Modulating the Physicochemical Properties and Internal Structure of Maize Starches with Differing Amylose Contents via Non-Covalent Interaction with Tea Polyphenols

TL;DR

This study explores how tea polyphenols interact with maize starches of different amylose content, altering their structure and thermal stability.

Contribution

The novel contribution is the systematic investigation of how tea polyphenol complexation affects starch structure and thermal behavior based on amylose content.

Findings

Tea polyphenol incorporation significantly enhances thermal resistance, especially in high-amylose starches.

TP complexation reduces relative crystallinity and strengthens hydrogen bonding in starch matrices.

SEM shows increased density and interconnected micro–nano structures with higher amylose content.

Abstract

Starch–polyphenol interactions play a critical role in regulating the structural organization and thermal behavior of starch-based systems. In this study, maize starches with different amylose contents were used to systematically investigate how tea polyphenol (TP) complexation influences starch structure and thermal stability. Starch–TP complexes were prepared under thermal-induced conditions and characterized using thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM). TGA results showed that increasing amylose content slightly reduced the thermal stability of native starches, whereas TP incorporation significantly enhanced thermal resistance, particularly in high-amylose systems. XRD analysis indicated that TP complexation did not affect the crystal structure of starch but led to a pronounced reduction in relative crystallinity, with low-amylose complexes exhibiting predominantly amorphous behavior and high-amylose complexes retaining partial nanocrystalline organization. FTIR spectra revealed reduced short-range molecular order and strengthened hydrogen bonding interactions after TP binding. DSC analysis demonstrated increased gelatinization temperatures accompanied by decreased enthalpy changes, reflecting restricted molecular mobility and delayed solvation of nanocrystalline domains. SEM observations further showed a transition toward denser and more interconnected micro–nano structures with increasing amylose content. Overall, TP preferentially interacts with amylose-rich regions through non-covalent interactions, promoting structural reorganization and enhanced thermal stability of the starch matrix. These findings provide new insight into amylose-dependent starch–polyphenol interactions and offer guidance for designing thermally stable starch-based functional materials.

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1. Introduction

Starch, a fundamental carbohydrate in the human diet, finds extensive applications in the food sector owing to its widespread availability and beneficial properties [1,2]. Starch plays a crucial role in defining the textural and nutritional properties of numerous food items. Within starch granules, two main types of polysaccharides exist: amylose and amylopectin. The ratio of amylose to amylopectin significantly influences the physicochemical properties, processing capabilities, and functional attributes of starch [3,4]. The ability to modify starch properties is vital for improving its performance in various food applications, particularly in the context of enhancing nutritional value and tailoring product quality. The interaction between polyphenols and starch has the potential to influence the physicochemical properties and functional attributes of starch, ultimately enhancing the overall quality of starch-based products, given the diverse physiological effects of polyphenols [5,6,7]. The exploration of the mechanisms through which polyphenols impact the physicochemical attributes of starch is becoming more crucial for the enhancement of food applications geared towards fostering healthy eating patterns.

Tea polyphenols (TP), including catechins, flavonoids, and other phenolic compounds, are naturally occurring substances in tea leaves [8]. Beyond their functional roles in food systems, phenolic compounds are widely recognized as important antioxidants that contribute to physiological antioxidant defense and adaptive responses to various biotic and abiotic stresses, owing to their redox properties and ability to modulate oxidative processes [9,10]. Numerous studies have investigated the impact of these polyphenols on the physicochemical properties and in vitro digestibility of starches from various botanical sources [5,11,12]. The observed modifications in starch properties are largely attributed to the ability of TP to bind with starch molecules, primarily through non-covalent interactions such as hydrogen bonding, hydrophobic interactions, and electrostatic forces [13]. These interactions can lead to the formation of two distinct types of complexes: the V-type inclusion complex, where the phenolic compound is partially encapsulated within the hydrophobic helix of the starch, and the non-inclusion complex, characterized by interactions between the hydroxyl and carbonyl groups of the polyphenols and the starch, resulting in intermolecular aggregate formation [6]. Together, these complexes significantly alter the structural and functional behavior of starches.

