Functional Additives Enhance Freeze–Thaw Stability and Retard Retrogradation in Wheat–Potato Starch Gels and Crystal Dumpling Wrappers

Abstract

Crystal dumpling wrapper production is hampered by rapid surface dehydration, severe freeze-cracking propensity, and storage-induced retrogradation. Modulation of blended starch properties through functional additives was investigated. This study systematically evaluated the impact of hydroxypropyl distarch phosphate (HPDSP), trehalose (TRE), guar gum (GG), and composite phosphates (CP) on physicochemical and structural properties of wheat–potato starch composite gel. Concurrently, the effects of additives on the cracking rate of crystal dumplings and texture of wrappers were investigated. Analysis revealed that apparent viscosity was increased by all additives except CP. Different additives significantly improved the freeze–thaw stability of the composite gel during the first three cycles. GG maintained enhanced freeze–thaw stability throughout the entire freeze–thaw cycle (dehydration shrinkage rate: 2.69–40.55%). Multivariate analytical techniques (SEM, FTIR, XRD, DSC) collectively indicated that the additives effectively inhibited starch retrogradation, whilst HPDSP showed the strongest retrogradation inhibition. CP enhanced water-retention capacity and produced a softer blended gel (hardness at 21 days was 100.56 gf). Furthermore, additives significantly reduced the freezing cracking rate of crystal dumplings and improved the textural properties of dumpling wrappers.

1. Introduction

Crystal shrimp dumplings, an iconic Cantonese dim sum delicacy, have been gaining popularity due to their exceptional translucency and elastic, cohesive texture. Traditional dumpling wrappers are prepared primarily from wheat flour, whose gluten network confers elasticity and moisture retention. Conversely, crystal wrappers are formulated from wheat starch and potato starch, which are gelatinized by scalding water treatment prior to shaping [1]. Wheat starch provides the rigid skeleton, while potato starch confers the system excellent transparency and water-holding properties owing to its high amylopectin content [2]. However, starch-based crystal dumpling wrappers are susceptible to surface dehydration during processing and suffer severe cracking during long-term freezing [3]. The dumplings experienced repeated freeze–thaw cycles arising from temperature fluctuations during distribution and storage, which disrupted the starch matrix and impaired their sensory and physicochemical properties [4]. Therefore, it is imperative to improve starch functionality and enhance the freeze–thaw stability of crystal dumpling wrappers. On the other hand, partially gelatinized starch scalded in boiling water undergoes retrogradation during storage. This process involves differential reorganization and recrystallization of amylose and amylopectin, which triggers gel network deterioration and textural degradation [5]. Targeted improvement of the properties of the composite starch system is essential for upgrading crystal dumpling wrappers, given that end-product quality is dictated by raw-material properties.

Contemporary food matrices are increasingly formulated as multi-starch blends, rendering the study of single-starch functionalities insufficient to meet industrial requirements. Consequently, systematic characterization of composite starch systems has become a prominent research focus. A binary blend of high-amylose maize starch and wheat starch at a 1:1 mass ratio yielded a gelatinized paste with markedly suppressed retrogradation and conferred moderate thickening and gelling capacities upon exposure to 120 °C steam [1]. Our previous study focused on the effects of incorporating purple rice starch on the gelatinization and retrogradation of wheat starch. The results indicated that substituting 15% (w/w) of wheat starch with purple rice starch delayed the loss of gel transparency and simultaneously reduced both the hardness value and the retrogradation indices [6].

The utilization of food additives is widely considered a physical modification method which is convenient, effective, and environmentally friendly to further improve the properties of starch. The incorporation of food additives, including inorganic salts, hydrocolloids, and oligosaccharide, has been demonstrated to effectively increase the compactness and cohesiveness of the structure [7]. Pectin has been demonstrated to enhance the freeze–thaw stability of waxy rice starch and improve the water-holding capacity of the product [8]. Moreover, the adverse impacts of freezing are substantially mitigated by modified starch, which acts to suppress ice-crystal growth and to regulate water migration [9,10].

We selected four additives known to enhance either freeze–thaw stability of frozen foods or dough quality, yet still under-explored in pure starch systems. Research on the effects of additives on the wheat starch–potato starch binary starch gel system remains relatively limited. Hydroxypropyl distarch phosphate (HPDSP) is a doubly modified starch synthesized by hydroxypropyl etherification, which exhibits strong hydration capacity and reduced dehydration shrinkage during freezing [11]. Trehalose (TRE) maintained food stability by reducing water mobility in frozen matrices, thereby preserving food stability and exhibiting potential for mitigating freeze-cracking rates [12]. Guar gum (GG) extracted from the seeds of the guar plant is a widely used stabilizer and thickener with excellent water solubility and low cost [13]. The incorporation of GG and its acid hydrolysate into pearl millet starch improved the water-holding capacity, swelling power, and solubility of the starch [14]. Composite phosphates (CP) are often used as moisture retainers in frozen foods and are also effective in regulating starch gelatinization [15,16].

The aim of this study was to systematically investigate the effects of four additives (HPDSP, TRE, GG, and CP) on the physicochemical properties (gelatinization, retrogradation, rheology, microstructure) and freeze–thaw stability of wheat–potato starch composite gels and explore the underlying mechanism. Furthermore, the effects of additives on the quality of dumpling wrappers (freezing cracking rate, and textural properties) were analyzed. These findings offered mechanistic insight into how distinct additives modulate the quality attributes of crystal dumpling wrappers and simultaneously established a scientific basis for the rational, additive-mediated regulation of binary or ternary starch systems in food manufacturing.

