The effects of low‐molecular‐weight fucoidan on the quality of frozen Litopenaeus vannamei

Abstract

BACKGROUND

Aquatic products are highly valued for their rich nutrient content and unique flavors. However, they are particularly vulnerable to microbial contamination during transportation and storage, which can lead to spoilage. Fucoidan exhibited a broad spectrum of biological activities. Nevertheless, research on its application as an antifreeze agent remains limited. The practical application of high‐molecular‐weight fucoidan is constrained to some extent by its high viscosity and poor solubility. Conversely, low‐molecular‐weight fucoidan (WIF) prepared via physical, chemical, or enzymatic degradation methods may offer superior permeability and biological activities, making it a promising, novel, and highly efficient cryoprotectant.

RESULTS

In this study, WIF was prepared through irradiation degradation using fucoidan as the material. The relative molecular weights of WIF obtained via 50 kGy γ‐ray irradiation ranged from 4.97 × 10³ to 3.25 × 10⁴ Da. It was found that the irradiation degradation method reduced the molecular weight and altered the original chemical composition. Following irradiation, the protein content decreased from 0.94% ± 0.02% to 0.24% ± 0.04%, whereas the reducing sugar content increased from 0.69% ± 0.01% to 6.25% ± 0.36%. Results indicated that soaking treatments with various concentrations of WIF solution exhibited excellent cryoprotective properties and water holding capacity in frozen Litopenaeus vannamei. Notably, after 180 days of frozen storage, the group treated with 15 g L⁻¹ WIF maintained a myofibrillar protein Ca²⁺‐ATPase activity of 0.0338 U mg⁻¹ prot and a total sulfhydryl content of 30.06 μmol g⁻¹ prot, both of which were significantly higher than those of the other groups (P < 0.05). Furthermore, at day 180, the surface hydrophobicity of the 15 g L⁻¹ WIF group was 61.35 μg mg⁻¹ prot, significantly lower than that of the control groups (P < 0.05).

CONCLUSION

These findings demonstrate that WIF effectively inhibits the decline of Ca²⁺‐ATPase activity, the oxidation of sulfhydryl groups, protein carbonylation, the exposure of hydrophobic residues, and the accumulation of malondialdehyde. In conclusion, WIF with lower molecular weight shows superior antifreeze and water retention abilities compared to high‐molecular‐weight fucoidan. © 2026 Society of Chemical Industry.

INTRODUCTION

Aquatic products are distinguished by their palatable meat, high protein content, and richness in unsaturated fatty acids and trace elements. However, the elevated levels of water and protein in the muscle tissue of these products make them susceptible to quality deterioration during frozen storage, leading to significant economic losses. ¹ During the freezing and storage processes, the formation and growth of ice crystals can result in the loss of juice from aquatic products. ² Additionally, microorganisms may exploit the nutrients present in these products, contributing to spoilage. ³ Furthermore, variations in storage conditions – such as changes in pH – and lipid oxidation can trigger protein denaturation, further exacerbating quality degradation throughout freezing and storage. ⁴

At present, the methods employed to address the deterioration of aquatic product quality during freezing and storage primarily include physical preservation technologies and chemical preservatives. Physical preservation techniques encompass refrigeration, ice temperature storage, and vacuum packaging. ⁵ Compared to chemical preservation methods, physical preservation technologies do not involve the use of chemical additives, thereby offering a higher level of safety. Physical preservation techniques are environmentally friendly, clean, and highly efficient, with short processing times and significant effects. ⁶ However, during prolonged cold storage, aquatic products may still experience quality deterioration such as juice loss, protein denaturation, and lipid oxidation. In addition to physical preservation techniques, preservative methods for aquatic products represent a significant category of countermeasures. ⁷ Aquatic products preservation technology offers advantages such as low cost and ease of operation. Based on their characteristics, these methods can be classified into several categories: salt‐based antifreeze and water retention agents, sugar‐based antifreeze and water retention agents, ⁸ phenolic antifreeze and water retention agents, and protein‐based antifreeze and water retention agents. ⁹ Among these options, phosphate‐based antifreeze water retention agents exhibit excellent performance in terms of both freezing resistance and color protection. However, excessive use of phosphates can lead to the development of metallic flavors in aquatic products and may pose health risks to consumers. ¹⁰ Phenolic compounds not only demonstrate strong antioxidant activity but also inhibit bacterial growth by disrupting microbial cell structures and affecting energy metabolism within microorganisms. However, phenolic compounds are relatively unstable and highly sensitive to light, heat, and pH. During processing, they are prone to oxidation and discoloration, ¹¹ which can result in brown or yellow deposits on the surface of seafood products. Furthermore, while most natural phenolic compounds are highly water‐soluble, they are poorly fat‐soluble. When treating high‐fat fish, these compounds struggle to penetrate uniformly into the fatty tissue, leading to inconsistent preservation results. Traditional sugar‐based antifreeze and water retention agents (such as sucrose or sorbitol) are characterized by high sweetness levels. ¹² However, they also contribute significantly to caloric content. Consequently, there is a growing need for natural alternatives that offer low sweetness and reduced caloric value. This has led to the development and application of natural sugar substitutes such as chitosan, alginate, and carrageenan, which serve as effective low‐calorie antifreeze and water retention agents. ¹³

Among marine‐derived polysaccharides, fucoidan has garnered significant attention for its potent antioxidant and antimicrobial activities. ¹⁴ However, due to several key industrial challenges –such as high molecular weight and high viscosity – its commercial application in the aquatic conservation market remains in its infancy. Traditional high‐molecular‐weight fucoidan often exhibits high viscosity and poor water solubility, which severely limits its ability to penetrate dense muscle tissues, thereby reducing its cryoprotective efficacy. Furthermore, the lack of standardized application protocols has hindered its large‐scale industrial integration. To overcome these commercial barriers, low‐molecular‐weight fucoidan (WIF) has emerged as a highly promising alternative. It effectively resolves the tissue permeability issues while retaining the essential advantages of being low‐calorie and low‐sweetness. Given the seafood industry's increasing demand for clean‐label and health‐conscious preservation methods, validating the efficacy of WIF is of immense industrial necessity. Therefore, in this experiment, various concentrations of WIF were employed to investigate the changes in the quality of Litopenaeus vannamei kernels during freezing and subsequent storage following immersion treatment. The effects of WIF on the quality of the frozen kernels were comprehensively assessed by monitoring moisture content changes, sensory evaluation, whiteness measurement, pH levels, Ca²⁺‐ATPase activity, total sulfhydryl content, carbonyl content, surface hydrophobicity, peroxide value, and thiobarbituric acid (TBA) value. We hypothesize that WIF can inhibit ice crystal recrystallization by forming a hydrogel within the shrimp muscle. Furthermore, WIF can penetrate more easily between myofibrils, binding to polar residues in protein molecules to stabilize their structure. These hypotheses were verified by measuring relevant indicators in the shrimp meat, aiming to provide a theoretical basis for superior freeze‐protection and water‐retention effects.

