Gelation mechanism analysis of low-salt silver carp (Hypophthalmichthys molitrix) surimi enhancing by ultrasonication combined apple pectin, citrus pectin, or peach gum

Abstract

This study investigated the effects of ultrasound (150 W, 10 min) combined with hydrocolloids (0.2 % apple pectin, 0.2 % citrus pectin, or 0.2 % peach gum) on the gel properties of low-salt silver carp surimi. Results showed hydrocolloids alone significantly improved the gel properties of low salt surimi, with peach gum achieving notable gel strength (5299.52 g·mm) and reduced weight loss (8.93 %). Ultrasound synergistically enhanced hydrocolloid effects, increasing whiteness and hardness while reducing water mobility. The citrus pectin-ultrasound combination demonstrated optimal performance: enhanced hydrophobic interactions and disulfide bonds, 27.18 % porosity, and sensory quality approaching conventional high-salt surimi. The storage modulus revealed that ultrasonication-assisted citrus pectin modification significantly strengthened the protein network and improved thermostability in low-salt surimi systems. Structural characterization revealed increased β-sheet content and denser network formation. These findings demonstrate that ultrasound-assisted hydrocolloid incorporation effectively compensates for salt reduction while maintaining superior gel properties, providing a practical approach for developing high-quality low-salt surimi products.

Highlights

• •Ultrasonic treatment alone promoted the formation of gel structures.
• •Peach gum enhanced gel strength and reduced weight loss significantly than pectin in low salt surimi.
• •Ultrasonication enhanced the connection between hydrocolloids and myofibril.
• •Ultrasound combined with citrus pectin developed the most dense and uniform microstructure.

1. Introduction

Surimi products, such as fish balls, fish sausages, and crab sticks, are highly valued for their high protein content, low fat ratio, and amino acid composition analogous to that of the human body, ensuring high protein digestibility (Zhang, Chen, et al., 2023). However, the overexploitation of marine fish stocks poses significant ecological risks, prompting a shift toward farmed freshwater fish as the primary raw material for surimi production, particularly in China (Jiao et al., 2024). Silver carp (Hypophthalmichthys molitrix) dominates Chinese freshwater aquaculture thanks to its low cost and ecological sustainability. However, under fluctuating rearing conditions its muscle shows unusually high endogenous protease activity, which—together with other factors—impairs gel-forming ability relative to that of marine fish species (Xie et al., 2024).

The gel properties of surimi primarily depend on the gelation of salt-soluble proteins, particularly myofibrillar proteins (MP). Typically, 2–3 % salt is added to surimi to solubilize MP and facilitate the formation of a three-dimensional gel network (Liu et al., 2025). Excessive salt intake is linked to cardiovascular diseases, hypertension, and kidney failure. The World Health Organization (WHO) recommends a daily sodium intake of less than 6 g for adults and advocates for reduced sodium levels in processed foods (Greer et al., 2020). However, reducing salt content in surimi products compromises their quality, including water retention and gel strength. Consequently, developing low-salt surimi products with acceptable texture and flavor has become a critical challenge in the meat processing industry.

Recent studies have explored various strategies to enhance the gel properties of low-salt surimi, including the incorporation of exogenous additives (salt substitutes, proteins, hydrocolloids) and non-thermal processing techniques (ultrasound, microwave). Among these, pectin has emerged as the most versatile biopolymer: apple pomace and citrus peel pectins are already commercial by-products, and their established hypo-tensive and pre-biotic credentials give surimi a clean-label health bonus (Wang et al., 2021). The functional performance of pectin is closely tied to its structural properties. For example, apple pectin (AP), as characterized by Liu et al. (2022), contains 80.39 % galacturonic acid and 1.34 % protein, with a molecular weight of 711.9 kDa. These attributes enable AP to form homogeneous gel networks in silver carp surimi, significantly improving gel-forming capacity and exhibiting a dose-dependent enhancement of hardness (Buda et al., 2021). In contrast, citrus pectin (CP) has a lower molecular weight (155.2 kDa) and galacturonic acid content (2.20 %), with a protein content of 1.34 %. This distinct composition facilitates the formation of complexes with MP via hydrophobic and hydrogen bonding, thereby enhancing water retention and gel stability (Yan et al., 2024). While pectin is ubiquitous in the plant kingdom, its commercial production relies heavily on a limited number of sources. Currently, CP accounts for 85.5 % of commercial pectin output, with AP contributing 14.0 % (Chan et al., 2017). Peach gum (PG) is a natural plant exudate secreted from the trunks and branches of peach trees, is produced abundantly in China, with an annual yield reaching up to 10 billion tons. Despite its high availability, PG remains significantly underutilized in current industrial applications (Zhu et al., 2019). PG is composed primarily of polysaccharides and has been traditionally used in Chinese medicine for its therapeutic properties, including the treatment of dysentery and diabetes, as well as its role as a pharmaceutical excipient (Zeng et al., 2022). In the food industry, PG is valued for its excellent rheological properties, such as high viscosity and gel-forming ability, which contribute to a smooth and lubricious mouthfeel. For example, PG has been successfully incorporated into dairy products to enhance texture and stability (Song et al., 2022). In the context of surimi, PG has demonstrated potential to improve gelation properties by acting as a filler within the protein matrix, thereby enhancing water retention and mechanical strength.

However, the effectiveness of hydrocolloids is often limited by the poor solubility of myofibrillar proteins under low-salt conditions, underscoring the need for complementary technologies such as ultrasonication to unlock their full potential. Ultrasonication is another promising technology for improving low-salt surimi gelation. Ultrasound enhances MP solubility, promotes hydrogen bonding and hydrophobic interactions, and facilitates the formation of superior gel networks. Gao et al. (2021) reported that ultrasonic pre-treatment promoted the dispersion of silver carp proteins, which improved the gelation properties of surimi during heat induction. The synergistic effects of ultrasonication and hydrocolloids have been demonstrated in low-salt meat products, comparative studies examining how different hydrocolloids interact with ultrasound to influence both the gel properties and potential mechanism in low-salt surimi remain limited (Gao, Hu, et al., 2023). This lack of research further constrains the ability to develop, refine, and implement effective process designs and optimizations, as well as to apply the findings in practical applications.

This study comprehensively investigated the combined effects of ultrasonication and three hydrocolloids (AP, CP, PG) on the gelation properties of low-salt surimi. We characterized gel quality through multiple parameters including gel strength, texture profile, water-holding capacity (WHC), color attributes, rheological behavior, and sensory characteristics. To elucidate the underlying mechanisms, we employed Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR) analysis, chemical interaction measurements, and microscopic techniques including scanning electron microscopy (SEM) and confocal laser scanning microscopy (CLSM). The findings provide fundamental insights for developing improved low-salt surimi products.

