling time was prolonged, both the hardness and chewiness of the two potato varieties exhibited a decreasing trend, which stabilized after 10 min of boiling. The adhesiveness initially showed an upward trend (from 0 to 15 min) followed by a decline. At the peak (15 min), the adhesiveness of XS-6 was significantly higher than that of JZ-226 (p < 0.01). This phenomenon is likely attributable to the release of viscous molecules during the gelatinization process of potato starch, which subsequently leads to an increase in the potato’s adhesiveness. However, when the boiling process lasts for a specific period, the cellular structure of potatoes is completely disrupted, leading to the exudation of water and starch degradation, which ultimately results in a decrease in hardness, chewiness, and adhesiveness. This trend is consistent with the findings of Hu et al. [2] and Liang et al. [21]. Based on the above results, the fresh potato samples and those boiled for 15 min and 25 min were ultimately selected for the extraction and further analysis of starch and pectin.

3.2. Analysis of Physicochemical Properties of Potato Starch

Determining the physicochemical properties of potato starch enables the evaluation of its gelatinization characteristics, facilitating the prediction and regulation of the final product quality, including appearance, texture, mouthfeel, and stability [22]. The results of the relevant indicators in this study are presented in Figure 2A–F. As shown in Figure 2A, the moisture content of JZ-226 showed an overall declining trend during the boiling process, which may be attributed to its relatively compact starch structure, making moisture escape more easily. However, the moisture content of XS-6 initially exhibited a significant increase, followed by a significant decrease. This might be attributed to the fact that its starch granules absorbed water and swelled (a process of gelatinization) during the initial stage (15 min) of boiling, while excessive boiling (25 min) resulted in the breakage or retrogradation of starch chains, which facilitated the release of moisture. In general, the aforementioned differences may be associated with the amylose/amylopectin ratio, particle size, or chemical composition of potato starches from different varieties.

Significant changes (p < 0.05) in the amylose content of both JZ-226-CK and XS-CK were observed during the boiling process (Figure 2B). Specifically, boiling led to a continuous decrease in the amylose content of both samples. The unboiled JZ-226-CK and XS-6-CK had amylose contents of 33.14% and 25.97%, respectively. At the end of boiling, JZ-226-25 and XS-6-25 showed contents of 20.01% and 19.05%, respectively. This may be because amylose has a relatively small molecular weight and a linear structure [23]. During the gelatinization and swelling process of starch granules induced by boiling, amylose is more inclined to dissolve and leach out from the granules than the highly branched amylopectin, which consequently leads to a reduction in its content. In addition, Huong et al. [24] also proposed that during the boiling process, the starch granules containing amylose are susceptible to breakage, leading to a reduction in starch viscosity and ultimately resulting in the formation of gels with a softer texture. This aligns with the findings of this study.

WSI can be employed to assess the extent of overflow of the soluble components predominantly composed of amylose in starch. In Figure 2C, the WSI of JZ-226 and XS-6 both exhibited a significant increase during the boiling process, which is a direct manifestation of the substantial leaching of potato amylose. This result indicates that the gelatinization of potato starch caused by boiling facilitates the free swelling of starch granules and enables them to absorb more water [25].

As shown in Figure 2D, the SP of JZ-226 and XS-6 declined during the boiling process. This aligns with the findings of Lin et al. [26], indicating that the excessive gelatinization caused by prolonged boiling had irreversibly damaged the structure of amylopectin, reduced its molecular weight, weakened its ability to form a strong gel network, and ultimately led to a significant decrease in swelling volume [27].

The determination of transparency primarily reflects the clarity and transparency of the paste–liquid system formed after starch gelatinization [28]. A high transparency indicates that during the gelatinization process of starch, the starch granules are uniformly and thoroughly disintegrated. Low transparency (high turbidity) implies a large amount of light-scattering substances in the paste liquid, such as fragments of incompletely gelatinized starch granules, amylose molecules that have redissolved and repolymerized after leaching from granules, and insoluble impurities like lipids and proteins [29]. As can be observed from Figure 2E, compared with CK, the transparency of JZ-226 significantly decreased at 15 min of boiling and increased after 25 min, but still remained lower than that of CK. This may be attributed to the formation of short chains in amylopectin or alterations in the gel structure. However, the transparency of XS-6 continuously decreased with the increase in boiling time, which may suggest that it has a higher thermal sensitivity.