2. Materials and Methods

2.1. Materials

Wheat starch (WS, starch content: 88.88%; moisture content: 10.59%; protein content: 0.30%; amylose content: 26.84%) and potato starch (PS, starch content: 79.47%; moisture content: 16.84%; protein content: 0.10%; amylose content: 35.06%) were provided by Guangdong Xinxiongji Food Technology Co., Ltd. (Jiangmen, China). Hydroxypropyl distarch phosphate (HPDSP) was obtained from Hangzhou Prostar Starch Co., Ltd. (Hangzhou, China). Trehalose (TRE) was purchased from Dezhou Huiyang Biotechnology Co., Ltd. (Dezhou, China). Guar gum (GG) was procured from Beijing Guaran Science and Technology Co., Ltd. (Beijing, China) Composite phosphate No. 2 (CP), composed of sodium tripolyphosphate, sodium pyrophosphate, sodium hexametaphosphate, disodium dihydrogen pyrophosphate, potassium dihydrogen phosphate and trisodium phosphate, was purchased from Xuzhou Hengshi Food Co., Ltd. (Xuzhou, China). All other chemicals were of analytical grade.

2.2. Preparation of Composite Gels and Freeze-Dried Gel Samples

The wheat–potato starch composite with a ratio of 2:1 (wheat starch/potato starch) was selected as the base formulation based on preliminary experiments evaluating molding performance and freeze-cracking resistance. The additive concentrations were selected based on concentration gradients established by referring to previous studies [17] [18], and the final concentrations were determined using the freeze–thaw stability and gel hardness of starch gels as the main evaluation criteria in preliminary experiments. HPDSP, TRE, GG and CP were added at 8%, 6%, 6%, and 0.4% (w/w) concentration, respectively. Single starches and mixed starch samples without additives were used as the control groups. Starch pastes and freeze-dried starch gels were prepared according to the methods of Li et al. [19] with moderate modifications. Starch and additives were incorporated into distilled water and stirred continuously at 160 rpm/min at 25 °C for 30 min to prepare an 8% (w/w) starch suspension. The mixture was heated in a water bath at 95 °C for 30 min with magnetic stirring. After cooling, a portion of the starch paste samples was transferred to a 4 °C refrigerator for storage durations of 0, 1, 7, 14, and 21 days. All samples were freeze-dried, milled, and sieved using a 100-mesh screen.

2.3. Determination of Rheological Properties

Rheological properties were measured according to the method of Li et al. [20] with slight modifications. Starch pastes obtained from the procedure in Section 2.2. were promptly transferred to a Rheometer (MCR 502, Anton Paar, Ostfildern, Germany) with aluminum parallel plates measuring 40 mm in diameter and featuring a 1 mm gap, and all measurements were conducted at 25 °C. All rheological experiments were performed in three independent replicates, with freshly prepared samples used for each replicate.

2.3.1. Steady-State Rheology

The apparent viscosity of starch pastes was measured throughout a shear rate range from 0.1 to 100 s⁻¹. The obtained data were fitted to power-law equations for a more comprehensive characterization of the steady-state shear rheological behavior, as depicted in Equations (1) and (2):

$$\tau =K{\dot{\gamma }}^{n-1}$$

(1)

$${\displaystyle \frac{\tau }{\dot{\gamma }}}=\eta =K{\dot{\gamma }}^{n-1}$$

(2)

In these equations, τ denotes shear stress (Pa), γ is shear rate (s⁻¹), K represents the consistency coefficient (Pa·sn), n is the flow behavior index, and η stands for apparent viscosity (Pa·s).

2.3.2. Frequency Sweep

To determine the linear viscoelastic region (LVR), strain sweeps were performed from 0.01% to 100% at a constant frequency of 1 Hz. Subsequently, the storage modulus (G′), loss modulus (G″) and tan δ of composite starch gels were measured at a constant strain of 1% and a frequency scan ranging from 0.1 to 20 Hz.

2.4. Differential Scanning Calorimetry (DSC)

The thermal property of different starch samples was measured using differential scanning calorimetry (DSC4000, PerkinElmer, CT, USA) following the method of Lan et al. [21] with some modifications. The sample (5 mg) was weighed into an aluminum crucible, mixed with 15 μL of deionized water, and then equilibrated at 4 °C for 12 h to stabilize its moisture content. Analyses were conducted with a heating rate of 10 °C/min from 20 °C to 100 °C. A sealed empty stainless pan was used as the reference. Samples stored at 4 °C for 1, 7, 14, and 21 days were re-scanned under identical conditions. The enthalpy values for gelatinization (ΔH_g) and retrogradation (ΔH_r) were determined from the initial and re-heating curves, respectively. The percentage of retrogradation was calculated utilizing Equation (3):

$$R\left(\%\right)={\displaystyle \frac{∆H_r}{∆H_g}}\times 100$$

(3)

In this equation, ΔH_r represents the retrogradation enthalpy change (J/g), and ΔH_g denotes the gelatinization enthalpy change.

2.5. Measurement of Iodine-Binding Capacity

The iodine-binding capacity of different starch samples was determined using the method described by Li et al. [19] with minor modifications. Starch suspensions (1%, w/v) were gelatinized at 95 °C for 30 min; then, 100 µL of starch paste was mixed with 4.8 mL of distilled water and 100 µL of iodine reagent (0.08% I₂, 0.80% KI, w/v). The samples were incubated at 25 °C in darkness for 15 min. Absorption spectra (350–800 nm) were recorded on a Varioskan Flash microplate reader (Thermo Scientific, Waltham, MA, USA).

2.6. Measurement of Freeze–Thaw Stability

A freeze–thaw stability test was conducted using a method adapted from Meng et al. [22]. Starch pastes (10 g) made as described in Section 2.2 were transferred to a tube and stored at −18 °C for 20 h and then thawed at 25 °C for 2 h. The procedure was repeated for 1, 3, 5 and 7 cycles. Thawed samples were centrifuged at 3500× g for 15 min and the remaining weight was recorded. The syneresis was quantified as the percentage of supernatant weight relative to the total gel weight.