MATERIALS AND METHODS

Materials

Fucoidan from Kjellmaniella crassifolia Miyabe, named ‘F’ was supplied by National R&D Branch Center For Seaweed Processing (Dalian Ocean University, Dalian, China); potassium bromide was from Koufu Fine Chemical Research Institute (Tianjin, China); 2,2‐diphenyl‐1‐picrylhydrazyl (DPPH) was from Kasei Kogyo (Tokyo, Japan); salicylic acid, phenanthroline, and trichloroacetic acid were from Damao Chemical Reagent Factory (Tianjin, China); catechol violet and tetraethoxypropane were from Aladdin Bio‐Chemistry Technology Co., Ltd (Shanghai, China); the ultra‐trace Ca²⁺‐ATPase kit, total sulfhydryl (—SH) kit, and protein carbonyl content determination kit were from Jiancheng Institute of Bioengineering (Nanjing, China); and ethylenediaminetetraacetic acid (EDTA) was from Bodhi Chemical (Tianjin, China). All other reagents utilized in this study were of analytical grade purity.

Preparation of WIF

Referring to the experimental method described by Meng Y et al., ¹⁵ with slight modifications, 5 g of fucoidan was weighed and dissolved in distilled water at a solid‐to‐liquid ratio of 1:50 (m/v). The solution was then subjected to degradation through irradiation using ⁶⁰Co γ‐rays at a dose of 50 kGy under static room temperature conditions. The degradation process was conducted at a dose rate of 2 kGy h⁻¹. The irradiation was carried out with the technical support of the Institute for Farm Products Processing and Nuclear‐Agricultural Technology, Hubei Academy of Agricultural Science. After irradiation, the resulting sample solution was concentrated by rotary evaporation and subsequently lyophilized. Subsequently, the freeze‐dried product was ground to produce WIF, which was stored in a dry environment for future use.

Structural analysis of WIF

Chemical composition analysis

The phenol–sulfuric acid method was employed to quantify the total sugar content. The barium chloride–gelatin method was utilized to assess the sulfate group content. The carbazole colorimetric method was applied to determine the glucuronic acid content in fucoidan, using glucuronic acid as a standard. ¹⁶ The protein content in fucoidan was measured using the Caumas Brilliant Blue method, ¹⁷ while the reducing sugar content was evaluated through the 3,5‐dinitrosalicylic acid method. ¹⁸

Determination of molecular weights

The relative molecular weight distributions of F and WIF were determined using high‐performance gel permeation chromatography coupled with multi‐angle laser scattering (HPGPC‐MALS). The HPGPC‐MALS system comprised a high‐performance liquid chromatograph (model 1260, Agilent Technologies, Santa Clara, CA, USA), a Shodex OHpak SB‐803 HQ column in tandem with a Shodex OHpak SB‐802.5 HQ column (Resonac Corp., Tokyo, Japan), and an 18‐angle laser light scatterer. The mobile phase utilized was a 0.1 mol L⁻¹ NaNO₃ aqueous solution, delivered at a flow rate of 0.6 mL min⁻¹. The column temperature was maintained at 35 °C, the injection volume was set to 100 μL, and the total run time for molecular weight determination was 65 min.

The number‐average molecular weight (M _n) is a statistically averaged molecular weight calculated based on the number of molecules. It primarily reflects the influence of low‐molecular‐weight fractions in the system: ¹⁹

$$M_{\mathrm{n}}=\frac{\sum N_{\mathrm{i}}{M}_{\mathrm{i}}}{\sum N_{\mathrm{i}}}=\frac{W}{N}$$

where N _i is the number (or mole number) of molecules with molecular weight M _i; $\sum N_{\mathrm{i}}{M}_{\mathrm{i}}$ represents the total weight of all molecules (W); and $\sum N_{\mathrm{i}}$ is the total number of molecules ($N$).

Weight‐average molecular weight M _w is a molecular weight averaged statistically by molecular mass. Larger molecules contribute more significantly to M _w. This reflects the average ‘mass’ level of molecules in the sample and is closely related to the mechanical properties of polymers ²⁰ (such as strength and viscosity):

$$M_{\mathrm{w}}=\frac{\sum w_{\mathrm{i}}{M}_{\mathrm{i}}}{\sum w_{\mathrm{i}}}=\frac{\sum N_{\mathrm{i}}{M}_{\mathrm{i}}^2}{\sum N_{\mathrm{i}}{M}_{\mathrm{i}}}$$

where M _i is the total weight of molecules with molecular weight $M_{\mathrm{i}}(W_{\mathrm{i}}=N_{\mathrm{i}}{M}_{\mathrm{i}}$).

Infrared spectral analysis

Potassium bromide pressed tablets were prepared using the potassium bromide tablet method. This involved mixing 2 mg of F and WIF samples with 200 mg of potassium bromide, thoroughly grinding the mixture, and then applying a pressure of 18 N for 2 min to form the tablets. The resulting pressed tablets were analyzed using a Fourier transform infrared (FTIR) spectrometer. Scanning was conducted over a spectral range of 400–4000 cm⁻¹, with spectroscopy performed utilizing 200 mg of potassium bromide as the background plate.