The IBV is capable of characterizing both the ratio of amylose to amylopectin and the extent of cell damage upon disruption. A higher degree of cell damage is associated with a darker color [30]. Both the IBV of JZ-226 and XS-6 starches exhibited a trend of initially significant increase followed by a marked decrease during the boiling process. We speculate that this may result from starch degradation after heat treatment, with the generated short straight-chain or medium- to long-chain dextrins enhancing iodine binding capacity and deepening the blue color.

In general, the changes in the physicochemical properties of starch can partially reveal the texture changes in potatoes during the boiling process.

3.3. Analysis of Physicochemical Properties of Potato Pectin

Determining the physicochemical properties of potato pectin offers insights into its chemical structure and functional characteristics. The results of the physicochemical property indicators of potato pectin in this study are shown in Figure 3A–C. Determining the degree of esterification (DE) of pectin enables the clear identification of pectin types (such as high-ester pectin or low-ester pectin), reveals the properties of gels, and evaluates its solubility and stability in water. As shown in Figure 3A, during the boiling process, the degree of esterification (DE) of JZ-226 and XS-6 increased significantly. This trend is likely attributed to the preferential degradation of low-methoxyl pectin and free galacturonic acid induced by boiling, and the thermal inactivation of endogenous pectin methylesterase (PME) [31]. When the temperature surpasses 60–65 °C, potato PME rapidly inactivates within 5–10 min (D-value ~3–5 min at 70 °C), halting the enzymatic de-esterification reaction. Consequently, high-methoxyl pectin fragments are relatively retained due to thermal degradation selectivity, which in turn leads to an increase in the DE value [32]. Certainly, this phenomenon may lead to a reduction in the gel-forming ability of pectin, thereby promoting the softening of potato texture.

The determination of pectin components obtained through fractional extraction unveils the composition, structure, and degree of cross-linking of pectin substances in plant cell walls, particularly in the middle lamella and primary cell wall [33]. As depicted in Figure 3B, during the boiling process of the JZ-226 and XS-6 samples, the contents of AIR, ISP, and CSP significantly decreased, while the content of WSP markedly increased. Among them, JZ-226 showed a higher WSP content after 25 min of boiling, with more significant pectin degradation and transformation. This is consistent with the following viewpoint: boiling disrupts the structure of potato cell walls, triggering the degradation and transformation of pectin, facilitating the continuous breakdown and conversion of insoluble pectins (AIR, CSP, and ISP) into WSP pectin, and ultimately affecting the texture of potatoes (changing from hard to soft) [34]. Notably, CSP is deemed to be highly correlated with textural properties, primarily due to the fact that it has relatively short side chains and contains a substantial amount of RG (rhamnogalacturonan) structures. These features reduce the steric hindrance between molecules, which is conducive to the formation of the “egg-box model” by Ca²⁺. As a result, during processing (such as boiling), the cell wall of CSP can maintain a relatively intact state and is less prone to degradation [15]. The CSP content in JZ-226 and XS-6 significantly decreased with boiling time. At 25 min, XS-6 had an even lower CSP content, and this decrease was unfavorable for eggshell structure formation. In this study, as a reagent for pectin extraction, citric acid can effectively chelate Ca²⁺ in the cell wall and dissolve the egg-box model structure, thereby releasing those high-esterification-degree (HM) pectin fragments with relatively large molecular weights that were originally cross-linked by calcium bridges. This is likely the primary reason for the decrease rather than the increase in CSP content. Moreover, the decline in CSP content can lead to the softening of potato tissues. In general, boiling treatment could significantly influence the textural properties of potatoes by altering the molecular structure, solubility characteristics, and component distribution of pectin.