2. Materials and methods

2.1. Materials

Frozen silver carp surimi (AAA grade) was purchased in Jing li Aquatic Food Co., Ltd. (Hubei, China). Pectin (purity 99.0 %) was purchased from Andre Pectin Co., Ltd. (Shandong, China). Peach gum (purity 99.0 %) was purchased from Dingli Gum Industry Co., Ltd. (Shandong, China). All analytical-grade chemical reagents were purchased from Sinopharm Chemical Reagent Company Limited (Shanghai, China).

2.2. Preparation of surimi gel

Frozen surimi was thawed overnight at 4 °C, cut into small pieces, and minced for 3 min. Salt (3 g/100 g, NaCl/surimi) was added and mixed for 3 min, followed by the addition of deionized water to adjust the moisture content to 80 g/100 g (water/surimi) and mixed for 1 min. This sample served as the high-salt control group (CON). The low-salt group (LS) was prepared using the same procedure but with the addition of salt adjusted to 1 g/100 g (NaCl/surimi). The treatment groups were prepared by supplementing low-salt surimi with apple pectin (AP), citrus pectin (CP), or peach gum (PG) at an optimized concentration of 0.2 g/100 g (hydrocolloid/surimi), based on preliminary experimental results. The corresponding mixtures were designated as LA, LC, and LP, respectively. The 150 g samples were ultrasonicated for 10 min using an ultrasonic processor (KQ-3200, Kunshan Ultrasonic Instruments Co., Ltd.) at a frequency of 40 kHz and power of 150 W (on-time and off-time were set at 2.0 s and 4.0 s, respectively, with the water temperature maintained below 15 °C). Based on different treatment conditions, the ultrasonicated samples were designated as: ULS (ultrasound +1 g/100 g NaCl), ULA (ultrasound +0.2 g/100 g apple pectin), ULC (ultrasound +0.2 g/100 g citrus pectin), and ULP (ultrasonic +0.2 g/100 g peach gum). The samples were encased in polyethylene casings (100 mm height × 20 mm diameter) and underwent a two-step heat treatment (40 °C for 30 min followed by 90 °C for 20 min) to form the final gel products.

2.3. Gel strength measurement

Gel strength was quantitatively determined using a TMS-PRO texture analyzer (Food Technology Corporation, Sterling, VA, USA). Samples were cut into 25 mm high cylinders and positioned on the texture analyzer platform. Texture analysis was performed using a P/0.5 cylindrical probe under controlled conditions: a test speed of 60 mm/min, 30 % compression strain, and a 0.5 N trigger force.

2.4. Texture profile analysis

The textural properties of the samples (25 mm high cylinders) were measured using a TMS-PRO texture analyzer (Food Technology Corporation, Sterling, VA, USA). The analysis was performed at a test speed of 6 mm/min with a trigger force of 0.5 N and 50 % strain (Geng et al., 2024).

2.5. Water-holding capacity and weight loss measurement

The weight of the gel sample W1 was recorded by taking 1 g of the gel sample and wiping off the excess water on the gel surface, and the weight after centrifugation (4 °C, 10000 ×g, 10 min) was recorded as W2, and the WHC was calculated according to the following eq. (1).

$$\mathrm{WHC}\left(\%\right)=\text{W2}÷\text{W1}\times 100\%$$ (1)

The weight loss measurement was based on weighing a 10 mm thick gel sample, which was accurately weighed and recorded as G1. The gel sample was heated at 90 °C for 20 min, the water on the surface was wiped off, and then the weight, G2, was recorded and calculated using the following formula (2).

$$\text{Weight loss}\left(\%\right)=\left(\text{G1}-\text{G2}\right)÷\text{G1}\times 100\%$$ (2)

2.6. Whiteness measurement

The whiteness of gel samples was measured using a CR-400 colorimeter (Konica Minolta, Japan), The brightness (L*), red/green (a*), and yellow/blue (b*) were recorded. Whiteness values were calculated using the following equation (Wu, Xiong, et al., 2024):

$$\text{Whiteness}=100-{\left[{\left(100-{\mathrm{L}}^{\ast }\right)}^2+{\mathrm{a}}^{\ast 2}+{\mathrm{b}}^{\ast 2}\right]}^{1/2}$$ (3)

2.7. Low-field nuclear magnetic resonance (LF-NMR) and Magnetic resonance imaging (MRI)

The water mobility in surimi gels was measured using LF-NMR (Niumag Electric Co., Shanghai, China) according to the method of Li et al. (2024) with slight modifications. Approximately 2 g of each sample was packed into a 15 mm NMR tube and analyzed with the following parameters: repetition time (TW) = 5000 ms, number of scans (NS) = 4, and echo counts = 8000.

Water distribution in gels was analyzed using the built-in NMR imaging software (v3.0, Niumag Electric Co.), with proton density visualized as color-coded maps.

2.8. Dynamic rheology measurement

Following the protocol described by Ye et al. (2024) with slight modifications, rheological measurements were conducted using an MCR 302 rheometer (Anton Paar, Graz, Austria). Samples were tested between parallel plates (1 mm gap) under oscillatory shear at 0.1 Hz frequency and 2 % strain amplitude. Temperature was ramped from 20 °C to 90 °C at 5 °C/min while monitoring storage modulus (G') and loss (G") modulus.

2.9. Raman spectroscopy measurement

The gel samples were sectioned into 1–2 mm thick slices and subjected to cryofixation at −80 °C for 12 h, followed by lyophilization for 24 h. Raman spectroscopic analysis was performed using a confocal Raman spectrometer (laser wavelength: 532 nm) according to the methodology described by Xiong et al. (2021).

2.10. Fourier transform infrared spectroscopy

Surimi gel samples were lyophilized and mixed with potassium bromide (1:100, g/g). The IR spectra were then determined using a Nicolet iS-5 FT-IR spectrometer (Thermo Fisher Scientific, USA). The parameters were as follows: Wave number. The analytical parameters of the FT-IR spectra were as follows: Wave number in the range of 400 cm ⁻¹ - 4000 cm ⁻¹ The scanning frequency was 64 times, and the resolution was 4 cm ⁻¹.