Pre‐treatment of fresh L. vannamei

A total of 70 kg of fresh L. vannamei, with a size range of 10–15 shrimps per 500 g, were selected for this study. Following washing, rapid chilling using ice, deheading, and shelling processes, 48 kg of L. vannamei shrimp meat was obtained. The meat was randomly divided into eight groups, each containing 6 kg of shrimp samples from the South American white variety: (i) control group (water immersion treatment); (ii) positive control group: sodium pyrophosphate at a concentration of 15 g L⁻¹; (iii) F at a concentration of 5 g L⁻¹; (iv) F at a concentration of 10 g L⁻¹; (v) F at a concentration of 15 g L⁻¹; (vi) WIF at a concentration of 5 g L⁻¹; (vii) WIF at a concentration of 10 g L⁻¹; (viii) WIF at a concentration of 15 g L⁻¹. The material‐to‐liquid ratio for the samples to the soaking solution was maintained at 1:1 (w/v). After soaking the shrimps in the solution at a temperature of 4 °C for 1 h, they were removed and drained for 2 min before being quick‐frozen. Subsequently, they were packed into polyethylene bags and stored in freezing conditions at −18 °C.

Determination of frozen storage quality of L. vannamei

Effects of WIF on moisture changes of L. vannamei

Determination of thawing loss rate

Frozen L. vannamei were thawed at 4 °C for a duration of 8 h. Following this, the shrimps were removed and drained on gauze for 5 min. The surface moisture was then wiped off using additional gauze. Three parallel groups were established for each treatment condition. The thawing loss rate for each group of samples was calculated using the following formula:

$$\text{Thawing loss rate}\left(\%\right)=\frac{m_1-m_2}{m_1}\times 100\%$$

where m ₁ represents the mass of L. vannamei prior to thawing (g), and m ₂ denotes the mass of L. vannamei after thawing (g).

Determination of cooking loss rate

The frozen shrimps were thawed at 4 °C for 8 h, after which they were removed and drained on gauze for 5 min. The surface moisture was then wiped off using gauze. Subsequently, the thawed shrimps were heated in a steaming oven for 2 min, cooled to room temperature, and again wiped with gauze to eliminate any remaining surface water before being weighed. Three parallel groups were established for each treatment condition. The cooking loss rate of each sample group was calculated using the following formula:

$$\text{Cooking loss rate}\left(\%\right)=\frac{m_2-m_3}{m_2}\times 100\%$$

where m ₂ represents the mass of thawed shrimps (g) and m ₃ denotes the mass of cooked shrimps (g).

Determination of water holding capacity

Frozen white shrimps were thawed at 4 °C for 8 h, subsequently drained on sterilized gauze for 5 min, and surface moisture was gently blotted using the same gauze. Subsequently, the samples were wrapped in gauze and placed in a 50 mL centrifuge tube, followed by centrifugation at 4 °C and 3000 rpm for 15 min. After centrifugation, the samples were carefully removed from the gauze and weighed. Three parallel groups were established for each condition. The water holding capacity (WHC) of each group of samples was calculated using the following formula:

$$\mathrm{WHC}\left(\%\right)=\left(1-\frac{m_4-m_5}{m_4}\right)\times 100\%$$

where m ₄ represents the mass (g) of the thawed L. vannamei sample and m ₅ denotes the mass (g) of the centrifuged L. vannamei.

Determination of whiteness

The whiteness of the L. vannamei samples was assessed using a CR‐400 colorimeter (Konica Minolta, Tokyo, Japan). The second muscle of the shrimp served as the measurement object, which was neatly arranged on a white plate. The L*, a*, and b* values of the samples were recorded with the colorimeter, and the whiteness value ΔW for each sample was calculated using the following formula derived from the collected L* (indicating brightness), a* (representing red–green chromaticity), and b* (denoting yellow–blue chromaticity). The instrument was calibrated against a standard white reference, and three parallel groups were established within each set:

$$\mathrm{\Delta }W=100-{\left[{\left(100-{\mathrm{L}}^{*}\right)}^2+a^{*2}+b^{*2}\right]}^{1/2}$$

Measurement of sensory evaluation

The sensory panel consisted of ten trained individuals (five males and five females) with a background in food science. Prior to the evaluation, panelists underwent 20 h of training in accordance with ISO 8586 to harmonize their descriptive criteria for shrimp quality. All evaluations were conducted in a sensory laboratory compliant with ISO 8589, featuring individual booths and D65 standard lighting. Samples were labeled with random three‐digit codes and served at 40–50 °C. To minimize sensory fatigue, panelists rinsed their palates with distilled water and unsalted crackers between samples. Table 1

Table 1.

Sensory evaluation method for steamed Litopenaeus vannamei

	10–9 points	8–6 points	5–3 points	2–0 points
Color	Shrimp flesh is orange‐red on the back surface and white on the belly	Shrimp flesh is orange on the dorsal surface and slightly white on the belly	Shrimp flesh is light orange on the dorsal surface and slightly yellow in color on the abdomen	Shrimp meat without orange color on the back surface and yellowish color on the abdomen
Odors	Shrimp is full of flavor	Shrimp is light in flavor	Loss of fresh shrimp flavor, slightly fishy or foul odor	The fresh taste of shrimp meat disappears, with obvious fishy or foul odor
Flavor	With distinctive shrimp flavor	Slightly fresh flavor, no other off‐flavors (fishy or metallic)	Slightly fresh flavor, slightly off‐flavor (fishy or metallic)	No fresh taste, very strong odor (fishy or metallic taste)
Taste	Elastic and firm	Better elasticity and firmer meat	Slightly less elastic, slightly less firm flesh	Poor elasticity and firmness

Hematoxylin–eosin staining

The second segmental muscle from the back of shrimp in each treatment group at 0, 30, 60, 90, and 180 days of freezing was selected as the experimental subject for staining. The changes in muscle organization among the samples in each group were observed. The second muscle from the back of the shrimp was fixed using Davidson's fixative (composed of 50 mL of 37% formalin, 75 mL of alcohol, 25 mL of glacial acetic acid, and 75 mL of deionized water) for a duration of 24 h. Subsequently, the tissue was washed with 50% ethanol and dehydrated through a gradient series of ethanol before being paraffin‐embedded. The paraffin‐embedded tissues were then sectioned to observe structural changes in the muscle tissues using an optical microscope at a magnification level of 100×.

Determination of pH

The method described by Abreu et al. ²¹ was employed to determine the pH value of the shrimp samples. A total of 3 g of trimmed shrimp was weighed into a 100 mL centrifuge tube, followed by the addition of 30 mL of deionized water. The mixture was then homogenized thoroughly for 1 min at a speed of 2000 rpm. Subsequently, centrifugation was performed at 5000 rpm and at a temperature of 4 °C for a duration of 15 min. The pH value of the supernatant was measured, which represents the pH value of the shrimp samples.