2.7. Measurement of Gel Hardness

Following the method of Zou et al. [23] with slight modifications, the hardness of complex starch gels was measured by a texture analyzer (EZ-SX 500 N, Shimadzu, Kyoto, Japan) equipped with a cylindrical probe (diameter: 1.27 cm, labeled P/0.5). Starch pastes obtained in Section 2.2 were dispensed into cylindrical plastic molds (60 mm × 24 mm) and refrigerated at 4 °C for specified periods (0, 1, 7, 14, and 21 days). Samples were compressed at 1.0 mm/s to 50% strain (10.0 mm) with a 5.0 g trigger force.

2.8. Fourier-Transform Infrared (FTIR) Spectroscopy

The FTIR spectra of freeze-dried samples were collected using an Fourier-transform infrared (FTIR) spectrometer (IRXross, Shimadzu, Kyoto, Japan) ranging from 4000 to 400 cm⁻¹ with 64 scans at a resolution of 4 cm⁻¹. The region of 1200–800 cm⁻¹ was subjected to reanalysis and band intensity ratios at 1047/1022 cm⁻¹ and 1022/995 cm⁻¹ were obtained.

2.9. X-Ray Diffractometry (XRD)

The crystalline structure of freeze-dried samples was determined by an X-ray diffractometer (D8 Advance, Bruker AXS Inc., Karlsruhe, Germany). Samples were scanned from 4° to 40° (2θ) at 2°/min. Relative crystallinity (RC) was calculated and analyzed using MDI Jade 6.0 software.

$$RC\left(\%\right)={\displaystyle \frac{A_c}{A_c+A_a}}$$

(4)

In this equation, A_c and A_a represent crystalline and amorphous regions, respectively.

2.10. Scanning Electron Microscopy (SEM)

The microstructure of freeze-dried gels was observed by scanning electron microscopy (SEM) (HitachiTM4000 Plus, Tokyo, Japan). The samples were evenly spread on aluminum plates with conductive adhesive and then coated with gold under vacuum. Cross-section images were obtained at 500× magnification.

2.11. Quality of Crystal Dumpling Wrappers

2.11.1. Preparation of Dumpling Wrappers

The basic formula for dumpling wrappers in the control group consisted of 40 g of wheat starch, 20 g of potato starch, 32 g of distilled water, and 0.75 g of lard. Additives were incorporated based on the dry weight of the starches. Boiling water was poured into the starch–additive mixture to facilitate gelatinization. The mixture was then sealed and rested for 3 min. Lard was added and the dough was kneaded. The dough was rolled out to a final thickness of 2 ± 0.05 mm after lard incorporation and then shaped using a circular mold.

2.11.2. Measurement of Frozen Cracking Rate of Dumplings

The prepared crystal dumpling wrappers were filled with stuffing and immediately frozen at −80 °C for 30 min, then transferred to −18 °C for 30 days. The frozen cracking rate of dumplings was observed according to Gao et al. [24] with appropriate modifications [23]. Dumpling surfaces were visually inspected for cracks and classified according to crack size and number. Dumplings were scored on a 0–1 scale: 0 for intact surfaces, 1 for visible cracks, and 0.5 for intermediate morphology. The number of freeze cracks was recorded, and the frozen cracking rate was calculated using the established formula in Equation (5):

$$C\left(\%\right)={\displaystyle \frac{N_1}{N_2}}\times 100$$

(5)

In this equation, C represents the freeze-crack ratio (%), N₁ denotes the number of dumplings with cracks, and N₂ represents the total number of dumplings.

2.11.3. Textural Properties of Dumpling Wrappers

The texture of dumpling wrappers was determined using a texture analyzer (TA-XT Plus, Stable Micro Systems, London, UK). After boiling to optimal cooking time, dumpling wrappers were placed horizontally on the testing platform and compressed with a cylindrical probe. Surfaces were verified as smooth and bubble-free prior to texture profile analysis.

The test conditions were adjusted compared with the method of Liu et al. [25]: probe P36R, trigger force of 5.0 g, compression ratio of 70%, pre-test speed of 2.0 mm/s, test speed of 0.8 mm/s, post-test speed of 0.8 mm/s, and a 5 s interval between two compressions.

2.12. Statistical Analysis

All experiments were conducted three times (n = 3), and the results are presented as the mean ± standard deviation. Duncan’s multiple tests in SPSS 22.0 software (SPSS Inc., Chicago, IL, USA) were used for statistical analysis to detect significant differences (p < 0.05).

3. Results and Discussion

3.1. Rheological Properties

3.1.1. Steady-Shear Rheological Properties

The rheological behavior of the gelatinized system is presented in Figure 1. Samples with different additives showed a significant drop in apparent viscosity with increasing shear rate, indicating that they were non-Newtonian fluids with shear-thinning characteristics [26]. The power-law model was employed to characterize the rheological properties of the samples, and the corresponding parameters are listed in

Table 1. Power-law parameters for different starch gels.

Sample	K (Pa·sn)	n	R2
WS	40.96 ± 3.16 d	0.26 ± 0.03 c	0.994 ab
PS	32.53 ± 0.81 e	0.51 ± 0.00 a	0.993 ab
WS-PS	54.43 ± 2.04 c	0.26 ± 0.01 c	0.998 a
8% HPDSP-WS-PS	65.76 ± 2.18 b	0.31 ± 0.03 bc	0.995 ab
6% TRE-WS-PS	67.79 ± 5.21 b	0.30 ± 0.01 bc	0.996 ab
6% GG-WS-PS	91.60 ± 1.36 a	0.27 ± 0.00 c	0.998 a
0.4% CP-WS-PS	52.60 ± 1.94 c	0.36 ± 0.03 b	0.987 b

K, viscosity coefficient; n, fluid exponent; R², correlation coefficient. WS, PS, and WS-PS denote wheat starch gel, potato starch gel, and their composite gel, respectively; 8% HPDSP-WS-PS represents WS-PS with 8% hydroxypropyl distarch phosphate (HPDSP); 6% TRE-WS-PS, with 6% trehalose; 6% GG-WS-PS, with 6% guar gum; and 0.4% CP-WS-PS, with 0.4% composite phosphates. Different superscripts in the same column indicate significant difference (p < 0.05).