2.11. Intermolecular forces measurements

Following the method of Xiong et al. (2021) with slight modifications, molecular forces in gel samples were analyzed through sequential extraction. Briefly, 3 g of gel sample was incubated with 15 mL of the following solutions:0.05 mol/L NaCl (SA); 0.6 mol/L NaCl (SB); 0.6 mol/L NaCl +1.5 mol/L urea (SC); 0.6 mol/L NaCl +8 mol/L urea (SD); 0.6 mol/L NaCl +8 mol/L urea +0.5 mol/L β-mercaptoethanol (SE). The relative contributions of different molecular forces were calculated as: Ionic bonding = SB – SA; Hydrogen bonding = SC – SB; Hydrophobic interactions = SD – SC; Disulfide bonding = SE – SD. After incubation, samples were homogenized at 10,000 rpm and centrifuged at 10,000 ×g for 20 min. The supernatant protein content was determined using Coomassie Brilliant Blue assay.

2.12. Scanning electron microscopy

Following Zhu et al. (2023) with minor modifications, gel samples (5.0 mm × 5.0 mm × 10 mm) were fixed in 2.5 % glutaraldehyde for 24 h, dehydrated in anhydrous ethanol, and sputter-coated with gold. Microstructural observation was then conducted using scanning electron microscopy (Hitachi, Tokyo, Japan) at ×5000 magnification.

SEM images (×5000 magnification) were analyzed using ImageJ software (National Institutes of Health, Bethesda, MD, USA) to quantify gel porosity, calculated as:

$$\text{Porosity}\left(\%\right)=\left(\text{Pore area}/\text{Total image aera}\right)\times 100\%$$ (4)

2.13. Confocal laser scanning microscopy

The distribution of polysaccharides in the samples was examined using a FluoView™ FV3000 inverted microscope (Olympus, Tokyo, Japan). For staining, 1 g of sample was mixed with 40 μL of 0.01 % Calcofluor White solution (polysaccharide dye) (Xiong et al., 2021). After staining, the samples were thoroughly rinsed with distilled water to remove excess dye. The stained samples were then carefully mounted on single-well slides and covered with coverslips to minimize air bubble formation. Fluorescence imaging was performed using an excitation wavelength of 405 nm and emission wavelength of 430 nm.

2.14. Sensory evaluation

The sensory characteristics of surimi gels were evaluated with reference to the protocol of Wu, Zhao, et al. (2024). A trained panel consisting of ten sensory experts (5 males and 5 females, aged 23–28 years) with one year of experience in fish product evaluation participated in the study. Prior to assessment, gel samples were standardized to uniform dimensions and assigned random three-digit codes to eliminate bias. Panelists independently evaluated five key attributes (texture, saltiness, juiciness, smell, and color) using a standardized 100-point scoring system (Table 1).

Table 1.

Score criteria on sensory.

Project	Standard for evaluation	Score
Texture	Compact structure, uniform pores, pressure-resistant with full elastic recovery	20–25
	Uniform and dense pores, pressure-resistant but no elastic recovery	14–19
	Uneven surface pores, prone to cracking under pressure	8–13
	Highly porous, rough surface, fragile and cracks easily	1–7
Saltiness	Well-balanced umami with pleasant aftertaste	20–25
	Slightly mild saltiness	14–19
	Weak saltiness	8–13
	Nearly no detectable saltiness	1–7
Juiciness	Tender and juicy	20–25
	Moderately tender and juicy	14–19
	Slightly dry and less juicy	8–13
	Dry and tough with minimal juiciness	1–7
Smell	No noticeable fishy off-odor	13–15
	Slight fishy off-odor	9–12
	Distinct fishy off-odor	5–8
	Strong fish odor	1–4
Color	Bright white and glossy	9–10
	White with slight yellow	7–8
	Yellow	4–6
	Dark yellow	1–3

2.15. Statistical analysis

The data were subjected to statistical analysis using SPSS 27.0 software (SPSS Inc., Chicago, USA) and plotted using Origin 2022 (Origins Laboratories, Northampton, USA). Each measurement was performed at least three times, and all experiments were repeated three times. The differences in means were tested by Duncan's test (P < 0.05) and expressed as mean ± standard error (SE). Pearson correlation analysis and hierarchical clustering analysis (HCA) were performed using the plug-in application in Origin 2022.

3. Results and discussion

3.1. Gel strength

Gel strength, an indicator of surimi gel's resistance to deformation under external forces (Lin et al., 2024), varied significantly among treatment groups (Fig. 1A). Compared to the low-salt group (LS), the high-salt control (CON) exhibited greater gel strength (P < 0.05), attributed to the enhanced solubilization of salt-soluble proteins under high ionic strength conditions, which promotes the formation of an ordered gel network (Fu et al., 2012). Conversely, insufficient salt content hinders the extraction of MP, thereby preventing the formation of a structurally stable three-dimensional gel network (Zhao, Wei, et al., 2023). The addition of hydrophilic colloids (LA, LC, LP) significantly improved the gel strength of low-salt surimi (P < 0.05). Notably, PG increased the gel strength by 117 % (from 2443.53 g·mm to 5299.52 g·mm), outperforming AP and CP. This improvement likely stems from the ability of polysaccharides to fill voids within the gel network (Lan et al., 2023). Ultrasonication further enhanced gel strength, regardless of the presence of hydrophilic colloids (ULS, ULA, ULC, ULP). It was speculated that ultrasound disrupts myosin structures, increasing MP solubility and promoting intermolecular interactions during gelation. This hypothesis is supported by Gao, You, et al. (2023), which reported that ultrasonication under low-salt conditions facilitates the formation of non-disulfide bonds, improving gel properties. Remarkably, the ULC group achieved a gel strength of 6076.16 g·mm, surpassing even the high-salt control (P < 0.05). This suggests that ultrasonication synergizes with CP to enhance MP-hydrocolloid interactions, forming a denser and more ordered gel network (Lin et al., 2024). In conclusion, the addition of hydrocolloids, particularly PG, significantly improved the gel strength of low-salt surimi, while ultrasonication further enhanced this effect, with the ULC group surpassing even the high-salt control.

Fig. 1.

Effect of hydrocolloids and ultrasonication on low-salt surimi gels: (A) Gel strength; (B—F) Texture profile. Significant differences marked (*P < 0.05, **P < 0.01, ***P < 0.001). CON: 3 % salt surimi. LS: 1 % salt surimi. Low-salt surimi (1 % salt) with individual treatments: ULS: ultrasound alone; LA: 0.2 % apple pectin; LC: 0.2 % citrus pectin; LP: 0.2 % peach gum. Low-salt surimi with combined treatments: ULA: ultrasound +0.2 % apple pectin; ULC: ultrasound +0.2 % citrus pectin; ULP: ultrasound +0.2 % peach gum.