Extraction of myofibrillar proteins

Minced shrimp samples (5.0 g) were weighed into a 100 mL centrifuge tube and mixed with 10 volumes of Tris–HCl buffer (20 mmol L⁻¹, pH 7.0, containing 0.05 mol L⁻¹ KCl). The mixture was homogenized at 20 000 rpm for 1 min and then centrifuged at 10 000 rpm for 15 min at 4 °C. The supernatant was discarded, and the resulting pellet was mixed with 10 volumes of Tris–HCl buffer (20 mmol L⁻¹, pH 7.0, containing 0.6 mol L⁻¹ KCl). After homogenizing for 1 min, the mixture was extracted at 4 °C for 1 h, followed by centrifugation at 9000 rpm for 10 min at 4 °C. The obtained supernatant was the myofibrillar protein solution. The myofibrillar protein content was determined by the biuret method using bovine serum albumin (BSA) as the standard.

Effects of WIF on protein structure change

Determination of total protein sulfhydryl and carbonyl content and Ca²⁺‐ATPase activity

The activity of Ca²⁺‐ATPase was assessed using the Ultra Trace Ca²⁺‐ATPase Assay Kit. The total sulfhydryl (‐SH) content was measured with the Total Sulfhydryl (—SH) Assay Kit (microplate method). Additionally, the carbonyl content of proteins was evaluated using the Protein Carbonyl Content Assay Kit (assay of serum and tissues) employing an ultraviolet colorimetric method.

Determination of surface hydrophobicity

The fluorescent probe 8‐anilino‐1‐naphthalenesulfonic acid (ANS) was utilized to assess the hydrophobicity of protein surfaces. The supernatant protein concentration was diluted to 0.1, 0.2, 0.3, 0.4, and 0.5 mg mL⁻¹ using a buffer solution of 20 mmol L⁻¹ Tris–HCl (containing 0.6 mol L⁻¹ KCl at pH 7.0). A volume of 4 mL from each of the five concentrations of the supernatant protein solution was aliquoted, followed by the addition of 20 μL of an 8 mmol L⁻¹ ANS solution. The mixture was thoroughly mixed and allowed to stand for 10 min at room temperature. Fluorescence intensity measurements were conducted using a fluorescence photometer under specific conditions: excitation and emission wavelengths were set at 390 nm and 470 nm, respectively, with a gain setting of 50. The slope obtained from plotting fluorescence intensity against protein concentration served as an indicator for evaluating the hydrophobicity of the protein surface.

Effects of WIF on lipid changes of L. vannamei

Determination of the peroxide value of L. vannamei was conducted with reference to the method described by Drinić et al. ²² The TBA value of L. vannamei was determined using the method described by Villasante et al. ²³

Statistical analysis

Experimental data collected from three independent replicates were analyzed using SPSS version 27.0 (IBM Corp., Armonk, NY, USA) for one‐way analysis of variance (ANOVA) followed by Duncan's multiple range test, with statistical significance set at P < 0.05. All graphical representations were generated using Origin 2022 (OriginLab Corp., Northampton, MA, USA), ensuring standardized visualization of experimental results.

RESULTS AND DISCUSSION

Analysis of WIF structure

Analysis of WIF chemical composition

The chemical composition of polysaccharides is closely related to their biological activities, as illustrated in Table 2. Following irradiation at 50 kGy, WIF exhibited an increasing trend compared to F, with the exception of protein content, which showed a decreasing trend. The observed increase in total sugar content may be attributed to the degradation of a small portion of the polysaccharide unit structure and the breaking of glycosidic bonds during this process. ²⁴ This phenomenon is consistent with the changes in total sugar content observed in hot‐water‐degraded fucoidan by Saravana et al. ²⁵ Furthermore, the rise in sulfate group content may result from irradiation enhancing the exposure sites for WIF sulfate groups, thereby providing a protective advantage for reactive groups. Notably, protein content decreased from 0.94% ± 0.02% to 0.24% ± 0.04% post irradiation, indicating that γ‐irradiation effectively removed most proteins present in fucoidan without diminishing polysaccharide levels. After γ‐irradiation, there was a significant increase in reducing sugar content; specifically, it rose from 0.69% ± 0.01% to 6.25% ± 0.36%. This increase can be explained by alterations in glycosidic bonds due to γ‐irradiation that lead to either production of small amounts of reducing sugars or exposure of reducing groups within polysaccharides. ²⁶

Table 2.

Chemical composition of WIF obtained by irradiation‐induced depolymerization

Component	Total sugar content (%)	Sulfate content (%)	Glycuronic acid content (%)	Protein content (%)	Reducing sugar content (%)
F	52.26 ± 0.22a	34.73 ± 1.44a	2.85 ± 0.25a	0.94 ± 0.02a	0.69 ± 0.01a
WIF	54.20 ± 0.57b	40.44 ± 0.29b	4.76 ± 0.10b	0.24 ± 0.04b	6.25 ± 0.36b

Data are expressed as the mean ± standard deviation (n = 3); different lower‐case letters (a, b) in the same column indicate significant differences (P < 0.05). F, native fucoidan; WIF, low‐molecular‐weight fucoidan.

Monosaccharide composition of WIF

In this experiment, the monosaccharide composition of WIF was analyzed, and the results are presented in Table 3. The composition was predominantly fucose, which constituted over 85% of the total amount. This was accompanied by smaller quantities of mannose and glucose, as well as trace amounts of galactose and other monosaccharides. Notably, there were no changes observed in the monosaccharide composition of polysaccharides before and after irradiation. These results are consistent with the findings of Xiong et al., ²⁷ who used γ‐irradiation to degrade the polysaccharides from Morchella fruiting bodies and studied their monosaccharide composition. They found that the monosaccharide composition did not change due to irradiation. In comparison to native fucoidan, both mannose and galactose levels were elevated in low‐molecular‐weight fucoidan. Specifically, the fucose content increased from 84.74% to 86.87% post‐irradiation, indicating an enrichment of fucose within the irradiated product.

Table 3.