. All samples exhibited high correlation coefficients (R² > 0.98), indicating an excellent fit of the power-law model to the rheological data. The flow behavior index (n) was consistently below 1, confirming pseudoplastic and shear-thinning behavior. Consistency coefficients (K) directly correlated with thickening capacity, where higher values denoted enhanced viscosity [27]. The WS-PS showed increased apparent viscosity compared to single starch gels. Other additives significantly elevated apparent viscosity, besides CP. The addition of CP has no significant effect on the apparent viscosity of the gel. However, the interactions between GG and starch molecules strengthen interparticle cohesion and inhibit the leakage of amylose [28]. HPDSP establishes a denser and highly water-retentive covalent network at the molecular level through the synergistic interplay between its chemically cross-linked backbone and the hydrophilic groups introduced by etherification [11]. TRE increases viscosity and strengthens intermolecular forces through hydrogen bonding with starch molecules, thereby facilitating the formation of a denser gel network [29]. Zhang et al. [30] also reported that Huangjing polysaccharides increased the viscosity of sweet potato gel.

3.1.2. Dynamic Rheological Properties

As shown in Figure 1B, C, both the storage modulus (G′) and loss modulus (G″) of the composite gels increased with frequency in the tested range. G′ was significantly greater than G″, confirming that samples exhibited a solid-like state and showed a typical rheological behavior of weak gels [5]. TRE weakened the direct interactions between starch molecules through competitive hydration, resulting in a decrease in storage modulus G′, in contrast to other additives that increased both G′ and G″ of the gel system [31]. The starch gel system with GG showed the highest G′ and G″. This enhancement was ascribed to the high hydrophilicity of GG, which forms a stable three-dimensional network structure through hydrogen bonding with starch molecules [32]. CP may form non-covalent linkages with the hydroxyl groups of starch molecules, which could result in a more compact and stable gel network. As shown in Figure 1D, except for HPDSP, additive-containing samples displayed varying degrees of increased tan δ compared to WS-PS, and the GG group exhibited the most notable enhancement in viscous behavior. GG may potentially form a three-dimensional network via non-covalent bonding—predominantly hydrogen bonds—between polymer chains, functionally mimicking the viscoelastic properties of a gluten network [33]. HPDSP exhibited a low tan δ value, indicating that the system behaves more like a solid (elastic) material with a network that enhances structural integrity.

3.2. Iodine-Binding Capacity

Iodine binds to the hydroxyl groups on the outer surface of amylose helices, which leads to the formation of a linear polyiodide chain within the hydrophobic cavity and results in a colored amylose–iodine complex. The absorbance of this complex is measured between 350 and 800 nm [34]. The impacts of additives on the amylose content in starch gels during the gelatinization process can be characterized by iodine binding capacity, and the results are presented in Figure 2. The maximum absorption wavelength for all samples was around 630 nm, indicating that the additives primarily interacted with amylose during starch gelatinization [35]. The absorbance values around 630 nm for other additive groups were lower than those of the WS-PS group, except for the HPDSP-WS-PS group. This may be attributed to the fact that HPDSP releases a portion of amylose under high-temperature conditions, leading to a slight increase in absorbance. However, previous studies have demonstrated that HPDSP was uniformly dispersed throughout the starch gel, bound to leached amylose, thereby delaying the starch retrogradation process [11]. TRE and GG are capable of forming hydrogen bonds with amylose, resulting in inhibited iodine–starch interaction and a consequent reduction in the characteristic absorption peak [36]. This indicated that HPDSP, GG, and TRE primarily influenced starch retrogradation by controlling amylose leaching. The decrease in the characteristic absorption peak of CP can be attributed to the alteration of the starch surface potential, which limited the access of iodine to the binding sites and consequently reduced the iodine-binding capacity [37].

3.3. Freeze–Thaw Stability

The syneresis value was employed to evaluate the capacity of starch to resist undesirable physical changes during freeze–thaw cycles. The syneresis of starch gels subjected to different freeze–thaw cycles is demonstrated in Figure 3. The dehydration shrinkage of WS-PS increased from 6.00% to 76.13% from the first to the third freeze–thaw cycle. The syneresis of gels with additives decreased to varying degrees after 1–3 freeze–thaw cycles, indicating that additives had improved freeze–thaw stability within certain limits. HPDSP improved the freeze–thaw stability and freezing tolerance of starch gels through promoting hydrogen-bond formation between water and starch polymers (amylose and amylopectin), thus stabilizing the gel matrix against syneresis and ice-induced damage [38]. The cryoprotective effect of TRE is primarily attributed to its strong hydrogen-bonding capacity and high affinity for biomolecules, which effectively suppress ice crystal growth and inhibit starch chain rearrangement during freezing, preserving the structural integrity of the starch gel network [39]. The gel system containing 6% GG-WS-PS maintained better cryoprotective properties throughout the freeze–thaw cycles. Hydrophilic colloids exhibit high water-holding capacity due to interactions between abundant free hydroxyl groups and water molecules. Simultaneously, their ability to bind with starch inhibits ice crystal damage to the starch structure [17]. CP may have stemmed from its cross-linking with starch, which could have reinforced the gel network and enhanced water retention. The dehydration shrinkage values of the samples increased significantly after 5–7 freeze–thaw cycles. The additives had no significant effect on the freeze–thaw stability of the starch gel. This may be attributed to the gradual destruction of the starch gel network by large ice crystals during repeated freeze–thaw cycles, leading to structural collapse and a substantial reduction in water-holding capacity [40]. The freeze–thaw cycles employed in this study are not intended to directly simulate typical commercial processing and transportation scenarios. Instead, they serve as an accelerated stress testing method to systematically investigate the mechanism by which additives influence the freeze–thaw stability of starch gels under progressive stress conditions. Products typically undergo fewer freeze–thaw cycles (usually 1–2) under less controlled conditions in normal commercial distribution.