3.2. TPA

The textural properties of surimi gels, including hardness, springiness, cohesiveness, gumminess, and chewiness, are critical indicators of gel quality and consumer acceptance. The textural properties of all treatment groups are shown in Fig. 1B-F. The low-salt group (LS) exhibited significantly lower values for all textural parameters compared to the high-salt control (CON, P < 0.05). This decline was attributed to incomplete solubilization of myofibrillar proteins (MP) under low salt conditions, resulting in a loose and less cohesive gel network, consistent with the observed reduction in gel strength. Hydrocolloid addition (LA, LC, LP) or ultrasonication (ULS) alone significantly improved the hardness of low-salt surimi compared to the LS group (P < 0.05), with similar trends observed for springiness, cohesiveness, gumminess, and chewiness. Ultrasonication promotes MP unfolding, exposing internal reactive groups and enhancing intermolecular interactions (He et al., 2022). Hydrocolloids, such as pectin, absorb water and swell during heating, exerting pressure on the gel matrix to maintain elasticity. Zhao, Piao, et al. (2023) demonstrated that fucoidan incorporation reduced intermolecular spaces between proteins, resulting in a denser network structure. Among the hydrocolloid-treated groups, LP exhibited higher springiness and chewiness compared to LA and LC (P < 0.05). This difference may be attributed to the unique molecular structure of peach gum, which enhances gel elasticity. However, Buda et al. (2021) reported that pectin addition could dilute MP concentration, reducing gel hardness, highlighting the influence of hydrocolloid type and concentration on surimi gel properties. The combination of ultrasonication and hydrocolloids (ULA, ULC, ULP) further enhanced textural properties. Notably, the ULC group exhibited significantly higher hardness, springiness, cohesiveness, gumminess, and chewiness compared to the CON group (P < 0.05), demonstrating the superior performance of the ultrasonication-citrus pectin combination.

3.3. Water-holding capacity (WHC) and weight loss

WHC indicates the ability of the gel network to retain water, directly influences the tenderness, texture, and juiciness of surimi products (Yan et al., 2024). The effect of WHC in different treatment groups is shown in Fig. 2A. Compared to the CON group (3 % NaCl), WHC was significantly decreased in the LS group (1 % NaCl) (P < 0.05). This decline could be attributed to insufficient solubilization of myosin at lower salt concentrations, leading to incomplete unfolding of MP and the formation of a less stable gel network, which failed to effectively retain water (Zhao, He, et al., 2024). On the contrary, high salt conditions promote MP solubilization, facilitating the formation of chemical bonds during thermal induction and creating a denser gel network with enhanced water retention (Monto et al., 2024). The addition of hydrocolloids significantly improved the WHC of low-salt surimi (P < 0.05). Notably, the LP group achieved a WHC of 67.37 %, representing an 11.67 % increase compared to the LS group (60.33 %). This improvement is likely due to AP's ability to bind water and form a viscous sol-gel structure, which fills the protein network and enhances its density (Zhu et al., 2023). Additionally, the hydrophilic groups (-OH) in hydrocolloids further contribute to water retention within the gel network (Mao et al., 2025). The ULS group demonstrated an improved WHC of 63.23 %, compared to 60.33 % in the LS group. This enhancement is likely due to ultrasound-induced cavitation, which improves MP solubility and promotes protein cross-linking. Among all treatments, the ULC group achieved the highest WHC (71.53 %), which was significantly greater than that of the LC group (66.31 %, P < 0.05). This suggests that ultrasonication synergizes with CP to create a dense and homogeneous network structure, effectively retaining more water. He et al. (2023) reported similar findings that ultrasonic treatment enhanced intermolecular crosslinking through disulfide bonds and hydrophobic interactions, thereby improving water binding in surimi gels.

Fig. 2.

Effect of hydrocolloids and ultrasonication on low-salt surimi gels: (A) water holding capacity; (B) weight loss; (C) whiteness; (D) spin–spin relaxation times; (E) relative area percentages of T₂₁ + T₂₂, T₂₃ and T₂₄ peaks; (F) hydrogen proton density images. Different lowercase letters in the figure indicate significant differences among different surimi samples (P < 0.05). CON: 3 % salt surimi. LS: 1 % salt surimi. Low-salt surimi (1 % salt) with individual treatments: ULS: ultrasound alone; LA: 0.2 % apple pectin; LC: 0.2 % citrus pectin; LP: 0.2 % peach gum. Low-salt surimi with combined treatments: ULA: ultrasound +0.2 % apple pectin; ULC: ultrasound +0.2 % citrus pectin; ULP: ultrasound +0.2 % peach gum.

Proteins are denatured during the heating process thus causing water loss from the muscles. As shown in Fig. 2B, the LS group exhibited significantly higher weight loss compared to the CON group (P < 0.05). The addition of hydrocolloids significantly reduced weight loss in low-salt surimi (P < 0.05), likely due to their role as thickening agents that trap water within a three-dimensional network structure. Wei et al. (2024) demonstrated that κ-carrageenan effectively retains water within the gel network, reducing water loss rates. Ultrasonication, both alone and in combination with hydrocolloids, significantly reduced weight loss in low-salt surimi. Compared to the CON group, the ULC group exhibited a marked reduction in weight loss (P < 0.05). As demonstrated by Bai et al. (2025), ultrasound-induced cavitation disrupts chemical bonds within MP, promoting interactions among smaller protein fragments and leading to the formation of a denser gel network that effectively minimizes water loss. This consistent with the WHC results. These findings are consistent with the WHC results observed in the study. In conclusion, the synergistic application of hydrocolloids and ultrasonication significantly enhanced WHC and reduced weight loss, with the ULC group showing particularly superior performance.