Monosaccharide composition of WIF obtained by irradiation‐induced depolymerization

Sample	Monosaccharides (%)
	Man	Rha	Glc A	Gal A	Glc	Gal	Xyl	Fuc
F	6.55	0.42	0.78	1.17	4.64	0.54	1.20	84.74
WIF	6.31	0.47	0.66	0.89	3.59	0.87	0.34	86.87

Glc: Glucose; Gal: Galactose; Man: Mannose; Xyl: Xylose; Fuc: Fucose; Rha: Rhamnose; Glc A: Glucuronic acid; Gal A: Galacturonic acid; F, native fucoidan; WIF, low‐molecular‐weight fucoidan.

Determination of the molecular weights of F and WIF

The molecular weight distributions of fucoidan (F) and low‐molecular‐weight fucoidan (WIF) were determined using the HPGPC‐MALS method. The statistical data for the three peaks of F and the six peaks of WIF are presented in Table 4. Results showed that the relative molecular weights of F ranged from 2.33 × 10⁴ to 6.04 × 10⁵ Da, while those of WIF varied from 4.97 × 10³ to 3.25 × 10⁴ Da. Notably, the molecular weight of WIF obtained after γ‐ray irradiation degradation at a dose of 50 kGy was significantly lower than that of F. The polydispersity indices (M _w/M _n) of fucoidan F were calculated as 1.39 and 1.26. For low‐molecular‐weight fucoidan WIF, the M _w/M _n values were 1.19, 1.04, and 2.22. The polydispersity index M _w/M _n represents the breadth of polymer molecular weight distribution. A larger ratio indicates a broader distribution, while a value closer to 1 indicates a narrower distribution. Compared with F, WIF has a larger polydispersity index, suggesting that irradiation degradation leads to a more dispersed molecular weight distribution in low‐molecular‐weight fucoidan.

Table 4.

Molecular weight of native fucoidan (F) and low‐molecular‐weight fucoidan (WIF)

	F		WIF
	Peak 1	Peak 2	Peak 1	Peak 2	Peak 3
M w (g mol−1)	6.036 × 105 (±0.802%)	2.329 × 104 (±13.373%)	3.251 × 104 (±0.958%)	1.004 × 104 (±2.890%)	4.965 × 103 (±26.534%)
M n (g mol−1)	4.339 × 105 (±1.009%)	1.851 × 104 (±14.274%)	2.733 × 104 (±1.077%)	9.662 × 103 (±3.401%)	2.241 × 103 (±23.212%)
w (%)	83.6	15.4	27.2	42.6	25.7

Infrared spectral analysis of F and WIF

Infrared spectroscopy is one of the most important methods for characterizing substances. It provides extensive information about functional groups, assists in determining various molecular structures, offers insights into monosaccharide compositions and sugar ring conformations, and is widely employed to detect the structural features of polysaccharides. ²⁸

The characteristic peaks of F and WIF are summarized in Fig. 1. It was observed that both F and WIF exhibited O—H vibrations around 3400 cm⁻¹, C—H vibrations around 2945 cm⁻¹, C=O vibrations around 1650 cm⁻¹, and S=O vibrations around 1250 cm⁻¹, as well as C—O—C/C—OH stretching vibrations around 1060 cm⁻¹. These peaks are indicative of polysaccharide characteristics. Additionally, F and WIF displayed absorption peaks near 850 cm⁻¹, suggesting that the sulfate groups in F and WIF were attached to the fucoidan C‐4 position with axially bonded sulfate moieties. Furthermore, compared with F, WIF exhibited increased absorption intensity at approximately 1735 cm⁻¹ (attributed to C=O stretching vibration and ether C—O stretching) and at about 1125 cm⁻¹ (corresponding to C—O—H bending vibration within the pyranose ring). According to the studies by Bratož et al. ²⁹ and Sheppard et al., ³⁰ the characteristic peak of the carboxyl group C=O stretching vibration lies within the range of 1720–1740 cm⁻¹, which is in good agreement with our observations. However, WIF lacked a corresponding C—O stretching vibration on the ring at approximately 1166 cm⁻¹, indicating that γ‐ray irradiation‐induced degradation altered the original structure of fucoidan to some extent.

Figure 1.

Fourier‐transform infrared spectra of native fucoidan (F) and irradiation‐depolymerized low‐molecular‐weight fucoidan (WIF).

Effects of F and WIF on moisture changes of L. vannamei

Effects of F and WIF on thawing loss rate of L. vannamei

During frozen storage, free water in shrimp muscle is converted into ice crystals, whose formation and growth disrupt the integrity of cells and muscle fibers, inflicting irreversible mechanical damage that triggers myofibrillar‐protein denaturation and a consequent loss of functionality, ultimately leading to increased thawing loss. ³¹

As shown in Fig. 2(A), the thawing loss rate of all sample groups increased with prolonged frozen storage. After 180 days, the positive control, F‐supplemented, and WIF‐treated shrimp groups exhibited markedly lower thawing loss than the control group; notably, the positive control and 15 g L⁻¹ WIF groups registered 16.36% and 15.38%, respectively, which were significantly lower than the remaining treatments (P < 0.05), while the difference between these two groups was not statistically significant (P > 0.05). This effect is presumably attributable to the formation of ice crystals from freezable water during freezing, which damages the shrimp microstructure and increases moisture loss. WIF contains a large number of hydroxyl groups. These hydroxyl groups are hygroscopic and can bind water in aquatic products, thereby inhibiting ice crystal formation in L. vannamei. ³² These experimental results are similar to those reported by Kittiphattanabawon et al., ³³ who found that treating frozen L. vannamei with gelatin hydrolysate significantly reduced thawing and cooking losses in the shrimp and inhibited the increase in freezable water content. Guerreiro et al. ³⁴ investigated the cryoprotective potential of fucoidan and found that its shear‐thinning behavior and polyanionic nature facilitate interactions between polysaccharides and the water–ice interface, thereby inhibiting ice crystal growth.

Figure 2.

Effects of F and WIF on moisture changes of Litopenaeus vannamei with freezing storage time: (A) thawing loss; (B) cooking loss; and (C) water holding capacity.