Maize starch, one of the most widely used starches in the food industry, is available in a variety of amylose contents, ranging from waxy (low-amylose) to high-amylose starch. These variations provide a unique opportunity to study the differential effects of polyphenol complexation on starches with distinct amylose content [7]. High-amylose corn starch is known for its slow digestibility and strong tendency to retrograde, making it a candidate for producing resistant starch. In contrast, low-amylose starches, which are more prone to gelatinization and less prone to retrogradation, offer advantages in specific applications requiring rapid cooking or softer textures [14,15]. Understanding how TP with these maize starches with differing amylose contents could provide valuable insights into modulating starch properties for specific functional and nutritional outcomes.

Thus, this study investigates the effects of TP on the physicochemical and structural characteristics of maize starches with different amylose contents. Specifically, this study examines how TP influences key parameters such as gelatinization, rheological properties, thermal stability, and microstructure of maize starches with varying amylose levels. Additionally, the research explores the formation of TP-starch complexes and evaluates their effects on relative crystallinity and granule morphology through comparative analysis of diffraction features under identical experimental conditions. By analyzing the differences in behavior between high-amylose and low-amylose maize starches, the study aims to provide a deeper understanding of how amylose content modulates the interaction with tea polyphenols.

2. Materials and Methods

2.1. Materials and Chemicals

Waxy maize starch (WMS) with 2.50% amylose and normal maize starch (NMS) with 29.14% amylose were provided by Gaofeng Starch Technologies Co., Ltd. (Suzhou, Jiangsu, China). Two types of high-amylose maize starch (HMS1 and HMS2 with 55.00% and 70.00% amylose, respectively) were provided by Quanyin Xiangyu Biotechnology Co., Ltd. (Beijing, China). Tea polyphenols (TP, purity > 97%) was obtained from Shanghai Yuanye Bio-Technology Co., Ltd. (Shanghai, China). All other chemical reagents employed in this research were of analytical grade.

2.2. Preparation of Starch–TP Complexes

Utilizing a method previously established by Mao et al. [16], the complexes of starch and TP were prepared with some modifications. The first step involved dissolving 0.5 g of TP in 250 mL of ultrapure water, followed by the addition of 5.0 g each of WMS and NMS to the solution. Subsequently, the mixture was heated at 90 °C for 30 min with continuous stirring (60 rpm), resulting in the formation of WMS-TP and NMS-TP, respectively. HMS1 and HMS2 (5.0 g each) were introduced into the aforementioned TP dispersion and subsequently subjected to a heating process at 121 °C for 30 min with continuous stirring (60 rpm) in a high-pressure reaction kettle, resulting in the formation of HMS1-TP and HMS2-TP, respectively. After heating, all samples were allowed to cool naturally to room temperature under ambient conditions without controlled cooling. Subsequently, the cooled samples were freeze-dried and milled for further analysis. For comparison, the corresponding untreated native starches (WMS, NMS, HMS1, and HMS2) without TP addition were used as control samples. This design enabled direct evaluation of the effects of TP complexation across maize starches with different amylose contents. A schematic diagram of the starch–TP complexes preparation process is presented in Figure 1.

2.3. Rheological Properties

The rheological properties of maize starches with varying amylose contents and corresponding starch–TP complexes were assessed using a DHR-3 rheometer (TA Instruments Inc., New Castle, DE, USA) fitted with a parallel plate (40 mm) and set at a 1000 μm gap. Following preparation as detailed in Section 2.2, the starch and starch–TP sample pastes were allowed to cool to room temperature and were promptly transferred to the rheometer plate for testing. All rheological measurements were performed at a constant temperature of 25 °C, controlled by the rheometer temperature system. A frequency sweep test ranging from 0.1 rad/s to 100 rad/s was conducted at a strain of 1% (linear viscoelastic region). The storage modulus (G′) and loss modulus (G″) trends of starch–TP gels were determined [5].