3.4. Whiteness

The whiteness of surimi products directly affects consumer acceptability (Lan et al., 2023). Fig. 2C illustrates the variation of L*, a*, b*, and whiteness in each group. The data indicate that whiteness was primarily influenced by L* values. In a highly ionic concentration environment, proteins are fully solubilized and evenly distributed in the gel matrix, creating a uniformly smooth surface that reflects light effectively, thereby enhancing whiteness (Zhao, Wei, et al., 2023). Conversely, insufficient protein solubilization under low-salt conditions results in a loose gel network, which is less effective at reflecting light. Xu et al. (2022) demonstrated that a dense gel network can trap water on the protein surface, promoting light reflection and increasing whiteness. Additionally, Cando et al. (2016) found that high salt conditions could facilitate the formation of chemical bonds, further enhancing whiteness. The addition of hydrocolloids significantly improved the whiteness of low-salt surimi compared to the LS group (P < 0.05). Among the three hydrocolloids tested, CP had the greatest impact on whiteness, followed by PG and AP. The limited effect of AP may be attributed to its wheat-colored powder (Buda et al., 2021), while the smaller molecular weight of CP allowed it to bind more tightly to protein molecules despite its yellowish color (Liu et al., 2022). In our preliminary experiments, PG demonstrated a positive correlation between its concentration and gel whiteness. Hydrocolloids fill the pores within the gel network, creating a uniform surface texture that enhances light reflection and improves whiteness. Wei et al. (2024) reported that κ-carrageenan-treated gels formed a denser and more homogeneous network, increasing whiteness through light refraction. Ultrasonication further enhanced whiteness in low-salt surimi. The ULS group exhibited a significantly higher whiteness value of 80.31 compared to 76.72 for the LS group (P < 0.05). This improvement is likely attributable to ultrasound-induced improvements in gel texture and water retention, which enhance light reflection. The whiteness values of three hydrocolloids increased following synergistic ultrasonic treatment, suggesting a reinforcement of the gel network. This change in whiteness may be attributed primarily to enhanced light diffraction resulting from structural modifications within the gel matrix. Notably, the ULC group achieved the highest whiteness value of 81.57, which was higher than those of the CON group (80.37) and the LC group (80.86). Ultrasonication combined with CP dilutes pigment content and promotes the formation of a tightly ordered gel network, further enhancing water retention and light reflection. Zhao, Piao, et al. (2023) similarly observed that ultrasound combined with curdlan improved water retention and whiteness in gel samples. In conclusion, the combination of hydrocolloids and ultrasonication significantly improved the whiteness of low-salt surimi, especially in the ULC group. Despite variations in the effects of different hydrocolloids, all observed changes in whiteness remained within the acceptable range for consumers.

3.5. LF-NMR

To further investigate the water distribution and mobility inside the gel, the relaxation time (T₂) of surimi gels under different treatment conditions was measured by LF-NMR analysis. As shown in Fig. 2D, the relaxation times were categorized into four consecutive patterns: T₂₁ (0–1 ms) and T₂₂ (1–10 ms) represent bound water closely associated with macromolecules, T₂₃ (70–200 ms) denotes immobile water trapped in the gel network and T₂₄ (1000–3000 ms) corresponds to free water distributed freely in the gel matrix. Salt reduction, hydrocolloid addition, and their combination with ultrasonication significantly altered the T₂ relaxation profiles, indicating substantial impacts on water distribution and mobility (Fig. 2D). Compared to the CON group, the T₂₁ peak in the LS group shifted to a higher relaxation time, reflecting weakened protein-water interactions due to reduced salt content. This shift led to a significant decrease in immobile water (T₂₃) and an increase in free water (T₂₄), which is consistent with reports that low salt content disrupts gel network stability in silver carp surimi (He et al., 2023). The percentage stacking plots for different states of moisture presence in the gel matrix are shown in Fig. 2E. The addition of hydrocolloids (AP, CP, PG) reversed this trend, increasing immobile water by about 10 % and reducing free water (P < 0.05). This improvement is attributed to hydrocolloids' ability to enhance gel network stability through hydrophobic and hydrogen bonding interactions with MP (Yan et al., 2024). For instance, CP demonstrated superior water retention capacity, likely due to its lower molecular weight and higher density of hydrophilic groups (-OH and -COOH) (Yu et al., 2024). Ultrasonication further enhanced immobile water content in low-salt surimi. The ULS group exhibited a 6 % increase in immobile water compared with the LS group (P < 0.05), attributed to ultrasound-induced unfolding of MP structures and exposure of hydrophobic groups (Zou et al., 2021). When combined with hydrocolloids, ultrasonication synergistically increased the immobile water content. Notably, the ULC treatment achieved the highest content of 82.89 %, which exceeded both the high-salt control (CON, 79.14 %) and its counterpart without ultrasound (LC, 78.73 %). This synergistic effect is likely attributable to ultrasound-induced promotion of tighter protein-polysaccharide interactions, a phenomenon previously observed in curdlan-enhanced surimi gels by Zhang, Lu, et al. (2023). Furthermore, the concomitant increase in immobile water content aligns with the improved WHC, corroborating the enhancement of the gel network's stability.

3.6. MRI image

MRI is often considered complementary to LF-NMR, providing excellent visualization of internal structural changes and water distribution in gels. Fig. 2F shows the MRI images of surimi gels under different treatment conditions, where red indicates high water content (high hydrogen proton density) and blue corresponds to low water content (low hydrogen proton density). Compared to the CON group, the LS group exhibited the lower hydrogen proton density, indicating that reduced salt content impaired gel network formation and caused significant water loss. Under low-salt conditions, the reduction of negative charges on the protein surface diminishes inter-myofibrillar electrostatic repulsion, leading to insufficient water-holding space within the gel network and a consequent decline in WHC (Zhang, Chen, et al., 2023). Compared with the LS group, the ultrasonic and hydrocolloids (LA, LC, LP) treatment alone increased the red pixel intensity in MRI images, reflecting higher immobile water content. When combined with hydrocolloids (ULA, ULC, ULP), ultrasonication synergistically increased hydrogen proton density, with the ULC group showing the most intense red coloration (Fig. 2F). This suggests that ultrasound promotes tighter protein-polysaccharide interactions, as demonstrated by Zhang, Chen, et al. (2023) in curdlan-enhanced surimi gels.

3.7. Rheological properties

The protein network structure in surimi gels is maintained by interactions between protein molecules, including ionic bonds, hydrogen bonds, disulfide bonds, and hydrophobic interactions. The storage modulus (G') reflects the elastic deformation capacity of the gel, which is influenced by protein concentration and intermolecular interactions (Gao, You, et al., 2023). As shown in Fig. 3A, G‘remained constant below 35 °C for all gel samples, indicating the initial formation of a protein network stabilized by hydrogen bonds. As the temperature increased, G' sharply decreased around 40–45 °C and 50–55 °C due to the disruption of hydrogen bonds and activation of endogenous proteases, which degrade myosin and destabilize the gel network (Singh et al., 2020). Similar results were reported by Lan et al. (2023). From 60 °C to 80 °C, G' increased rapidly, which was attributed to protein repolymerization and the formation of a stable gel network. The final G' of the low-salt group (LS) was significantly lower than that of the high-salt control (CON), consistent with previous studies showing that low ionic strength reduces myosin solubility and leads to over-aggregation during heating, resulting in a weaker gel network (Zhao, Yang, et al., 2024). Zhao, Wei, et al. (2023) also noted that higher salt concentrations enhance myosin extraction and expose more hydrophobic and sulfhydryl groups, leading to higher G' values. The addition of hydrocolloids (LA, LC, LP) or ultrasonication (ULS) increased G' compared to the LS group, suggesting that these treatments promote myosin unfolding and direct myosin head aggregation. Gao, You, et al. (2023) demonstrated that konjac glucomannan and ultrasonication enhance beef myosin G' through hydrophobic interactions and disulfide bonds. The combination of ultrasonication and hydrocolloids (ULA, ULC, ULP) further increased G', with the ULC group exhibiting the highest value. This improvement is attributed to ultrasound-induced enhancement of myosin solubility and stronger interactions between myosin and hydrocolloids. Zou et al. (2021) reported that ultrasound exposes reactive groups in proteins, enhancing intermolecular interactions and promoting gelation.