Effects of F and WIF on cooking loss rate of L. vannamei

As shown in Fig. 2(B), the cooking loss rate of all samples increased with prolonged frozen storage. Increasing the concentrations of F and WIF progressively attenuated this rise, indicating a concentration‐dependent inhibitory effect on cooking loss. After 30 days of frozen storage, inter‐group differences became evident; the positive control exhibited a significantly lower cooking loss than the other groups (P < 0.05). Following 180 days of frozen storage, the positive control, 10 g L⁻¹ WIF and 15 g L⁻¹ WIF groups exhibited significantly lower cooking losses than the remaining groups (P < 0.05). This effect is presumably attributable to the ability of carbohydrate cryoprotectants to interact with proteins, thereby enhancing water‐binding capacity and reducing cooking loss. Moreover, the 15 g L⁻¹ WIF group displayed a cooking loss significantly lower than that of both the 15 g L⁻¹ F and 10 g L⁻¹ WIF groups (P < 0.05), whereas no significant difference was observed between the latter two (P > 0.05). These results demonstrate that, at equivalent concentrations, WIF was superior to F in reducing cooking loss. The lower‐molecular‐weight fucoidan (WIF) penetrated more readily between myofibrils, where it associated with the polar residues of proteins and bound water molecules trapped within the myofibrillar lattice, ³⁵ thereby stabilizing protein structure and achieving a superior cryoprotective and water‐retention effects. ³¹

Effects of F and WIF on WHC of L. vannamei

WHC is closely associated with the retention of water‐soluble nutrients and flavor compounds in shrimp meat, significantly impacting its quality. ³⁶ As illustrated in Fig. 2(C), the WHC of muscle tissue across all sample groups exhibited a decreasing trend with prolonged freezing time. Notably, by the 30th day of freezing, significant differences in WHC among the various sample groups emerged (P < 0.05). At 180 days of freezing, the samples from the group supplemented with WIF demonstrated a markedly higher WHC compared with those from other groups (P < 0.05). This enhancement may be attributed to the ability of WIF to form a hydrogel within shrimp muscle, which inhibits ice crystal formation and recrystallization, thereby increasing the overall WHC of the shrimp. Hansen et al. ³⁷ investigated the relationship between ice crystal size and thawing loss under different freezing methods, and confirmed that larger ice crystals lead to higher thawing loss, and that the disruption of muscle microstructure by ice crystals is the primary cause of juice drip loss. These findings are consistent with the conclusions of the present study.

WHC is primarily influenced by the extent of protein denaturation and hydrolysis in meat. ³⁸ The water retention effects of various soaking solutions on L. vannamei were evaluated based on cooking loss, thawing loss, and WHC. Excluding the positive control group, a concentration of 15 g L⁻¹ WIF demonstrated the most effective water retention performance. This treatment significantly mitigated thawing loss, cooking loss, and the reduction in WHC during the freezing and storage periods of L. vannamei. Consequently, this allowed for better preservation of moisture within the shrimp muscle tissues. This observation suggests that 15 g L⁻¹ WIF partially inhibited excessive polymerization of the myosin heavy chain and stabilized the protein structure, thereby achieving an enhanced water retention effect. ³⁹ Furthermore, WIF exhibited a superior protective effect on the WHC of L. vannamei compared with F.

Effects of F and WIF on the whiteness of L. vannamei

As presented in Table 5, the whiteness of frozen L. vannamei samples across all groups exhibited a gradual decline with prolonged freezing time. After 180 days of freezing, the reduction in whiteness for samples in all groups was less pronounced than that observed in the control group, with the exception of the 10 g L⁻¹ F group. Notably, the decrease in whiteness for samples treated with 10 g L⁻¹ WIF and 15 g L⁻¹ WIF was lower than the reduction recorded for the positive control group. These findings suggest that low‐molecular‐weight fucoidan WIF is more effective than fucoidan F in inhibiting the loss of whiteness during freezing and storage of L. vannamei, thereby achieving superior color preservation effects.

Table 5.

Effects of native fucoidan (F) and low‐molecular‐weight fucoidan (WIF) on the whiteness of Litopenaeus vannamei during frozen‐storage duration

Group	0 day	14 days	30 days	60 days	90 days	180 days
Control	86.34 ± 0.79a	83.62 ± 2.57b	82.59 ± 1.68b	81.88 ± 0.85b	82.41 ± 0.35b	81.63 ± 0.26b
Positive control	86.41 ± 0.18a	85.38 ± 0.79a	83.02 ± 1.30b	82.61 ± 0.42b	83.12 ± 0.00b	82.67 ± 0.14b
5 g L−1 F	86.82 ± 0.79a	85.2 ± 0.38b	83.88 ± 0.85c	83.32 ± 0.22cd	82.51 ± 0.19de	82.21 ± 0.57e
10 g L−1 F	86.43 ± 0.50a	84.42 ± 0.66b	82.74 ± 1.58c	82.49 ± 0.06c	82.47 ± 0.02c	81.54 ± 0.88c
15 g L−1 F	85.71 ± 0.84a	84.66 ± 0.22a	82.19 ± 1.43b	82.33 ± 0.43b	82.57 ± 0.53b	81.87 ± 0.57b
5 g L−1 WIF	86.57 ± 0.31a	85.16 ± 0.84b	83.46 ± 0.49c	82.19 ± 0.19d	82.19 ± 0.14d	82.24 ± 0.72d
10 g L−1 WIF	85.46 ± 0.33a	85.62 ± 0.05a	83.57 ± 0.64b	82.79 ± 0.18c	82.68 ± 0.57c	82.60 ± 0.15c
15 g L−1 WIF	85.27 ± 0.83a	84.63 ± 0.35a	83.31 ± 0.38b	83.03 ± 0.43b	83.15 ± 0.02b	82.74 ± 0.31b

Data are expressed as mean ± standard deviation (n = 3); different lower‐case letters (a–d) within the same row indicate significant differences between values (P < 0.05).

Effects of F and WIF on the pH value of L. vannamei

The pH value significantly influences the color, elasticity, WHC, and other quality indicators of shrimp meat, serving as a crucial parameter for evaluating its freshness and overall quality. ⁴⁰ As illustrated in Fig. 3(A), with the extension of freezing time, the pH values across all sample groups exhibited a trend of initial decrease followed by an increase. This phenomenon can be attributed to the early‐stage storage processes where glycogen in aquatic products degrades into acids. Concurrently, ATP and its related compounds also undergo decomposition, leading to the accumulation of phosphoric acid, which results in a reduction of pH levels. ⁴¹ During extended storage, endogenous enzymes and microorganisms inherent in aquatic products actively break down proteins and amino acids into alkaline substances like amines, thereby causing the pH value of aquatic products to rise. Therefore, rising pH levels serve as an indicator reflecting the extent of deterioration in aquatic product quality. ⁴² Notably, a concentration of 10 g L⁻¹ F effectively inhibited the decline in pH during the frozen storage of L. vannamei. In contrast, 15 g L⁻¹ WIF demonstrated superior efficacy in mitigating increases in pH throughout their frozen storage period.