2.4. X-Ray Diffraction (XRD)

The freeze-dried starch–TP complexes and corresponding control samples were subjected to direct analysis with an X-ray Diffractometer (D2 PHASER, Bruker Inc., Bremen, Germany). The sample was scanned over a range of 4° to 40° (2θ) with an increment of 2°/min. The relative crystallinity (RC) was quantitatively determined by calculating the ratio of the crystalline area to the total area (comprising both crystalline and amorphous regions) through the utilization of Jade 6.0 software [17].

2.5. Fourier Transform Infrared (FT-IR) Spectroscopy

The infrared spectra of the freeze-dried starch–TP complexes and corresponding control samples were acquired using an FTIR spectrophotometer (IS10, Nicolet Inc., Madison, WI, USA). Each sample was co-ground with KBr and then compressed into a transparent sheet. The absorption spectra were acquired from 4000 cm^−1^ to 500 cm^−1^ through 64 scans at a resolution of 4 cm^−1^ [16]. After acquisition, the FTIR spectra were smoothed and baseline-corrected. Fourier self-deconvolution was performed using OMNIC 9.2 software (half-bandwidth: 19 cm^−1^; enhancement factor: 1.9). Peak heights at approximately 1047 cm^−1^, 1022 cm^−1^, and 995 cm^−1^ were determined from the deconvoluted spectra, and the ratios R_1047/1022_ and R_1022/995_ were calculated.

2.6. Differential Scanning Calorimeter (DSC)

Thermal properties of the freeze-dried starch–TP complexes and corresponding control samples were acquired using a differential scanning calorimeter (DSC3, Mettler Toledo, Greifensee, Switzerland). Placing approximately 5 mg of each sample in an aluminum pan, a double amount of water (10 μL) was then added. Before analyzing, seal the aluminum pan hermetically and let it equilibrate at 24−25 °C for 12 h. The DSC scanning procedure covered temperatures ranging from 30 °C to 140 °C, with a heating rate of 10 °C/min, while utilizing an empty pan as the reference standard. The onset (T_o_), peak (T_p_) and conclusion (T_c_) temperatures were recorded [18].

2.7. Thermogravimetric (TGA)

A thermogravimetric analyzer (STA449C/4/G, Netzsch, Bavaria, Germany) was utilized to investigate the mass loss (TG) and differential thermogravimetry (DTG) curves of freeze-dried starch–TP complexes and corresponding control samples. About 4 mg of each sample was placed in a platinum pan and subjected to heating from 30 °C to 600 °C at a rate of 10 °C/min in a nitrogen atmosphere with a gas flow of 20 mL/min [19].

2.8. Scanning Electron Microscopy (SEM)

The microstructures of freeze-dried starch–TP complexes and corresponding control samples were observed through scanning electron microscopy (Quanta 200, FEI Inc., Hillsboro, OR, USA). After being fixed to a specimen holder with double-sided tape, samples were coated with gold in a vacuum evaporator. The SEM analysis, conducted at an acceleration voltage of 10–20 kV, was utilized to observe the surface structure and morphology of these samples.

2.9. Statistical Analysis

The measurement of each sample was executed using a minimum of three freshly prepared, independent samples. The outcomes were communicated in reference to the mean and standard deviation. The experimental data were analyzed using analysis of variance (ANOVA) with the SPSS software (26.0; SPSS Inc., Chicago, IL, USA). The difference in means from the ANOVA was evaluated using Duncan’s multiple range test at a significance level of 5% (p < 0.05).