In the results of absolute molecular weight (Figure 3C), compared with the fresh samples (CK), the number-average molecular weight (Mn, Figure 3C(a)) and weight-average molecular weight (Mw, Figure 3C(b)) of JZ-226 and XS-6 significantly increased after boiling. This phenomenon can be explained by the following: Pectin in fresh potatoes exists as independent long-chain molecules, primarily in the form of protopectin, which is a high-molecular-weight polymer, accompanied by a small amount of soluble pectin freely present between cells. Thus, the pectin extracted at this stage is mainly the soluble pectin freely located in the intercellular spaces. However, boiling treatment can induce pectin degradation, which facilitates the cleavage and dissolution of protopectin, and prompts the originally insoluble high-molecular-weight protopectin fragments to dissociate from the cell wall network and subsequently convert into soluble pectin [35,36]. The results correspond to those presented in Figure 3B(a,b). At this time, the extracted pectin encompasses not only the freely existing soluble pectin between cells but also the degraded pectin, which consequently leads to a significant increase in its molecular weight.

The PDI describes the breadth of the molecular weight distribution of a polymer. When PDI ≈ 1, it indicates a highly uniform molecular weight, suggesting high purity and stable quality of the samples. When PDI > 1, it implies a broad molecular weight distribution, indicating non-uniformity of the sample, which contains an excessive amount of degraded small molecular fragments [37]. In Figure 3C(c), the PDI values of both fresh and boiled JZ-226 and XS-6 were greater than 1, which suggests an extremely broad molecular weight distribution, consistent with the multi-domain characteristics (HG/RG-I/RG-II) of pectin.

The monosaccharide composition information of pectin in JZ-226 and XS-6 is shown in

Table 1. The monosaccharide compositions of pectin extracted from the JZ-226 and XS-6 potato varieties after boiling for 0 (CK), 15, and 25 min.

Samples	Ara (%)	Rha (%)	Gal (%)	Glu (%)	Gal-UA (%)
JZ-226-CK	2.423 ± 0.005 b	1.183 ± 0.012 b	62.627 ± 0.434 c	31.123 ± 0.304 a	2.643 ± 0.165 c
JZ-226-15	4.170 ± 0.024 a	2.817 ± 0.026 a	80.077 ± 0.116 a	7.130 ± 0.124 b	5.800 ± 0.071 b
JZ-225-25	4.173 ± 0.132 a	2.750 ± 0.014 a	79.487 ± 0.852 b	7.540 ± 0.724 b	6.050 ± 0.067 a
XS-6-CK	2.653 ± 0.029 b	1.680 ± 0.028 b	61.367 ± 0.474 b	30.217 ± 0.690 a	4.077 ± 0.275 c
XS-6-15	4.353 ± 0.128 a	3.477 ± 0.081 a	77.307 ± 0.612 a	7.023 ± 0.076 b	7.660 ± 0.372 a
XS-6-25	4.680 ± 0.078 a	3.277 ± 0.033 a	76.803 ± 0.158 a	8.887 ± 0.215 b	6.353 ± 0.160 b

Values are the means of three replicates ± standard deviation (SD). Within the same variety in the same column, values with different letters differ significantly (p < 0.05) in arabinose (Ara), rhamnose (Rha), galactose (Gal), glucose (Glu), and galacturonic acid (Gal-UA).

. Both the pectin samples were predominantly composed of arabinose (Ara), rhamnose (Rha), galactose (Gal), glucose (Glu), and galacturonic acid (Gal-UA), with Gal having the highest proportion. Moreover, there was no significant difference between the two varieties. This finding is consistent with the research conducted by Xie et al. [38]. During boiling, the contents of Ara, Rha, Gal, and Gla-UA increased significantly, while that of Glu decreased notably. The increase in the contents of Ara and Rha may be associated with the exposure and release of the side chains (rich in Ara/Rha) of the RG-I backbone under the effect of boiling, which makes them more accessible for extraction. Meanwhile, the significant increase in Gal content indicates that boiling can promote the degradation of starch or cellulose bound to pectin, thereby releasing more free Gal. Gal-UA, as a component of the pectin backbone, significantly increased in content after boiling treatment. This might be attributed to the fact that excessive high-temperature boiling time can inhibit the degradation of the pectin backbone by pectinase, thereby making it easier to extract pectin molecules with intact structures, high galacturonic acid content, and large molecular weights [39]. In contrast, the marked decrease in Glu content suggests its hydrolysis during boiling.