Figure 3.

Influence of frozen‐storage duration on the sensory attributes of Litopenaeus vannamei treated with fucoidan of varying molecular weights and concentrations: (A) pH; (B) overall sensory score; (C,D) color score; (E,F) odor score; (G,H) flavor score; and (I,J) taste score.

Effects of F and WIF on the sensory evaluation of L. vannamei

As illustrated in Fig. 3(B), the total sensory scores of boiled L. vannamei samples across all groups exhibited a decreasing trend, with the scores falling within low‐score range by day 90. After 180 days of freezing, the color of samples from the 10 g L⁻¹ WIF, 15 g L⁻¹ WIF, and positive control groups was significantly superior to that of the other groups (P < 0.05). Additionally, the odor of samples from both the positive control group and the 15 g L⁻¹ WIF group was significantly better than that of the control group (P < 0.05). However, it is noteworthy that the flavor quality of samples from the positive control group surpassed that of the 15 g L⁻¹ WIF group. This difference can be attributed to the brown coloration and distinct seaweed flavor imparted by fucoidan and low‐molecular‐weight fucoidan in their soaking solutions. Considering the impact this has on consumer acceptance, physical adsorption methods may be used for decolorization in the future. For example, when preparing the WIF soaking solution, pretreatment with activated carbon or macroporous resin could be employed. These materials can effectively adsorb large‐molecule pigment impurities while preserving the polysaccharide components that confer freeze resistance. To eliminate odors, masking agents such as cyclodextrin can be added for encapsulation, or extracts from rosemary, ginger, and other sources can be combined to enhance aroma and achieve the goal of odor removal. The taste of the shrimp samples with 15 g L⁻¹ WIF was the best. WIF addition inhibited ice crystal growth and reduced water loss, thereby preventing deterioration in color and elasticity. ⁴³ Furthermore, the antioxidant properties of WIF may also play a crucial role in preventing declines in organoleptic quality associated with lipid oxidation.

Effects of F and WIF on protein denaturation of L. vannamei

Effects of F and WIF on Ca²⁺‐ATPase enzyme activity

Ca²⁺‐ATPase activity reflects the denaturation extent of myosin in muscle, as it specifically characterizes the myosin head region. ⁴⁴ The higher the activity, the lower the degree of actomyosin denaturation, and thus the better the quality of the shrimp. The change in Ca²⁺‐ATPase activity during frozen storage is presumably attributable to the two reactive thiol groups located within its active region; these thiols are readily oxidized to form disulfide bonds, thereby inducing conformational changes that alter the enzyme's activity. ⁴⁵ As illustrated in Fig. 4(A), Ca²⁺‐ATPase activity declined progressively with prolonged frozen storage. After 30 days, inter‐group differences became evident: the 15 g L⁻¹ F group exhibited significantly higher myofibrillar Ca²⁺‐ATPase activity than all other treatments (P < 0.05). By day 180, the 15 g L⁻¹ WIF group retained an activity of 0.0338 U mg⁻¹ prot, which was significantly higher than those of the other groups (P < 0.05) and markedly different from the control (P < 0.01). These results indicate that WIF effectively retarded protein denaturation in frozen shrimp, presumably because the free hydroxyl groups of fucoidan form hydrogen bonds at the protein surface, thereby stabilizing the native conformation. ⁴⁶ WIF influences Ca²⁺‐ATPase activity through two primary mechanisms: direct inhibition and indirect modulation. Direct inhibition occurs when the strong negative charges of the sulfate groups interfere with the enzyme's phosphorylation‐related conformational transitions via electrostatic interactions. ⁴⁷ Indirect modulation involves Ca²⁺ chelation, the calcium‐sensing receptor signaling pathway, and antioxidant effects, through which WIF affects the functional state of Ca²⁺‐ATPase by regulating global cellular calcium metabolism. ⁴⁸ Furthermore, its low‐molecular‐weight conferred higher permeability and enhanced metal ion chelating capacity, thereby improving its regulatory efficiency toward Ca²⁺‐ATPase.

Figure 4.

Effects of freezing duration on proteins in Litopenaeus vannamei treated with fucoidan of different molecular weights and concentrations: (A) Ca²⁺‐ATPase activity; (B) total sulfhydryl content; (C) carbonyl content; and (D) surface hydrophobicity.

Effects of F and WIF on the total hydrophobic content of L. vannamei

Sulfhydryl groups serve as critical structural and functional components of proteins. During the frozen storage of aquatic products, these sulfhydryl groups undergo oxidation to form disulfide bonds, leading to a reduction in total sulfhydryl content. ⁴⁹ The alteration in sulfhydryl content can be indicative of the extent of cysteine oxidation within proteins. ⁵⁰ As illustrated in Fig. 4(B), the total sulfhydryl content of myofibrillar proteins across all sample groups exhibited a decreasing trend with extended freezing duration. On day 30 of frozen storage, significant differences in total thiol content emerged among the groups. The control group had significantly lower total sulfhydryl content than the others (P < 0.05). In contrast, the positive control and 15 g L⁻¹ WIF groups exhibited significantly higher total thiol content than the rest (P < 0.05). By day 90 of freezing, there was a marked decline in total sulfhydryl content across all sample groups. After 180 days of frozen storage, the total sulfhydryl contents for samples from the positive control group and both WIF treatment concentrations (10 and 15 g L⁻¹) were measured at 30.64, 29.99, and 30.06 μmol g⁻¹ prot respectively – values that were significantly greater than those of the other groups (P < 0.05). These findings suggest that medium‐ and high‐concentration WIF immersion treatments effectively inhibited the oxidation of myofibrillar protein's sulfhydryl groups into disulfide bonds during freezing storage periods, thus contributing positively to mitigating protein oxidation reactions. Zhang et al. ⁵¹ reported that treatment of frozen shrimp with carrageenan oligosaccharides significantly reduced the formation of protein carbonyls and dityrosine, maintained higher total sulfhydryl content, and effectively inhibited the degradation and oxidation of myofibrillar proteins. These findings are consistent with the results of the present study.