3. Results and Discussion

3.1. Rheological Characteristics

The rheological properties of starch systems, specifically G′ and G″, are notably impacted by the interaction between maize starch with different amylose content and TPs, as demonstrated in Figure 2. The modifications result from the interaction between the structural properties of amylose and the capability of polyphenols to bind with starch molecules through non-covalent bonds like hydrogen bonding and hydrophobic interactions [20,21]. Specifically, TPs interact with starch molecules, especially amylose, through hydrogen bonding and hydrophobic interactions. This interaction further increases G′ by stabilizing the starch network and promoting the formation of a more structured gel. Polyphenols can strengthen the amylose network, especially in high-amylose starch, by forming complexes with the starch molecules, leading to an even higher storage modulus. In high amylose starch, the presence of TPs tends to significantly increase G′, reflecting a strong gel network with enhanced elasticity. In low amylose starch (higher amylopectin content), the effect of polyphenols on G′ may be less pronounced, but still observable due to the interaction with the branched structure of amylopectin.

As the amylose content increases, the G″ also increases. This is because the linear amylose chains contribute to viscous energy loss during deformation. The molecular friction and movement of amylose chains lead to some dissipation of energy in the system, but the rise in G″ is generally smaller compared to G′. The presence of tea polyphenols can increase G″ by interacting with both amylose and amylopectin, creating some resistance to molecular motion and thus increasing viscous energy loss. However, the overall increase in G″ is still usually smaller than the increase in G′, meaning the system behaves more elastically. Thus, the interaction between amylose and tea polyphenols leads to stronger, more elastic starch gels, particularly in high amylose systems, whereas low amylose systems experience a smaller but noticeable effect.

3.2. X-Ray Diffraction (XRD) Analysis

XRD patterns of native maize starches with different amylose contents and their corresponding starch–TP complexes are presented in Figure 3. For clarity, native starches (WMS, NMS, HMS1, and HMS2) are shown in Figure 3a, while the corresponding starch–TP complexes are shown in Figure 3b. As shown in Figure 3a, WMS and NMS exhibit distinct diffraction peaks at approximately 2θ = 15.02°, 17.08°, 18.12°, and 23.05°. These diffraction features are commonly observed in cereal starches and are associated with long-range ordered regions arising from nanocrystalline domains embedded within the starch granules rather than ideal crystalline solids. According to recent crystallographic perspectives, such diffraction patterns originate from the collective organization of nanocrystalline domains embedded within an amorphous matrix, which contribute to the observed X-ray scattering behavior [22]. In contrast, HMS1 and HMS2 display additional low-angle diffraction features near 2θ = 5.63°, together with peaks at approximately 17.11°, 22.04°, and 24.08°, which are consistent with diffraction features associated with hexagonal nanocrystalline domains. These variations suggest differences in the spatial arrangement and packing density of intrinsic ordered domains within starch granules with higher amylose contents, which may influence the extent of long-range organization detectable by XRD [23]. These diffraction features reflect differences in the dominant packing arrangements of intrinsic ordered domains within the starch granules. Under identical experimental conditions, a gradual reduction in diffraction peak intensity is observed with increasing amylose content among the native starch samples (Figure 3a), suggesting a relative decrease in long-range ordered organization within the starch granules. After complexation with TP, all starch samples exhibit a clear attenuation of diffraction peaks (Figure 3b), indicating disruption of long-range ordered nanocrystalline domains, corresponding to orthorhombic structures in WMS and NMS and hexagonal structures in HMS1 and HMS2. The extent of this disruption depends strongly on starch composition. For WMS-TP and NMS-TP, the characteristic diffraction peaks observed in the native starches are markedly weakened or nearly absent, suggesting a substantial loss of long-range ordered organization. In contrast, HMS1-TP and HMS2-TP retain discernible diffraction peaks, although with significantly reduced intensity compared with their native counterparts. These results indicate that starches with lower amylose content are more susceptible to TP-induced disruption of nanocrystalline domains and transition toward predominantly amorphous structures, whereas higher-amylose starches exhibit greater resistance to the loss of long-range nanocrystalline order. Based on the XRD results, WMS-TP and NMS-TP exhibit predominantly amorphous diffraction patterns, indicating substantial disruption of nanocrystalline order after TP complexation. In contrast, HMS1-TP and HMS2-TP retain weakened but detectable diffraction features, suggesting partial preservation of nanocrystalline domains. This difference can be attributed to the distinct internal organization of starch granules with varying amylose contents, which governs the accessibility and stability of intrinsic ordered domains upon interaction with polyphenols. Overall, the XRD results demonstrate that TP complexation leads to a reduction in long-range ordered organization within starch granules, with the magnitude of this effect being modulated by amylose content.