Fig. 3.

Effect of hydrocolloids and ultrasonication on low-salt surimi gels: (A) storage modulus; (B) loss modulus. CON: 3 % salt surimi. LS: 1 % salt surimi. Low-salt surimi (1 % salt) with individual treatments: ULS: ultrasound alone; LA: 0.2 % apple pectin; LC: 0.2 % citrus pectin; LP: 0.2 % peach gum. Low-salt surimi with combined treatments: ULA: ultrasound +0.2 % apple pectin; ULC: ultrasound +0.2 % citrus pectin; ULP: ultrasound +0.2 % peach gum.

The loss modulus (G") reflects the viscosity of the surimi gel. As shown in Fig. 3B, all surimi gel samples exhibited similar rheological patterns, with G' significantly higher than G", confirming their viscoelastic solid-like behavior (Wu, Zhao, et al., 2024). The increase in G' and G" for hydrocolloid-modified or ultrasonically treated samples compared to the LS group indicates enhanced viscoelasticity. The ULC group showed the highest G' and G" after heating, demonstrating the significant impact of ultrasound and citrus pectin on surimi gel viscoelasticity. In conclusion, the combination of hydrocolloids and ultrasonication significantly improved the viscoelastic properties of low-salt surimi, with the ULC group demonstrating the highest G' and G" values.

3.8. Raman spectroscopy

Fig. 4A reveals the Raman spectrum of the protein, the amide I band (1600–1700 cm⁻¹) in Raman spectra provides critical insights into protein secondary structure. The spectral bands 1650–1658 cm⁻¹, 1665–1680 cm⁻¹, 1680–1690 cm⁻¹, and 1660–1665 cm⁻¹ corresponded to α-helix, β-sheet, β-turn, and random coil of the secondary structure of the protein, respectively (Zhao, Yang, et al., 2024). Protein secondary structure composition was determined by deconvoluting the amide I band using PeakFit software (v4.12) with Gaussian fitting. As shown in Fig. 4B, the CON group exhibited higher β-sheet content compared to the LS group, attributed to high salt concentrations promoting myosin solubilization and β-sheet formation. Similar results were reported by Feng et al. (2018), who observed that increasing NaCl concentration significantly increased β-sheet content while reducing α-helix content, indicating myosin unfolding. The addition of hydrocolloids (LA, LC, LP) generally decreased α-helix content and increased β-sheet content compared to the LS group. This phenomenon can be attributed to the glycation-induced unfolding of myosin and the disruption of α-helix structures by hydrocolloids. Hydrocolloids enhance hydrogen bonding, stabilizing β-sheet structures and promoting protein cross-linking. For instance, κ-carrageenan has been shown to increase β-sheet content while reducing β-turn content, leading to a more compact gel network (Lan et al., 2023). Ultrasonication of low-salt surimi also increased β-sheet content and decreased α-helix content, likely due to the cavitation effect disrupting hydrogen bonds that stabilize α-helix structures (Zhao, He, et al., 2024). Combined with hydrocolloids (ULA, ULC, ULP), ultrasonication further raised the β-sheet fraction; the ULC gel contained the highest proportion, significantly exceeding that of the CON group (P < 0.05). This suggests that ultrasonication enhances intermolecular interactions, facilitating the formation of a dense gel network. Similar findings were reported by Gao, You, et al. (2023), who found that ultrasound pretreatment combined with konjac glucomannan increased the β-sheet content in beef myosin. Therefore, the changes in the secondary structure of the ULC gel samples directly correlated with enhancements in the three-dimensional gel network, consistent with our WHC results (Fig. 2A) and G' results (Fig. 3A).

Fig. 4.

Effect of hydrocolloids and ultrasonication on low-salt surimi gels: (A) Raman spectra; (B) protein secondary structure; (C) FTIR spectra; (D) intermolecular forces. CON: 3 % salt surimi. LS: 1 % salt surimi. Low-salt surimi (1 % salt) with individual treatments: ULS: ultrasound alone; LA: 0.2 % apple pectin; LC: 0.2 % citrus pectin; LP: 0.2 % peach gum. Low-salt surimi with combined treatments: ULA: ultrasound +0.2 % apple pectin; ULC: ultrasound +0.2 % citrus pectin; ULP: ultrasound +0.2 % peach gum.

3.9. FTIR

FTIR spectra were used to analyze the chemical changes in the intermolecular interactions of protein-hydrocolloid systems in surimi gels. As shown in Fig. 4C, the absorption peaks at 3000–3500 cm⁻¹, corresponding to N—H and O—H stretching vibrations, reflect the O—H stretching bands of amide A. These peaks are closely related to intramolecular and intermolecular interactions, particularly hydrogen bonding and hydrophobic interactions (Zhang, Lu, et al., 2023). The amide I band (1650 cm⁻¹), primarily induced by C O stretching vibrations, and the amide II band, related to N—H bending and C—N stretching vibrations. Compared to the LS group, the lower peaks of the amide A band in hydrocolloid-treated (LA, LC, LP) and ultrasonicated groups (ULS) indicate stronger protein-water interactions, suggesting enhanced protein-water binding in low-salt surimi with hydrocolloids. This effect was further amplified by the combination of ultrasonication and hydrocolloids, likely due to ultrasound-induced enhancement of hydroxyl interactions between proteins and hydrocolloids (Yu et al., 2024).