3.4. Gel Hardness

The increase in hardness of gelatinized starch during storage is closely associated with the retrogradation of starch [41]. As shown in

Table 2. Gel hardness of different starch gels with additives at different storage times (0, 1, 7, 14, and 21 days).

Sample	0 Day (gf)	1 Day (gf)	7 Days (gf)	14 Days (gf)	21 Days (gf)
WS	51.31 ± 0.74 b	86.27 ± 0.50 c	98.61 ± 1.26 c	112.70 ± 0.74 d	156.60 ± 11.79 b
PS	39.43 ± 2.61 e	112.26 ±2.38 a	129.91 ± 11.57 a	146.84 ± 0.22 a	171.19 ± 0.95 a
WS-PS	62.50 ± 0.51 a	93.28 ± 0.30 b	125.09 ± 0.81 ab	140.69 ± 0.62 a	154.84 ± 0.22 b
8% HPDSP-WS-PS	51.50 ± 1.07 b	78.41 ± 1.72 e	106.39 ± 1.14 c	124.17 ± 0.44 c	140.96 ± 4.73 d
6% TRE-WS-PS	42.77 ± 1.19 d	82.20 ± 0.62 d	117.37 ± 7.04 b	131.98 ± 2.46 b	144.60 ± 0.75 c
6% GG-WS-PS	44.72 ± 0.75 cd	74.49 ± 0.83 f	98.32 ± 1.44 c	110.77 ± 1.16 d	136.44 ± 1.44 e
0.4% CP WS-PS	45.38 ± 0.33 c	62.06 ± 1.59 g	70.39 ± 2.76 d	84.74 ± 1.33 e	100.56 ± 0.59 f

WS, PS, and WS-PS denote wheat starch gel, potato starch gel, and their composite gel, respectively; 8% HPDSP-WS-PS represents WS-PS with 8% hydroxypropyl distarch phosphate (HPDSP); 6% TRE-WS-PS, with 6% trehalose; 6% GG-WS-PS, with 6% guar gum; and 0.4% CP-WS-PS, with 0.4% composite phosphates. Different superscripts within a column indicate significant differences (p < 0.05).

, the hardness of starch gels stored at 4 °C increased with storage time (1, 7, 14, and 21 days). Samples exhibited low gel hardness on day 0, with pure potato starch gel demonstrating the lowest value, likely attributable to its inherently poor gelling ability [42]. The rapid increase in hardness within the first 7 days of storage was primarily attributed to the reorganization of amylose that had leached from starch granules during gelatinization. The continuous increase in gel hardness from day 7 to day 21 was likely due to the retrogradation of amylopectin [43].

The hardness of the WS-PS group increased from 62.50 gf on day 0 to 154.84 gf on day 21. The hardness of gels with additives was significantly reduced, which implied that additives had a retarding effect on the retrogradation of starch gels. HPDSP retarded starch retrogradation not only by creating steric hindrance to limit amylose rearrangement but also by binding leached amylose through hydrogen bonds [11]. Gels containing CP consistently exhibited the lowest hardness values during storage (100.56 gf on day 21). This phenomenon may be attributed to CP’s dual functionality: acting as a humectant to reduce moisture loss, while simultaneously forming complexes with amylose chains that inhibited starch retrogradation. GG interacted with starch, disrupting molecular mobility and the alignment and packing of starch chains, thereby retarding long-term retrogradation. Concomitantly, it enhanced the gel’s water-holding capacity, yielding markedly lower hardness [44].

3.5. Thermal Properties

Figure 4A shows the starch gelatinization curve, and

Table 3. Thermal properties of starch gels containing various additives.

Sample	To (°C)	Tp (°C)	Tc (°C)	ΔHg (J/g)
WS	57.13 ± 0.20 e	62.98 ± 0.40 c	71.50 ± 0.11 e	18.07 ± 0.17 b
PS	59.47 ± 0.02 a	64.47 ± 0.11 ab	74.07 ± 0.02 a	28.25 ± 0.43 a
WS-PS	57.90 ± 0.01 cd	64.06 ± 0.02 b	72.38 ± 0.01 c	16.88 ± 0.10 b
8% HPDSP-WS-PS	57.66 ± 0.02 d	64.28 ± 0.06 b	72.17 ± 0.04 d	14.44 ± 0.23 d
6% TRE-WS-PS	58.69 ± 0.30 b	64.86 ± 0.42 a	73.62 ± 0.01 b	16.13 ± 0.06 c
6% GG-WS-PS	58.19 ± 0.17 c	64.21 ± 0.14 b	72.41 ± 0.06 c	12.94 ± 0.06 e
0.4% CP-WS-PS	59.28 ± 0.03 a	64.97 ± 0.24 a	72.34 ± 0.20 cd	15.70 ± 0.31 c

WS, PS, and WS-PS denote wheat starch gel, potato starch gel, and their composite gel, respectively; 8% HPDSP-WS-PS represents WS-PS with 8% hydroxypropyl distarch phosphate (HPDSP); 6% TRE-WS-PS, with 6% trehalose; 6% GG-WS-PS, with 6% guar gum; and 0.4% CP-WS-PS, with 0.4% composite phosphates. Significant differences (p < 0.05) among means within a column are indicated by different letters, as determined by Duncan’s test.

summarizes the key thermal parameters of the composite gel. The WS-PS group exhibited intermediate T_o, T_p, and T_c values compared to single starch gels. TRE and CP significantly increased the initial gelatinization temperature (T_o), indicating that the temperatures required for starch gelatinization increased, delaying starch gelatinization. TRE delayed starch gelatinization through competitive water binding, which lowered water activity and restricted granule swelling [45]. The cross-linking of CP with starch reinforced the molecular network, restricting the granules’ ability to hydrate and swell [15]. The addition of HPDSP and GG did not significantly affect the thermal transition temperature of WS-PS.