3.3. FT-IR Analysis

The analysis of the molecular structure and interactions between starch and TPs can be effectively conducted using FT-IR spectroscopy. FT-IR can provide insights into the changes in functional groups and structural rearrangements. The FT-IR spectra of native maize starch with varying amylose levels and the complexes formed with TPs are depicted in Figure 4. Within the range of 3500–3200 cm^−1^, a broad band was observed in all samples, which was related to −OH stretching vibrations [5]. The starch–polyphenol complexes exhibited a more intense absorption band at 3500–3200 cm^−1^ compared with the corresponding native starches. This enhancement is attributed to the disruption of crystalline regions during gelatinization, which exposes additional hydroxyl groups and increases their vibrational activity. Moreover, the hydroxyl groups inherent to TPs further contribute to the overall −OH stretching absorption, collectively resulting in a strengthened band intensity. The absorption peak at 2928 cm^−1^ corresponds to the asymmetric stretching vibration of methylene (−CH_2_−) groups. This band is typically observed in starch and other polysaccharides, as they contain −CH_2_− groups in their glucose monomers. The peak around 1640 cm^−1^ is more likely associated with C−O−O stretching vibration in a carbohydrate group [24]. As shown in Figure 4, the FT-IR spectra of all starch–TP complexes exhibited no newly emerged characteristic peaks relative to the native starches. The absence of additional vibrational signals suggests that no detectable covalent bonding occurred and that the interactions are mainly mediated by non-covalent forces. A similar observation was also reported by Li et al. [5].

Amylose and amylopectin contribute differently to the hierarchical organization of starch granules, with amylopectin double helices being the primary structural motif responsible for short-range molecular order, while higher amylose contents are generally associated with a less ordered granular structure. In the present study, Fourier self-deconvoluted FTIR spectra were used to comparatively evaluate short-range molecular organization in starches and starch–TP complexes (Figure 4c,d). After spectral smoothing, baseline correction, and Fourier deconvolution, three overlapping bands in the 800–1200 cm^−1^ region were more clearly resolved, centered at approximately 1047, 1022, and 995 cm^−1^. These absorption features have often been used as qualitative indicators of changes in local molecular organization within starch systems, although they do not provide direct crystallographic information. The band near 995 cm^−1^ is generally associated with vibrational modes related to double-helix conformations, while variations at approximately 1022 and 1047 cm^−1^ are considered reflective of differences in molecular packing and hydrogen bonding environments [25]. The absorbance ratios of 1047/1022 and 1022/995 were therefore used to evaluate the short-range molecular order (R_1047/1022_) and the degree of double-helix structure (R_1022/995_) within the starch granules [26]. For all four starch samples, complexation with TPs led to a significant decrease in the R_1047/1022_ ratio (Table 1), indicating a disruption of short-range molecular order within the granules. In contrast, the R_1022/995_ consistently decreased upon TP addition, suggesting a relative stabilization or preservation of helix-related structures associated with the 995 cm^−1^ band. This effect was particularly pronounced in the amylose-rich starches (HMS1 and HMS2), likely due to stronger hydrogen bonding and greater conformational compatibility between amylose chains and TP. Overall, TP complexation simultaneously disrupted local crystalline order while promoting the maintenance of double-helix structures, with these effects becoming more evident as amylose content increased.