Based on the results, it can be concluded that as boiling time increases, the DE and Mw of potato pectin increase significantly, while hardness decreases accordingly, showing a significant negative correlation. The core mechanism for this is as follows: Heating preferentially degrades low-ester pectin, leading to an increase in the DE. It also disrupts the protopectin network, causing the dissolution of high-molecular-weight fragments and an increase in Mw/Mn. More critically, calcium ion-cross-linked CSP degrades continuously into WSP, disrupting the cell wall’s “egg-box” structure and directly reducing hardness. Compared with XS-6 pectin, JZ-226 pectin shows marked differences in key molecular features, potentially significantly affecting its interaction mechanism with other potato components (e.g., starch). The following mechanisms may explain the variety differences: Firstly, after boiling, JZ-226 has a significantly higher galactose content than XS-6. The numerous exposed hydroxyl groups on galactose side chains can form a dense hydrogen-bond network with starch. Secondly, JZ-226 retains more calcium-cross-linked CSP components. Its “egg-box structure” acts as a molecular scaffold, effectively anchoring starch granules and limiting their swelling [40]. Meanwhile, during boiling, JZ-226 releases more water-soluble pectin. This pectin has a polydispersity characteristic, containing both high-molecular-weight protopectin fragments and low-molecular-weight degradation products (PDI > 1), forming a multi-scale composite network that acts on both the surface and interior of starch granules [41]. Additionally, the moderate hydrophobicity from its increased esterification degree enhances its affinity for the helical structure of starch. In contrast, XS-6 experiences more thorough CSP degradation, has a lower galactose content, and releases less WSP, resulting in weaker interaction sites and physical entanglement with starch compared to JZ-226. In summary, the increase in pectin DE and molecular weight is essentially a “dissolution effect” of cell wall deconstruction. The negative correlation between this increase and hardness changes is mainly driven by CSP cross-linking loss.

3.4. Structural Characterization of Starch–Pectin Complex

3.4.1. SEM Observation of the Complex

The SEM images of the starch–pectin complexes are presented in Figure 4. Through observation, we found that the gelatinization time significantly influenced the microstructure of starches of JZ-226 and XS-6, which was manifested as follows: At the initial stage of gelatinization (15 min), JZ-226-15 and XS-6-15 (Figure 4A,G) formed a relatively stable, porous, lamellar, and continuous network structure. Moreover, compared with XS-6, the network structure of JZ-226-15 was more stable. At the later stage of gelatinization (25 min), the structure of JZ-226-25 (Figure 4B) became loose and rough, exhibiting a “sponge-like” appearance, while the structure of XS-6-25 (Figure 4H) showed enlarged voids and even collapse. This phenomenon may be related to the differing amylose contents of JZ-226 and XS-6. Moreover, these structural changes can directly lead to a decline in the stability of the starch gel network and a reduction in hardness.

Compared with JZ-226-15 and XS-6-15, both JZ-226-S-P-15 (Figure 4C) and XS-6-S-P-15 (Figure 4I) formed more uniform and distinct network structures. This may be attributed to the fact that the addition of pectin facilitated the formation of the starch gel network structure. This is likely because within the suitable pH range (the experimental environment is neutral, pH ≈ 7), the carboxyl groups in pectin moderately dissociate, forming negatively charged carboxylate anions (-COO⁻). The repulsion of negative charges causes pectin molecular chains to extend, providing ample binding sites for interaction with starch molecules. Meanwhile, this pH does not break the glycosidic bonds in starch molecules, enabling amylose to fully extend and form partial helices while amylopectin retains its branched structure. Consequently, the two components combine to form a network structure [42]. However, as gelatinization time increased, the gel network voids in JZ-226-SP-25 (Figure 4D) and XS-6-S-P-25 (Figure 4J) decreased, forming a lamellar structure. This could be attributed to the fact that, within the gelatinization system of the complex, pectin competitively binds with water molecules along with starch molecules, altering the moisture state of the system, and thereby indirectly influencing the rearrangement process of starch molecules [43].