3.10. Chemical forces

Protein structures are stabilized by four primary interaction forces: ionic bonds, hydrogen bonds, hydrophobic interactions, and disulfide bonds. As shown in Fig. 4D, the high-salt control group (CON, 3 % NaCl) exhibited significantly lower ionic bond content (P < 0.05) than the low-salt group (LS, 1 % NaCl). This phenomenon can be attributed to exogenous Na⁺ and Cl⁻ ions to charged amino acid residues, which disrupts native ionic interactions between MP while simultaneously inducing protein solubilization and conformational changes that expose previously buried functional groups (Wang et al., 2024). Conversely, hydrogen bonds increased markedly (P < 0.05) in the CON group, a critical adaptation for maintaining gel matrix stability and WHC. This trend aligns with He et al. (2023), who showed that raising the ionic strength triggers an α-helix-to-β-sheet shift in myofibrillar proteins, promoting intermolecular hydrogen bonding. Notably, the observed increase in hydrogen bonds and concomitant decrease in ionic bonds in low-salt surimi, both with hydrocolloid addition alone and in combination with ultrasonication, suggesting that hydrocolloids disrupt ionic interactions while concurrently stabilizing the gel network. Hydrophobic interactions and disulfide bonds are the dominant forces governing surimi gel conformation. Individual ultrasonic treatment increased the hydrophobic interaction content in low-salt surimi from 0.51 mg/mL to 0.78 mg/mL. This discrepancy likely stems from sonication-induced exposure of hydrophobic groups. The addition of the three hydrocolloids increased the hydrophobic interaction content in the low-salt surimi to 0.85 mg/mL (LA), 0.94 mg/mL (LC), and 1.16 mg/mL (LP), respectively. This observation mirrors Yan et al. (2024), who found that κ-carrageenan induces unfolding and aggregation of scallop myofibrillar proteins, strengthening hydrophobic interactions and yielding a firmer gel network. Furthermore, hydroxyl groups in hydrocolloids may form additional hydrogen bonds with protein subunits, facilitating protein moiety exposure and delaying denaturation, which ultimately enhances hydrophobic interactions and disulfide bonds (Gao, You, et al., 2023). The synergistic application of ultrasonication and hydrocolloids further enhanced both hydrophobic interactions and disulfide bond formation, driven by promoted protein molecular cross-linking. Specifically, the hydrophobic interaction and disulfide bond content in the ULC group reached 1.80 mg/mL and 4.02 mg/mL, respectively. He et al. (2023) corroborated this mechanism, showing that ultrasonication improves protein dispersion, thereby strengthening interactions with β-glucan and yielding a more densely ordered gel network in silver carp surimi. The observed increase in disulfide bonds may also reflect intensified myosin head-head and tail interactions during gelation. The ULC group demonstrated significantly enhanced hydrogen bonding, hydrophobic interactions, and disulfide bonding relative to CON (P < 0.05), accounting for its superior gel strength (Fig. 1A) and WHC (Fig. 2A).

3.11. SEM

As shown in Fig. 5, the low-salt group (LS) exhibited a highly porous (43.43 %) and loose structure compared to the high-salt control (CON). This is attributed to reduced myosin solubility and excessive protein aggregation under low salt conditions, leading to an irregular gel network. In contrast, increasing salt concentration promoted myosin dissolution and enhanced protein interactions, resulting in a denser and more robust gel network. Improvements in the gel network structure were observed following both hydrocolloid addition (LA, LC, LP) and ultrasonic treatment alone (ULS) compared to the LS group, with LC and ULS exhibiting porosities of 34.29 % and 37.18 %, respectively. The ULS group exhibited significantly lower porosity (P < 0.05), likely owing to ultrasound-induced protein dispersion and the formation of a denser network. Hydrocolloids, acting as fillers between protein molecules, prevented excessive aggregation and contributed to a smoother and more homogeneous gel structure (Zhao, Wei, et al., 2023). Mi et al. (2024) further demonstrated that hydrocolloids enhance intermolecular cross-linking through disulfide bonds and hydrophobic interactions, leading to tighter gel networks with improved water retention. The combination of ultrasonication and hydrocolloids (ULA, ULC, ULP) resulted in a more stable and homogeneous gel network. Notably, the ULC group demonstrated the most compact microstructure, with a porosity of 27.18 %, which was lower than that of the CON (32.72 %) and LC (34.29 %) groups. This improvement is primarily due to ultrasound-induced enhancement of CP solubility and structural unfolding, which promotes tight binding between protein molecules and CP, ultimately leading to the formation of an ordered and dense network. Similar findings were reported by Lin et al. (2024), who observed that ultrasound enhanced the binding of pork myosin to κ-carrageenan, forming an excellent gel network.

Fig. 5.

Effect of hydrocolloids and ultrasonication on the microstructure and porosity of low-salt surimi gels (Magnification 5 k ×) in each treatment group. CON: 3 % salt surimi. LS: 1 % salt surimi. Low-salt surimi (1 % salt) with individual treatments: ULS: ultrasound alone; LA: 0.2 % apple pectin; LC: 0.2 % citrus pectin; LP: 0.2 % peach gum. Low-salt surimi with combined treatments: ULA: ultrasound +0.2 % apple pectin; ULC: ultrasound +0.2 % citrus pectin; ULP: ultrasound +0.2 % peach gum.

3.12. CLSM

To validate the proposed conjecture, the microstructure of samples in a non-dehydrated state was analyzed using CLSM (Fig. 6). Calcofluor White-stained polysaccharide networks (bright regions) revealed distinct morphological differences among treatments. In ultrasonicated samples containing hydrocolloids (ULA, ULC, ULP), proteins formed large coacervates tightly bound to polysaccharides. Notably, the ULC group exhibited the brightest fluorescence intensity and a uniformly dense polysaccharide network, whereas ULA and ULP displayed relatively looser matrices and dimmer fluorescence. This suggests that citrus pectin possesses superior binding affinity to MP under ultrasonic conditions. These microstructural observations correlate with the improved water entrapment and gel stability in ULC, as the denser network restricts water mobility. This finding aligns with previous studies demonstrating that polysaccharides can modulate biopolymerization processes through protein interactions, thereby influencing protein aggregation behavior and altering the structural continuity of protein networks (Zhang, Chen, et al., 2023). Supporting evidence comes from Gao, You, et al. (2023), which reported that ultrasonication enhanced protein cross-linking with konjac dextran, ultimately strengthening the network structure of beef myofibrillar protein gels.

Fig. 6.

CLSM images of surimi gels with different treatments. The scale bar indicates 10 μm. CON: 3 % salt surimi. Low-salt surimi (1 % salt) with combined treatments: ULA: ultrasound +0.2 % apple pectin; ULC: ultrasound +0.2 % citrus pectin; ULP: ultrasound +0.2 % peach gum.