3.4. DSC Analysis

Differential scanning calorimetry (DSC) was employed to characterize the thermal behavior of maize starches and their complexes. As shown in Table 2, the thermal properties of native maize starches were strongly dependent on amylose content. With increasing amylose content, the T_o_, T_p_, and T_c_ increased significantly, whereas the gelatinization enthalpy (ΔH) showed a decrease. Specifically, T_o_ increased from 66.70 °C for WMS to 73.28 °C for HMS2, while ΔH decreased from 11.44 J·g^−1^ to 6.20 J·g^−1^. The increase in gelatinization temperatures indicates that starch granules with higher amylose content require greater thermal energy to initiate and complete gelatinization. This behavior reflects restricted granule swelling and reduced molecular mobility in high-amylose systems, where amylose-rich amorphous regions constrain the solvation and thermal destabilization of nanocrystalline domains.

Complexation with TP markedly altered the thermal behavior of all starch samples. After TP complexation, all starch samples exhibited a substantial upward shift in gelatinization temperatures (approximately 123–133 °C), indicating markedly enhanced thermal stability. In this study, gelatinization is defined as the thermally induced solvation and destabilization of nanocrystalline domains within starch granules, which originate from the ordered packing of hydrated, hydrogen-bond-stabilized amylopectin double-helical structures. The observed elevation in gelatinization temperature therefore reflects an increased thermal energy requirement to destabilize nanocrystalline domains after TP incorporation. This enhanced thermal stability can be attributed to strong non-covalent interactions between TPs and amylose- and amylopectin-rich regions, particularly hydrogen bonding, together with competition for available water. These effects collectively restrict molecular mobility within the granule and delay the thermal disruption of ordered structures. Despite the increase in gelatinization temperatures, starch–TP complexes exhibited significantly lower gelatinization enthalpy (ΔH) values (2.92–4.39 J·g^−1^) compared with their native counterparts. The reduction in ΔH indicates a decrease in the extent or cooperativity of nanocrystalline domains that undergo thermal destabilization during gelatinization. This finding suggests that TP complexation partially disrupts or dilutes native nanocrystalline regions prior to DSC heating, thereby reducing the amount of energy required for their subsequent solvation and destabilization. Notably, the ΔH values of starch–TP complexes did not show a monotonic dependence on amylose content. HMS1-TP exhibited the highest ΔH, whereas NMS–TP showed the lowest value. This non-linear trend implies that intermediate amylose levels may favor partial retention of ordered amylopectin-based domains or the coexistence of residual ordered structures within the starch–TP matrix. Taken together, the DSC results demonstrate that TP complexation simultaneously enhances the thermal stability of maize starch while reducing the effective amount or cooperativity of native ordered domains. The magnitude of these effects is clearly modulated by amylose content. These conclusions are fully consistent with the XRD and FTIR analyses presented in this study, which show disruption of native ordered structures accompanied by strengthened intermolecular interactions within the starch matrix. Notably, these findings differ from those reported in a previous study, in which decreased gelatinization temperatures were observed for starch–TP complexes prepared under non-gelatinized conditions at ambient temperature [27]. This discrepancy can be mainly attributed to differences in the preparation methods. In the previous study, starch–TP complexes were formed through adsorption-based interactions without gelatinization, whereas the present work employed a thermal-induced gelatinization process, including high-temperature and high-pressure treatment for high-amylose starches. Such conditions facilitate deeper penetration of TPs into the starch matrix and promote stronger intermolecular interactions, leading to fundamentally different structural rearrangements and thermal responses.