In groups of JZ-226-S-P-25-S (Figure 4E) and XS-6-S-P-25-S (Figure 4K) with pre-gelatinized starch added, we observed that the incorporation of starch could lead to the attachment of flocculent substances within the system, facilitating the formation of a fragmented structure in the complex and causing the disappearance of the original gel network. This phenomenon was more pronounced in XS-6-S-P-25-S. Furthermore, we found that compared with the aforementioned JZ-226-S-P-25-S and XS-6-S-P-25-S groups, the addition of pre-gelatinized pectin could promote the formation of lamellar structures in JZ-226-S-P-25-P (Figure 4F) and XS-6-S-P-25-P (Figure 4L) while retaining a portion of the gel network. However, the structural damage was more severe than that observed in JZ-226-S-P-25 and XS-6-S-P-25. This indicates that during the gelatinization process, pectin and starch continuously interact with each other. However, the prolongation of gelatinization time leads to the disruption of starch structures and the degradation of pectin, weakening their mutual interactions and the ability to form a gel network structure, ultimately resulting in the loss of their functional properties.

3.4.2. CLSM Observation of the Complex

The CLSM results for the JZ-226 and XS-6 potatoes are shown in Figure 5. The fluorescence distribution maps of starch–pectin complexes from these two potato varieties exhibited significant differences. When uncooked (Figure 5A,E), distinct red starch granules and green pectin solutions were observable under the microscope. The merged bicolor images showed that pectin adhered to the edges of starch granules, resulting in the formation of yellow regions. In terms of sample distribution, the JZ-226 complex was relatively concentrated, while the XS-6 complex was more dispersed. In the early stage of gelatinization (15 min), both starch and pectin in JZ-226-S-P-15 (Figure 5B) and XS-6-S-P-15 (Figure 5G) formed a continuous phase of interpenetrating networks (the yellow areas in the figures). This region serves as the “two-phase interface zone” between starch and pectin, representing the interactions of contact, permeation, and interweaving between the starch phase and the pectin phase. In the later stage of gelatinization (25 min), the yellow-colored regions in JZ-226-S-P-25 (Figure 5C) and XS-6-S-P-25 (Figure 5H) decreased. Simultaneously, phenomena such as the swelling and rupture of starch granules and the decentralization of fluorescence distribution were observed, indicating a weakening of the interactions between starch and pectin.

Furthermore, we found that the addition of pre-gelatinized starch could induce a two-phase separation state in JZ-226-S-P-25-S (Figure 5D) and XS-6-S-P-25-S (Figure 5I). After the addition of pre-gelatinized pectin, we observed that there were small patches of yellow regions in JZ-226-S-P-25-P (Figure 5E) and XS-6-S-P-25-P (Figure 5J), which were similar to those in Figure 5C,H. This suggests that upon reaching 15 min of gelatinization for complexes of the JZ-226 and XS-6, the reaction between pectin and starch within the complex system remains incomplete. Subsequent addition of pre-gelatinized pectin and continued gelatinization for 25 min results in its ongoing reaction with the starch in the complex system, thereby maintaining the stability of the gel.

Previous studies have demonstrated that starch and pectin primarily form a gel network structure through non-covalent bond interactions, which helps maintain the stability of the system [44], and the stronger this interaction is, the more stable the system becomes. As evident from Figure 5, the extent of interaction in the complexes of JZ-226 was greater than that of XS-6. This may be the reason why the hardness values of JZ-226 were higher than those of XS-6.