ΔH represents the energy required to disrupt the double-helical structures in both crystalline and amorphous regions of starch, thus serving as a measure of the overall crystal structure stability [5]. The addition of additives decreased the gelatinization enthalpy of starch to varying extents compared to the WS-PS group, indicating a decrease in starch structural order and delaying the starch rearrangement process. The introduction of hydroxypropyl groups increased starch hydrophilicity, enabling the chains to absorb water and swell more readily and thus decreasing the enthalpy required for pasting [11]. The interactions of CP or GG with starch during gelatinization suppressed chain mobility and rearrangement, which led to a reduction in enthalpy [46].

The gelatinized starch molecules rearranged and aggregated to form ordered crystalline structures when stored at 4 °C. The enthalpy of retrogradation and gelatinization was analyzed again to obtain the degree of retrogradation (R) of starch, which is used to quantify the retrogradation of starch gels [47]. As shown in Figure 4B, the R value of the mixed gels with additives was reduced compared with the WS-PS group on days 7, 14, and 21. The most significant reduction was observed in the mixed gel with HPDSP. This reduction may be attributed to HPDSP’s exceptional water absorption and thickening properties, which restricts starch molecular chain mobility, thereby reducing intermolecular entanglement and amylose recrystallization [48]. These results provided further evidence that the additives could slow down the starch retrogradation process to a certain degree.

3.6. Short-Range Molecular Order

Figure 5 presents the FTIR spectra of composite starch gels within the 4000–800 cm⁻¹ range. No obvious spectral alterations or new chemical bonds/functional groups emerged upon addition of additives, suggesting non-covalent interactions between additives and starch. A broad absorption peak was detected at 3100–3600 cm⁻¹, mainly attributed to O–H stretching vibrations. Starch, HPDSP, and TRE possess abundant hydroxyl groups that established extensive intra- and intermolecular hydrogen-bond networks within the complex, yielding a broad and intense band [49]. The absorption peak at 2930 cm⁻¹ was assigned to the C–H asymmetric stretching vibration, while the peak at 1647 cm⁻¹ was associated with the O–H bending vibration of water in the amorphous regions of starch [50]. Additives improved the water-retention capacity of the starch gels, elevating the moisture content and thereby intensifying this band.

The ratio of band intensities at 1047 cm⁻¹ to 1022 cm⁻¹ (R₁₀₄₇/₁₀₂₂, DO) and the ratio of band intensities at 1022 cm⁻¹ to 995 cm⁻¹ (R₁₀₂₂/₉₉₅, DD) serve as indicators for analyzing the alterations in the recrystallized and amorphous regions of starch [44]. These ratios are indicative of the short-range ordered structure and double-helical structure of starch, respectively. As shown in

Table 4. Ratio of FTIR peak intensities at 1047 cm⁻¹/1022 cm⁻¹ of composite starch gels containing additives after storage at 4 °C for different durations (0, 1, 7, 14, and 21 days).

Sample	0 Day	1 Day	7 Days	14 Days	21 Days
WS	1.446 ± 0.03 ab	1.770 ± 0.04 b	1.835 ± 0.02 a	2.085 ± 0.04 ab	2.170 ± 0.01 b
PS	1.475 ± 0.01 a	1.818 ± 0.04 a	1.840 ± 0.05 a	2.154 ± 0.05 a	2.249 ± 0.02 a
WS-PS	1.418 ± 0.03 bc	1.752 ± 0.01 b	1.809 ± 0.01 b	2.033 ± 0.06 bc	2.141 ± 0.02 b
8% HPDSP-WS-PS	1.320 ± 0.01 d	1.604 ± 0.02 d	1.637 ± 0.03 d	1.891 ± 0.02 d	1.930 ± 0.00 e
6% TRE-WS-PS	1.397 ± 0.02 c	1.690 ± 0.01 c	1.754 ± 0.01 b	1.991 ± 0.02 c	2.048 ± 0.01 c
6% GG-WS-PS	1.370 ± 0.01 e	1.663 ± 0.01 e	1.678 ± 0.01 c	1.946 ± 0.01 cd	1.995 ± 0.03 d
0.4% CP WS-PS	1.384 ± 0.01 c	1.688 ± 0.01 c	1.747 ± 0.02 b	1.980 ± 0.03 c	1.997 ± 0.01 d

WS, PS, and WS-PS denote wheat starch gel, potato starch gel, and their composite gel, respectively; 8% HPDSP-WS-PS represents WS-PS with 8% hydroxypropyl distarch phosphate (HPDSP); 6% TRE-WS-PS, with 6% trehalose; 6% GG-WS-PS, with 6% guar gum; and 0.4% CP-WS-PS, with 0.4% composite phosphates. Different superscripts in the same column denote significant difference (p < 0.05).

and

Table 5. Ratio of FTIR peak intensities at 1022 cm⁻¹/995 cm⁻¹ of composite starch gels containing additives after storage for different durations (0, 1, 7, 14, and 21 days).

Sample	0 Day	1 Day	7 Days	14 Days	21 Days
WS	1.515 ± 0.03 a	1.615 ± 0.01 b	1.633 ± 0.01 a	1.665 ± 0.06 ab	1.679 ± 0.03 ab
PS	1.540 ± 0.01 a	1.666 ± 0.05 a	1.666 ± 0.02 a	1.671 ± 0.01 a	1.691 ± 0.02 a
WS-PS	1.494 ±0.04 a	1.591 ± 0.01 b	1.622 ± 0.03 b	1.635 ± 0.02 ab	1.643 ± 0.01 ab
8% HPDSP-WS-PS	1.354 ± 0.03 ab	1.459 ± 0.01 f	1.497 ± 0.01 e	1.562 ± 0.01 c	1.594 ± 0.02 b
6% TRE-WS-PS	1.428 ± 0.01 a	1.561 ± 0.01 cd	1.607 ± 0.04 c	1.625 ± 0.05 abc	1.632 ± 0.02 ab
6% GG-WS-PS	1.368 ± 0.01 ab	1.499 ± 0.02 ef	1.521 ± 0.02 d	1.593 ± 0.00 bc	1.605 ± 0.09 ab
0.4% CP-WS-PS	1.406 ± 0.02 a	1.533 ± 0.01 de	1.545 ± 0.01 d	1.600 ± 0.01 abc	1.632 ± 0.01 ab

WS, PS, and WS-PS denote wheat starch gel, potato starch gel, and their composite gel, respectively; 8% HPDSP-WS-PS represents WS-PS with 8% hydroxypropyl distarch phosphate (HPDSP); 6% TRE-WS-PS, with 6% trehalose; 6% GG-WS-PS, with 6% guar gum; and 0.4% CP-WS-PS, with 0.4% composite phosphates. Different superscripts in the same column denotes significant difference (p < 0.05).