3.5. TGAAnalysis

To further investigate the effect of TP on the thermal stability of maize starches differing in amylose content, TGA was employed to examine the thermal degradation behavior of various maize starches as well as their TP complexes. The TG curves and DTG curves were further analyzed to elucidate the thermal decomposition characteristics. The TG curves of the samples are shown in Figure 5a,b, and all samples exhibited two main stages of thermal degradation. The first mass-loss stage occurred at approximately 100 °C and was mainly attributed to the evaporation of moisture present in the sample [28]. The second stage appeared in the temperature range of 30–400 °C, corresponding to the main thermal degradation of starch, including the breakdown of glycosidic linkages [29,30]. The DTG curves (Figure 5c,d) further revealed the degradation rates of the samples and the positions of their main thermal decomposition peaks, where the peak temperature is commonly regarded as an indicator of starch thermal stability [31]. As shown in Figure 5a, maize starches with different amylose contents exhibited distinguishable thermal degradation behaviors. WMS and NMS displayed slightly higher residual weights below 300 °C than HMS1 and HMS2, suggesting relatively lower initial thermal stability of high-amylose starches, which may be associated with differences in chain architecture and the higher proportion of linear amylose. In the main degradation region (≈300–380 °C), all starches underwent rapid mass loss, corresponding to the cleavage of glycosidic linkages and depolymerization of the starch backbone. Consistently, the DTG curves (Figure 5c) exhibited a single dominant decomposition peak for each sample. The peak temperature was slightly lower in high-amylose starches, indicating their increased susceptibility to thermal scission under inert conditions.

Compared with all maize starches (Figure 5a,c), the incorporation of TP markedly altered the thermal degradation behavior of all maize starches (Figure 5b,d). As shown in Figure 5b, starch–TP complexes exhibited higher residual weights below 300 °C than their corresponding native starches, indicating enhanced thermal stability in the low- and intermediate-temperature regions. This stabilizing effect was particularly pronounced in high-amylose starches (HMS1-TP and HMS2-TP), suggesting that TP complexation is more effective in reinforcing starch matrices rich in linear amylose chains. In the main degradation region (≈300–380 °C), all starch–TP complexes still underwent rapid mass loss; however, both the onset of thermal decomposition and the DTG peak temperatures (Figure 5d) were shifted to higher values compared with native starches. The upward shift in DTG peak temperature reflects an increased resistance to thermal scission following TP incorporation, indicative of strengthened intermolecular interactions within the starch matrix. Notably, the stabilizing effect of TPs was more evident in high-amylose systems, where a larger shift in peak temperature and a broader DTG peak were observed, implying a more heterogeneous and constrained degradation process. The enhanced thermal stability of starch–TP complexes can be attributed primarily to the formation of strong non-covalent interactions, particularly hydrogen bonding, between TP and starch chains. These interactions restrict chain mobility, hinder heat-induced depolymerization, and delay the cleavage of glycosidic linkages. Consistent with DSC results showing elevated gelatinization temperatures and FTIR/XRD analyses revealing reduced long-range crystallinity but partially preserved short-range ordering, the TGA results collectively confirm that TP complexation induces a more thermally resistant yet structurally reorganized starch matrix. This structural reinforcement is especially pronounced in high-amylose starches, highlighting the critical role of amylose content in modulating starch–polyphenol interactions and their thermal consequences.

3.6. SEM Analysis

The microstructure of maize starch–TP complexes with different amylose contents at multiple magnifications is shown in Figure 6. At 500× magnification (Figure 6(a1,b1,c1,d1)), all starch–TP complexes exhibited disrupted and aggregated morphologies rather than intact native starch granules, suggesting that the incorporation of TP was associated with pronounced structural reconstruction regardless of amylose content. WMS-TP formed relatively loose and irregular aggregates with visible interstitial voids, indicating a low degree of structural compactness. In contrast, NMS-TP showed more continuous agglomerates with improved particle cohesion. Notably, high-amylose starch–TP complexes (HMS1-TP and HMS2-TP) displayed markedly denser and more consolidated structures with reduced apparent porosity, suggesting enhanced matrix formation at the microscale. At 2000× magnification (Figure 6(a2,b2,c2,d2)), surface morphological differences among the samples became more evident. WMS-TP exhibited rough and fragmented surfaces with weak interparticle fusion, whereas NMS-TP presented partially fused domains and improved surface continuity. By comparison, HMS1-TP and HMS2-TP showed extensive surface fusion accompanied by pronounced surface corrugation, suggesting a higher degree of structural integration in starch–TP complexes with higher amylose contents. At 20,000× magnification (Figure 6(a3,b3,c3,d3)), distinct differences in nanoscale organization were observed. WMS-TP displayed discontinuous and unevenly distributed microdomains, while NMS-TP exhibited moderately interconnected nanostructures. In contrast, high-amylose starch–TP complexes formed more homogeneous and densely packed nano structural features with reduced interdomain gaps, suggesting a more integrated internal organization. At the highest magnification of 100,000× (Figure 6(a4,b4,c4,d4)), these differences were further amplified, with WMS-TP showing loosely arranged and irregular nanoscale features, whereas HMS1-TP and especially HMS2-TP exhibited tightly interconnected and continuous nano structural networks. Overall, the progressive transition from loose and heterogeneous assemblies to dense and highly interconnected micro–nano structures with increasing amylose content suggests that amylose plays a critical role in regulating the hierarchical organization of starch–TP complexes.