3.4.3. FT-IR Spectroscopy Analysis of the Complex

In this study, FT-IR spectroscopy (Nicolet, WI, USA) was employed to analyze the characteristic changes in the molecular structures of starch and starch–pectin complexes from JZ-226 and XS-6 during the cooking and gelatinization process. As shown in Figure 6, characteristic peaks were observed at 3400 cm⁻¹, 2930 cm⁻¹, 1624 cm⁻¹, 1157 cm⁻¹, and 1018 cm⁻¹ for all the treatment groups of the two potato varieties. Among them, the peak at 3400 cm⁻¹ corresponds to the hydroxyl group, and its variation reveals the dynamic reorganization process of the hydrogen-bonding network. The characteristic peak at 2930 cm⁻¹ represents the vibration of carbon-hydrogen bonds, reflecting conformational changes in the molecular chains. The peak at 1642 cm⁻¹ corresponds to the carbonyl group, and its shift indicates the influence of water molecules on the chemical bonding environment. The characteristic peaks at 1157 cm⁻¹ and 1018 cm⁻¹ denote the polysaccharide fingerprint region, arising from coupled vibrations of multiple bonds (e.g., C-O, C-C, and C-OH stretching), indicating a transition of sugar chains from an ordered to a disordered state. This phenomenon is related to the changes in the C-O-C bonding environment caused by the rupture of starch granules and the release of molecular chains [45]. Compared with the pure starch groups of JZ-226 and XS-6, no new functional groups emerged in the starch–pectin complexes of the two varieties after boiling and gelatinization. This indicates that no new chemical bonds were formed between starch and pectin. The interaction between them was primarily through non-covalent hydrogen bonds (-OH) of the pectin side chains [46].

In the FT-IR analysis related to starch, the short-range orderliness (1047/1022) and the degree of double helices (1022/995) are primarily used to characterize the crystalline structure and the degree of molecular arrangement order in starch granules. A higher 1047/1022 ratio indicates stronger short-range orderliness (e.g., local crystallization or hydrogen-bond arrangement) of starch molecular chains. A higher 1022/995 ratio suggests a more intact double-helix structure (e.g., amylose helices) of starch [47]. By analyzing the ratios of characteristic peaks of JZ-226 and XS-6, we found that as the boiling time prolonged, both the short-range orderliness (1047/1022) and the integrity of the double-helix structure (1022/995) of JZ-226-25 and XS-6-25 significantly decreased. This confirms that boiling disrupts the crystalline regions of starch granules, leading to the destruction of the ordered arrangement of starch molecular chains and the dissociation of the double-helix structure [48]. Notably, compared with JZ-226-15, JZ-226-25, XS-6-15, and XS-6-25, the short-range orderliness (1047/1022) of JZ-226-S-P-15, JZ-226-S-P-25, XS-6-S-P-15, and XS-6-S-P-25 was significantly decreased. This is likely because pectin can physically disrupt and limit the movement, aggregation, and ordered alignment of starch molecular chains (especially amylose) by competing with starch for water and forming interactions, consequently reducing the short-range orderliness.

Furthermore, after the addition of pre-gelatinized starch, the short-range orderliness in the JZ-226-S-P-25-S and JZ-226-S-P-25-P systems decreased sharply. This indicates that the added pre-gelatinized starch may strongly interfere with and disrupt the locally ordered arrangement of residual molecular chains in the systems by introducing a large number of amorphous gelatinized starch fragments. Moreover, compared with JZ-226-S-P-25 and XS-6-S-P-25, the short-range orderliness in JZ-226-S-P-25-S and JZ-226-S-P-25-P increased but remained lower than that in JZ-226-25 and XS-6-25. This indicates that the newly added pectin molecules persistently interact with free starch chains in the systems, promoting the formation of new locally ordered regions and thus limiting the excessive disordering of starch molecules during continuous boiling.

In general, the reduction in short-range order and the degree of double helices in the complexes indicate that cooking-induced gelatinization disrupts the crystalline structures that maintain firmness, leading to a weaker gel network and ultimately resulting in a significant decrease in the hardness and chewiness of boiled potato cubes.

3.4.4. XRD Analysis of the Complex

XRD analysis was employed to reveal the changes in the crystal structure of JZ-226 and XS-6 starches and their starch–pectin complexes during the boiling process. As shown in Figure 7, both JZ-226-CK and XS-6-CK exhibited a B-type crystal structure. After gelatinization, the starch samples of JZ-226-15, JZ-226-25, XS-6-15, and XS-6-25 all transformed into a V-type crystal structure. However, the crystal forms of all the complexes of JZ-226 and XS-6 remained B-type after gelatinization, indicating that the addition of pectin did not lead to the formation of a new crystal type in the complexes.