, the DO of the WS-PS group increased from 1.418 to 2.141, while the DD increased from 1.494 to 1.643 over the 21-day period. The increases in DO and DD values during storage were indicative of the development of short-range ordered and double-helix structures within the gel, confirming the occurrence of starch retrogradation. The decrease in DO and DD values in the composite gel system indicated that the additives effectively retarded the development of ordered structures during retrogradation. CP-starch interactions delayed the development of short-range order by increasing chain rigidity and restricting molecular mobility through electrostatic interaction [51]. This finding was consistent with the results from DSC. Wang et al. [52] also found similar results in their study on the impact of Porphyra haitanensis polysaccharides on the retrogradation of three different crystalline-structured starches.

3.7. Crystalline Structures of Gels

Numerous studies confirm that native wheat starch exhibits strong diffraction peaks near 15° and 23° (2θ), characteristic of A-type crystallinity, whereas potato starch displays characteristic peaks at approximately 5.5°, 15°, 17°, 20°, 22°, and 24° (2θ), consistent with B-type crystalline structure [53,54]. The XRD patterns of composite starch gels are shown in Figure 6. The original starch crystal structure was decomposed after heating at high temperatures, and all pasted samples showed similar peak profiles. This resulted from thermal disruption of hydrogen bonds between crystalline and amorphous starch regions, leading to crystalline structure loss. Furthermore, no new diffraction peaks appeared with the addition of the additives, indicating that no new crystalline phases were formed. Weak diffraction peaks emerged at 17° and 20° in gel samples during storage, corresponding to the B + V type. The B-type peaks likely originated from the structural reorganization of amylose and amylopectin during retrogradation, while V-type peaks primarily arose from complexation between amylose and lipid molecules [55]. The increasing crystallinity of all starch samples during storage reflected progressive recrystallization, indicating transformation of amorphous regions into crystalline domains [56]. The addition of additives reduced the relative crystallinity of the samples, consistent with the decrease in DO and DD values measured by FTIR, indicating that they inhibited starch rearrangement to varying degrees. HPDSP was the most effective, with a relative crystallinity of 16.83% at 21 days, which was 3.12% lower than that of the WS-PS group.

3.8. Microstructure of Composite Gels

The cross-sectional morphology of the composite starch gels is shown in Figure 7. Both honeycomb and irregular laminar structures were present on the gel, confirming that the samples were fully gelatinized [19]. The porous structure formed as a result of water sublimation during the sample lyophilization process. On day 0, potato starch gels exhibited smaller, more continuous pores than wheat starch gels due to higher swelling power and viscosity. WS-PS group exhibited significantly greater homogeneity and porosity than pure wheat or potato starch gels. In contrast, additive-containing gels developed more homogeneous and denser microstructures. The pore structure of starch gels coarsened, exhibiting significant enlargement of internal pores and partial disruption of the network structure, attributable to continued amylopectin retrogradation after prolonged storage [23]. The gel incorporating HPDSP maintained a denser and more uniform microstructure with smaller pores and a more cohesive network during storage. This enhanced structural stability was likely attributable to the introduced hydrophilic groups (hydroxypropyl, phosphate ester), which increases steric hindrance and hydration capacity, thereby significantly inhibiting starch chain recrystallization [11]. TRE competed for hydration by forming hydrogen bonds with starch chains and water molecules, inhibiting the rearrangement of starch molecules, but its structure was lax and open. GG formed a more continuous and viscoelastic composite network with starch, encapsulating starch granules and limiting their rearrangement [57]. CP gels exhibited a more uniform microstructure due to a combination of factors: the inhibition of starch swelling and gelatinization, enhanced electrostatic repulsion, and the suppression of short-term amylose recrystallization.

3.9. Quality of Crystal Dumpling Wrapper

3.9.1. Frozen Cracking Rate of Dumplings

The gluten-free, starch-rich crystal dumpling wrapper displayed an inherently loose and porous matrix that readily lost moisture during freezing. Surface microcracking and internal structural damage in frozen dumpling wrappers result from dehydration and the mechanical action of ice crystals on the starch network, respectively. This accelerates amylopectin retrogradation and exacerbates phase separation, collectively impairing product integrity. As indicated in Figure 8, additives exerted distinct effects on the freeze-crack rate of dumplings following 30 days of frozen storage. The WS-PS group exhibited a 16.66% cracking incidence. Additives significantly reduced freezing damage: HPDSP lowered incidence to 8.33%, TRE-containing formulations to 13.89%, and GG and CP formulations to 7.50% and 9.37%, respectively. GG exhibited strong water-absorption and water-holding capacities, binding water in both dumpling wrapper and filling to form a protective hydration layer. This layer reduced water migration and loss, lowering risks of wrapper cracking and filling shrinkage caused by dehydration during freezing. Moreover, GG served as a gluten substitute to stabilize the system’s structure [58]. Upon full hydration, HPDSP acquired adhesive properties that reinforced intermolecular starch bonding, enhancing wrappers’ strength and toughness [59].The two highly reactive hydroxyl groups of TRE may form hydrogen bonds with water molecules, generating a protective hydration layer. This layer immobilized moisture within the wrapper matrix and restricted water mobility during frozen storage, thereby mitigating freezing-induced fracture [60]. CP interacted with starch molecules to change the gel structure, enhancing stability and water retention [15].