It should be noted that the starch–polyphenol interactions investigated in this study were established under thermally induced conditions using simplified model systems. This experimental design was intentionally adopted to isolate the influence of intrinsic starch composition, particularly amylose content, on the formation and structural evolution of starch–polyphenol complexes. Within this controlled framework, the observed modifications in gelatinization behavior, thermal stability, and rheological properties demonstrate that mixing starch with polyphenols can translate into distinct physical responses that are directly relevant to food processing operations such as heating, baking, and extrusion. However, in real food matrices, additional factors including mechanical stresses, competing biopolymers, and complex processing environments may further influence starch–polyphenol interaction behavior and the resulting functional properties. Moreover, while the present work provides comprehensive physicochemical and structural characterization at the bulk scale, direct molecular-level evidence regarding binding mechanisms and the spatial distribution of polyphenols within amylose- and amylopectin-rich domains remains to be elucidated. Although this study focused on maize starches and tea polyphenols, the comparative trends observed (particularly the governing role of amylose content) may offer useful insight for other starch systems and polyphenolic compounds with similar structural features. Future studies incorporating molecular-scale analyses, binding affinity measurements, advanced spectroscopic techniques, and more complex food matrices will be essential for translating these mechanistic findings into practical strategies for designing starch-based foods with tailored physical and functional properties.

4. Conclusions

This study systematically examined the effects of tea polyphenol (TP) complexation on the structural organization and thermal behavior of maize starches differing in amylose content using a thermally induced simplified model system. The results demonstrate that amylose content is a key factor governing starch–polyphenol interactions and the resulting structural and thermal responses. DSC analysis showed that TP incorporation increased gelatinization temperatures while decreasing enthalpy values, indicating restricted granule swelling and delayed solvation of nanocrystalline domains. Consistent with this behavior, XRD results revealed a reduction in long-range nanocrystalline order after TP complexation, with low-amylose starches transitioning toward predominantly amorphous structures and high-amylose starches retaining partial nanocrystalline organization. FTIR analysis suggested attenuation of short-range molecular order accompanied by relative preservation of double-helix-related conformations, particularly in amylose-rich systems. TGA further confirmed enhanced thermal stability after TP incorporation, with stronger stabilization observed in high-amylose starches. These structural and thermal changes are attributed to strong non-covalent interactions, especially hydrogen bonding, between TPs and amylose- and amylopectin-rich regions, which reduce molecular mobility and promote a reorganized, thermally more resistant starch matrix. Overall, TP complexation weakens native nanocrystalline organization while reinforcing thermal stability through hierarchical structural reorganization, and these effects become increasingly pronounced with increasing amylose content under the experimental conditions employed. The findings provide mechanistic insight into amylose-dependent starch–polyphenol interactions and offer a framework for designing thermally stable starch-based materials via controlled polyphenol incorporation. Further investigation in complex food matrices and under realistic processing conditions is required to extend these conclusions beyond the present model system.