In the comparison of starches subjected to different boiling times, we found that compared with JZ-226-15 and XS-6-15, the diffraction peak intensities of JZ-226-25 and XS-6-25 near 17° (the main diffraction peak of the starch crystal structure) and 22° decreased. Moreover, this phenomenon was more pronounced in JZ-226. In addition, compared with JZ-226-CK and XS-6-CK, the relative crystallinities of all the starch–pectin complexes of JZ-226 and XS-6 after boiling decreased significantly (

Table 2. The short-range orderliness, double-helix degree, and relative crystallinity of the starch extracted from JZ-226 and XS-6 potato varieties.

Samples	R1047/1022	R1022/995	Crystallinity (%)
JZ-226-CK	1.023 ± 0.004 c	0.982 ± 0.012 a	10.432 ± 0.221 a
JZ-226-15	1.093 ± 0.001 a	0.863 ± 0.009 c	5.393 ± 0.272 b
JZ-226-25	1.084 ± 0.004 b	0899 ± 0.005 b	3.351 ± 0.034 c
JZ-226-S-P-15	1.021 ± 0.163 a	0.866 ± 0.089 a	5.532 ± 0.043 b
JZ-226-S-P-25	1.100 ± 0.072 a	0.885 ± 0.047 a	3.404 ± 0.210 c
JZ-226-S-P-25-S	1.122 ± 0.158 a	0.768 ± 0.099 a	3.363 ± 0.323 c
JZ-226-S-P-25-P	1.180 ± 0.025 a	0.841 ± 0.032 a	3.562 ± 0.301 c
XS-6-CK	1.089 ± 0.003 b	1.002 ± 0.006 a	8.623 ± 0.170 a
XS-6-15	1.106 ± 0.071 a	0.768 ± 0.065 b	5.244 ± 0.232 b
XS-6-25	1.139 ± 0.016 ab	0.851 ± 0.012 b	4.051 ± 0.021 c
XS-6-S-P-15	1.161 ± 0.048 a	0.756 ± 0.056 bc	5.451 ± 0.233 b
XS-6-S-P-25	1.138 ± 0.007 ab	0.809 ± 0.006 ab	4.402 ± 0.230 c
XS-6-S-P-25-S	1.097 ± 0.018 a	0.851 ± 0.025 a	2.493 ± 0.172 b
XS-6-S-P-25-P	1.166 ± 0.022 b	0.722 ± 0.026 c	2.682 ± 0.184 b

Values are the means of three replicates ± standard deviation (SD). Within the same variety in the same column, values with different letters differ significantly (p < 0.05).

). Compared with the relative crystallinities of JZ-226-15, JZ-226-25, XS-6-15, and XS-6-25, those of JZ-226-S-P-15, XS-6-S-P-15, JZ-226-S-P-25, and XS-6-S-P-25 increased. This indicates that the addition of pectin enhanced the relative crystallinity of starch. Furthermore, this phenomenon can be explained by the following aspects: Pectin molecules compete with starch for the free water in the system, reducing the available water within starch granules. Also, pectin forms a deposition layer on the surface of starch granules, which restricts the swelling of starch granules during the gelatinization process. As a result, the extent of heat-induced damage to the internal long-range ordered crystal structure inside the starch granules is significantly reduced [49].

Additionally, compared with JZ-226-S-P-25 and XS-6-S-P-25, the diffraction peak intensities of JZ-226-S-P-25-S, JZ-226-S-P-25-P, XS-6-S-P-25-S, and XS-6-6-S-P-25-P all exhibited a decreasing trend, and this change was more pronounced in JZ-226. This is manifested by the near-disappearance of the diffraction peak of JZ-226-S-P-25-P at 17°, indicating that its starch has transformed from a (semi-)crystalline state to an amorphous state, thereby reaching the gelatinization endpoint. However, XS-6-S-P-25-P retained some degree of ordered structure, maintaining the diffraction peak at that position. This may be due to the fact that the interaction intensity between pectin and starch in the JZ-226-S-P-25-P complex system is higher than that in the XS-6-S-P-25-P, resulting in the formation of a large number of non-crystalline complexes in the system, which severely disrupted the original crystals.