3.9.2. Textural Properties of the Dumpling Wrappers

Texture property is commonly employed to objectively assess food quality by characterizing texture-related mechanical properties. For cooked dumpling wrappers, parameters such as hardness, springiness, and chewiness are key indicators of texture that strongly correlate with consumer acceptance.

Table 6. Effects of additives on the textural properties of crystal dumpling wrappers.

Sample	Hardness (g)	Springiness	Chewiness (mJ)	Resilience
WS-PS	28,281.34 ± 498.50 a	0.88 ± 0.01 c	19,863.27 ± 353.60 b	0.68 ± 0.01 c
8% HPDSP-WS-PS	26,253.30 ± 798.35 b	0.94 ± 0.17 a	22,240.58 ± 247.55 a	0.86 ± 0.00 a
6% TRE-WS-PS	25,057.64 ± 15.53 c	0.91 ± 0.03 ab	17,323.39 ± 685.01 c	0.73 ± 0.10 b
6% GG-WS-PS	23,097.00 ± 257.60 d	0.93 ± 0.02 a	17,076.36 ± 12.52 c	0.76 ± 0.03 b
0.4% CP WS-PS	18,362.86 ± 280.43 e	0.88 ± 0.00 c	19,092.42 ±1 88.04 bc	0.72 ± 0.00 b

Different superscripts within a column indicate significant differences (p < 0.05). WS, PS, and WS-PS denote wheat starch gel, potato starch gel, and their composite gel, respectively; 8% HPDSP-WS-PS represents WS-PS with 8% hydroxypropyl distarch phosphate (HPDSP); 6% TRE-WS-PS, with 6% trehalose; 6% GG-WS-PS, with 6% guar gum; and 0.4% CP-WS-PS, with 0.4% composite phosphates.

presents the textural characteristics of crystal dumpling wrappers with different additives.

Hardness is a core parameter reflecting the softness or firmness of crystal dumpling wrappers. The hardness of all additive-treated groups was significantly reduced compared to the control group (WS-PS, 28,281.34 g). The CP-treated group exhibited the lowest hardness (18,362.86 g), representing a reduction of approximately 35% relative to the control. This effect arose from superior water retention and CP–starch interactions that promoted cross-linking, preserving viscoelasticity and softening the matrix [61]. An appropriate reduction in hardness contributes to improved mouthfeel by making them easier to bite through during chewing, thereby avoiding the undesirable eating experience associated with excessive firmness. Springiness reflects the ability of crystal dumpling wrappers to recover after deformation and is closely related to the smoothness and chewiness of the product. The springiness values of the HPDSP and GG-treated groups were 0.94 and 0.93, respectively, which were significantly higher than that of the WS-PS. Enhanced springiness contributes to better shape retention and improved mouthfeel of dumpling wrappers after cooking, thereby preventing quality deterioration caused by structural loosening. Chewiness comprehensively reflects the synergistic effect of hardness, springiness, and cohesiveness, and serves as an important indicator for evaluating the textural firmness of crystal dumpling wrappers [62]. The HPDSP-WS-PS exhibited the highest chewiness value, which was significantly higher than that of the WS-PS. Resilience reflects the ability of a sample to recover its original height after compression and is associated with the instantaneous rebound sensation of crystal dumpling wrappers. The HPDSP-treated group exhibited the highest resilience value, indicating that the gel network formed by HPDSP possesses enhanced instantaneous rebound capacity. Incorporation of HPDSP or TRE reduced wrapper hardness and improved chewiness, attributable to their superior water-binding capacity that yielded a more uniform moisture distribution and a softer texture [63]. In dough, GG formed a network structure that enhanced the elasticity and toughness of dumpling wrappers [64].

3.10. Mechanism of Interaction Between Starch and Additives

Figure 9 reveals the interaction between additives and starch, along with a potential mechanism for delaying starch retrogradation. Under heating conditions, wheat starch and potato starch granules absorb water and swell, disrupting their crystalline structures. Starch retrogradation primarily occurs during the cooling and storage stages following gelatinization and is fundamentally characterized by the re-association of dispersed starch molecules through hydrogen bonding to form ordered crystalline structures [5]. The presence of additives effectively interferes with this process via multiple intermolecular forces. Specifically, HPDSP retards starch retrogradation through steric hindrance and hydrogen bonding interactions with leached amylose [11]. Both GG and TRE compete for water molecules via hydrogen bonding and also interact with starch through hydrogen bonds, thereby effectively inhibiting starch chain reassociation [57]. Furthermore, CP hinders starch chain rearrangement through electrostatic repulsion, while synergistically delaying the hardening and retrogradation of starch gels via enhanced water-holding capacity [65].

4. Conclusions

This study provides the systematic and comparative elucidation of how four functionally distinct additives, namely HPDSP, TRE, GG, and CP, modulate the physicochemical properties, freeze–thaw stability, and retrogradation behavior of wheat–potato dual starch gels, with direct validation in a crystal dumpling wrapper system. With the exception of CP, all additives enhanced the apparent viscosity of the gels by forming hydrogen bonds, thereby strengthening intermolecular forces. Additives exhibited good protective effects during the first three cycles and GG maintained enhanced cryoprotective performance throughout the entire freeze–thaw process. CP enhanced water retention capacity, enabling the gels to maintain lower hardness even after 21 days of storage. Additives displayed distinct anti-retrogradation effects, among which HPDSP showed the strongest inhibitory effect on starch retrogradation.

In practical application, the incorporation of additives significantly reduced the freeze-cracking rate of dumplings during frozen storage, while also exerting varying effects on the textural properties of the wrappers. These differential effects and mechanisms provide a theoretical basis for the targeted selection of additives to achieve specific functional outcomes. While this work clarifies the individual action of each additive, future studies should focus on investigating synergistic multi-additive systems to further optimize the performance and commercial potential of starch-based frozen food products.