3.13. Sensory evaluation analysis

Sensory evaluation is a crucial indicator of a product's desirability and acceptability among consumers. As shown in Fig. 7A, the low-salt group (LS) exhibited lower scores for texture, saltiness, umami, smell, and color compared to the high-salt control (CON). This decline is attributed to insufficient MP solubilization under low salt conditions, leading to a loose gel structure and loss of juices, which negatively impacted texture, flavor, and color (Wu, Zhao, et al., 2024). Hydrocolloid addition (LA, LC, LP) or ultrasonication (ULS) improved the texture and color of low-salt surimi compared to the LS group, consistent with the results of TPA tests and whiteness measurements. Moreover, the incorporation of hydrocolloids was found to enhance the perceived saltiness of low-salt surimi. This result is consistent with the work of Lan et al. (2023), who demonstrated that κ-carrageenan significantly increased saltiness perception in low-salt golden pompano surimi. In contrast to the efficacy of hydrocolloids, the combination of ultrasound and hydrocolloids did not yield a synergistic enhancement in saltiness. This observation aligns with the findings of Barretto et al. (2020), which indicated that ultrasonication has only a minimal direct effect on salty taste perception. The combination of ultrasonication and hydrocolloids (ULA, ULC, ULP) improved the umami and smell characteristics of low-salt surimi, likely due to the formation of a stable gel network that inhibited the release of fishy off-odors.

Fig. 7.

Effect of hydrocolloids and ultrasonication on low-salt surimi gels: (A) indicator scores; (B) overall scores. Different lowercase letters in the figure indicate significant differences among different surimi samples (P < 0.05). CON: 3 % salt surimi. LS: 1 % salt surimi. Low-salt surimi (1 % salt) with individual treatments: ULS: ultrasound alone; LA: 0.2 % apple pectin; LC: 0.2 % citrus pectin; LP: 0.2 % peach gum. Low-salt surimi with combined treatments: ULA: ultrasound +0.2 % apple pectin; ULC: ultrasound +0.2 % citrus pectin; ULP: ultrasound +0.2 % peach gum.

Fig. 7B presents the overall sensory evaluation scores for the various surimi gels.

The ULC group achieved the highest total sensory score among all experimental groups (LS, LA, LC, LP, ULS, ULA, ULP). Importantly, there was no significant difference between the ULC group and the high-salt control (P > 0.05). This improvement is attributed to the enhanced texture, increased saltiness perception, and reduced fishiness achieved through the synergistic effects of ultrasonication and CP. Therefore, the combination of hydrocolloids and ultrasonication significantly improved the sensory properties of low-salt surimi, with the ULP group demonstrating overall acceptability comparable to the high-salt control.

3.14. Correlation analysis

Pearson correlation analysis revealed significant relationships between gel strength and key quality parameters (Fig. 8A). Gel strength showed strong positive correlations (P < 0.05) with WHC, resilience, hardness, immobile water content, G' value, β-sheet content, hydrogen bonds, hydrophobic interactions, disulfide bonds, and overall scores. Conversely, negative correlations (P < 0.05) were observed with weight loss, free water content, α-helix content, ionic bonds, and porosity. These findings corroborate previous reports on low-salt surimi gels (Lan et al., 2023). The synergistic effect of hydrophilic hydrocolloids and ultrasonication improved gel quality through three mechanisms: (1) enhanced network formation via increased protein cross-linking, (2) improved water immobilization through conversion of free to bound water, and (3) protein conformational changes from α-helix to β-sheet structures. These modifications promoted stronger intermolecular interactions (hydrogen bonds, hydrophobic interactions, and disulfide bonds) while reducing porosity. The resulting denser network structure accounted for the enhanced texture, WHC, and higher overall sensory scores of the treated gels.

Fig. 8.

Effect of hydrocolloids and ultrasonication on low-salt surimi gels: (A) Heat map of Pearson correlation analysis, where values with “*” were statistically significant (P < 0.05); (B) Thermogram analysis of important parameters. CON: 3 % salt surimi. LS: 1 % salt surimi. Low-salt surimi (1 % salt) with individual treatments: ULS: ultrasound alone; LA: 0.2 % apple pectin; LC: 0.2 % citrus pectin; LP: 0.2 % peach gum. Low-salt surimi with combined treatments: ULA: ultrasound +0.2 % apple pectin; ULC: ultrasound +0.2 % citrus pectin; ULP: ultrasound +0.2 % peach gum.

Hierarchical cluster analysis (Fig. 8B) further elucidated these relationships, with red indicating positive correlations and blue indicating negative correlations. The analysis revealed two primary clusters: one encompassing positive correlations between ultrasonication-hydrocolloid co-treatment and gel strength, springiness, hydrogen bonds, β-sheet, hardness, WHC, immobile water, G' value, hydrophobic interaction, and disulfide bonds; and the other representing negative correlations with weight loss, free water, ionic bonds, α-helix and porosity. These findings are consistent with those reported by Zhang, Lu, et al. (2023). Notably, the ULC group exhibited the most favorable gel properties, with the strongest correlations among the ULA, ULC, and ULP groups. This highlights the superior performance of the ultrasonication-citrus pectin combination in enhancing low-salt surimi gelation.

4. Conclusions

This study investigated the synergistic effect of ultrasound and hydrocolloids on improving low-salt surimi gel properties. The results demonstrated that both ultrasonic treatment and hydrocolloid addition independently enhanced gel strength, TPA, and WHC of low-salt surimi, while their combination yielded more pronounced improvements. Particularly, the ULC group exhibited a remarkable increase in gel strength from 2443.52 g·mm to 6076.15 g·mm. Comprehensive analyses including LF- NMR, Raman spectroscopy, and FTIR revealed that the ultrasound-citrus pectin combination promoted water migration from free to immobilized states and facilitated the α-helix to β-sheet transition, thereby strengthening hydrogen bonds and hydrophobic interactions within the gel network. SEM observations confirmed that the ULC group developed the most compact and homogeneous microstructure with the lowest porosity (27.18 %) compared to the CON group (32.72 %). Sensory evaluation indicated that the ULC treatment most closely resembled the high-salt control in terms of quality attributes. Furthermore, Pearson's correlation analysis and hierarchical cluster analysis established strong relationships between gel strength, water distribution, β-sheet content, hydrophobic interactions, disulfide bonds, and porosity, collectively explaining the superior performance of the ULC group. These findings provide valuable insights into the mechanism of ultrasound-assisted hydrocolloid modification in low-salt surimi gelation and offer practical guidance for developing reduced‑sodium surimi products without compromising quality.

CRediT authorship contribution statement

Shuguang Si: Writing – original draft, Validation. Danfeng Huang: Methodology. Ting Wang: Investigation. Yingying Sun: Supervision. Yunfang Qian: Writing – review & editing, Supervision. Xiao Liu: Writing – review & editing, Supervision, Funding acquisition.

Ethical statements

The sensory experiments conducted in this study received ethical clearance from our institution. All participants provided written informed consent after receiving detailed explanations of the study objectives, procedures, and potential benefits. Participation was voluntary, and subjects retained the right to withdraw at any time. All surimi samples met food safety standards for human consumption. Participant privacy and rights were strictly protected throughout the study.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Data availability

The data that has been used is confidential.