3.5. Speculated Mechanisms of the Interaction Between Starch and Pectin

We hypothesize the possible interaction mechanism between starch and pectin during potato boiling as follows (As shown in Figure 8): Potato pectin mainly interacts non-covalently with starch via a triple mechanism—hydrogen bonding, water competition, and surface deposition. Its side-chain hydroxyl groups form a hydrogen-bonding network with free hydroxyls on starch granule surfaces and leached amylose. Meanwhile, it competes for system water and forms a physical deposition layer on granule surfaces, restricting gelatinization swelling and the movement, approach, and ordered arrangement of starch molecular chains, thus significantly reducing short-range orderliness. After starch granule disintegration, starch and pectin form an interpenetrating continuous gel phase (the yellow interfacial region in CLSM) in the inter-granular space. This allows starch molecules to form a uniform porous layered structure, enhancing the system’s hardness and stability. Additionally, pectin degradation during boiling has a significant time-dependent disruptive effect on network structure formation. As the gelatinization time is prolonged to 25 min, continuous high temperature ruptures pectin molecular chains, reducing carboxyl group reactivity, directly weakening its ability to bind with leached amylose via electrostatic-hydrogen bonding (as shown by the significant reduction in yellow interpenetrating regions in CLSM images). Moreover, this degradation occurs simultaneously with the destruction of the starch structure, resulting in a synergistic negative effect: pectin not only loses its competitive binding ability to water molecules, causing an imbalance in the system’s moisture state, but also fails to effectively regulate the rearrangement of starch molecules. This transforms the originally uniform porous network structure (Figure 4C,I) into a lamellar or spongy loose structure (Figure 4D,J), with gel cavity collapse and a decrease in hardness. This impact is particularly prominent in the XS-6 variety compared to JZ-226, indicating that the degree of pectin degradation is closely related to the initial network density. Notably, when pre-gelatinized pectin is added externally to the system, the newly formed pectin molecules can interact with residual starch, partially restoring network continuity (Figure 5E,J), conversely confirming that the degraded pectin has lost its functional activity to stabilize the gel. Additionally, JZ-226 complex formation yields more amorphous structures, suggesting stronger pectin-binding capacity; whereas the effect in XS-6 is weaker, with a smaller decrease in crystallinity. Overall, we hypothesize that pectin initially forms an interpenetrating gel phase with starch via non-covalent interactions like hydrogen-bonding network formation, water competition, and surface deposition, maintaining potato texture firmness [50]. As cooking time extends, pectin degrades continuously while starch structure is disrupted, ultimately causing gel network relaxation and collapse, and softening the potato texture.

4. Conclusions

This study investigated the changes in textural properties of two potato varieties (JZ-226 and XS-6) during the boiling process, and elucidated the interaction mechanism within their starch–pectin composite systems during boiling, and its impact on textural properties. The study found that the hardness and chewiness of both potato varieties significantly decreased during boiling, a phenomenon primarily attributed to starch gelatinization and cell wall degradation. Specifically, as the boiling time increased, amylose constantly leached out from the starch, while starch granules continuously absorbed water and swelled until they ruptured. This resulted in an increase in the starch water-solubility index and a decrease in swelling power, transparency, and iodine blue value, thereby transforming the starch from relatively hard solid granules into a soft gel. Meanwhile, during the boiling process, pectin underwent continuous degradation. This led to a significant rise in its degree of esterification, a progressive breakdown of protopectin (AIR), a steady decline in the contents of ionic-soluble pectin (ISP) and covalently bonded pectin (CSP), a continuous increase in the content of water-soluble pectin (WSP), and a notable rise in the molecular weight of pectin. Ultimately, the aforementioned phenomena caused the loosening of cell junctions and the collapse of tissue structure. Through further exploration of the interactions between starch and pectin, we found that at the initial stage of gelatinization (15 min), the addition of pectin inhibited the formation of short-range orderliness in starch, enabling the starch to form a relatively stable network structure. However, as the gelatinization time was prolonged, the polymer chains of the pectin-starch complex ruptured, and the gel network was disrupted, leading to a softening of the potato texture. Compared with XS-6, the pectin from JZ-226 exhibits a stronger ability to inhibit the short-range ordering of starch, forms stronger interactions with starch, and creates a more stable network structure. Consequently, JZ-226 demonstrates superior hardness retention capability compared to XS-6. These findings provide a theoretical basis for understanding the molecular foundation of textural changes during potato processing and offer technical support for the development of functional potato products.