Effects of F and WIF on total carbonyl content of L. vannamei

The carbonyl content is a critical indicator of protein oxidation. ⁵² As illustrated in Fig. 4(C), the initial myofibrillar protein carbonyl content across all sample groups ranged from 0.1076 to 0.1283 nmol mg⁻¹ prot. With extended freezing duration, the myofibrillar protein carbonyl content exhibited an upward trend in each group. On day 60 of frozen storage, significant differences in myofibrillar protein carbonyl content emerged among the groups. The positive control and 15 g L⁻¹ WIF groups showed significantly lower myofibrillar protein carbonyl content than the other groups (P < 0.05). After 180 days of freezing, a marked increase to 0.7917 nmol mg⁻¹ prot was observed in the carbonyl content of the control group, which was significantly higher than that of the 15 g L⁻¹ WIF group (P < 0.05). No significant differences were found regarding the carbonyl content among myofibrillar proteins from samples belonging to the positive control, 15 g L⁻¹ F, and 10 g L⁻¹ WIF groups (P > 0.05). This finding suggests that both 15 g L⁻¹ WIF and combinations of treatments at concentrations of either 15 g L⁻¹ F or 10 g L⁻¹ WIF effectively inhibited the carbonylation of myofibrillar proteins during freezing and storage processes for L. vannamei.

Effects of F and WIF on surface hydrophobicity of L. vannamei

Surface hydrophobicity reflects the extent of exposure of hydrophobic groups in proteins; oxidation of amino acid residues and unfolding of peptide chains can trigger this exposure, leading to an increase in surface hydrophobicity. ⁵³ As shown in Fig. 4(D), surface hydrophobicity in all groups increased with extended frozen storage. On day 30 of frozen storage, significant differences emerged among the groups, where the 15 g L⁻¹ WIF group exhibited significantly lower surface hydrophobicity than the other groups (P < 0.05). On day 180 of frozen storage, the surface hydrophobicity of the 15 g L⁻¹ WIF group was 61.35, which was also significantly lower than that of the other treatment groups (P < 0.05). This can be attributed to the strong hydrophilic interactions between WIF and myofibrillar proteins, which effectively inhibit the exposure of hydrophobic amino acid residues and prevent protein aggregation. ⁵⁴

By measuring the Ca²⁺‐ATPase activity, total sulfhydryl content, carbonyl content, and surface hydrophobicity of myofibrillar proteins from L. vannamei, the results indicated that soaking treatment with 15 g L⁻¹ WIF effectively inhibited the denaturation of myosin, sulfhydryl oxidation, protein carbonylation, and the exposure of hydrophobic amino acid residues, thereby achieving a significant protective effect against protein denaturation.

Effects of F and WIF on lipid changes in L. vannamei

Effects of F and WIF on the peroxide value

The peroxide value serves as an indicator of the content of peroxides, which are the primary oxidation products of lipids. ⁵⁵ As illustrated in Fig. 5(A), the initial peroxide values for each group ranged from 0.2208 to 0.2221 mmol kg⁻¹. Notably, the peroxide values across all sample groups exhibited an increasing trend with prolonged freezing time. This phenomenon may be attributed to the gradual enlargement and increased quantity of ice crystals within the shrimp during extended freezing periods. These ice crystals can deform or even rupture shrimp cells by exerting pressure on them, leading to cellular damage and subsequent lipid spillage that accelerates lipid oxidation. ⁵⁶ The same trend was also observed in Yan et al.'s research. ⁵⁷ On day 180 of frozen storage, the peroxide values of the 15 g L⁻¹ F and 15 g L⁻¹ WIF groups were significantly lower than those of the positive control group (P < 0.05). The peroxide values of these two groups were 0.6257 and 0.6256 mmol kg⁻¹, respectively, with no significant difference between them (P > 0.05). This indicates that high‐dose F and WIF soaking treatments effectively inhibited the formation of lipid peroxides in L. vannamei. Moreover, as the doses of F and WIF increased, their inhibitory effects on lipid peroxide generation in L. vannamei were enhanced.

Figure 5.

Effects of F and WIF on lipid changes in Litopenaeus vannamei during frozen storage time: (A) peroxide content; and (B) thiobarbituric acid (TBA).

Effects of F and WIF on TBA values

The TBA value serves as an index for evaluating the extent of lipid oxidation and is utilized to quantify malondialdehyde (MDA) derived from hydroperoxides, which are the primary products resulting from the reaction between polyunsaturated fatty acids and oxygen. ⁵⁸ As illustrated in Fig. 5(B), the initial MDA content across all groups ranged from 0.1148 to 0.1211 mg kg⁻¹. The MDA levels in each group exhibited an increasing trend with prolonged freezing time, with the least increase observed in the 15 g L⁻¹ WIF group, followed by moderate increase in the 15 g L⁻¹ F and positive control group. On day 180 of frozen storage, the 15 g L⁻¹ F and 15 g L⁻¹ WIF groups had significantly lower MDA content than the other groups (P < 0.05), at 0.1656 and 0.1605 mg kg⁻¹, respectively. Moreover, the difference in MDA content between these two groups was not significant (P > 0.05). This finding indicates that both high‐concentration F and WIF treatments effectively inhibited lipid peroxide production in L. vannamei samples, corroborating previous results.

Hydrogen atoms located near double bonds in fatty acids are abstracted during the initial stages of lipid oxidation. Metal ions facilitate the formation of free radicals, which subsequently react with oxygen to generate peroxyl radicals. These peroxyl radicals can then interact with other lipid molecules, leading to the production of hydroperoxides. ⁵⁹ The hydroxyl groups present in fucoidan (F) and low‐molecular‐weight fucoidan (WIF) serve as hydrogen donors, reducing the likelihood of oxygen radical generation. This mechanism effectively inhibits the formation of hydroperoxides resulting from the oxidation of polyunsaturated fatty acids, ultimately contributing to a decrease in MDA levels.

Correlation analysis

Correlation analysis is a statistical approach used to explore linear relationships among variables, thereby revealing potential associations and informing the formulation of new scientific hypotheses and future research directions. ⁶⁰ In this study, Pearson correlations among thawing loss (TL), cooking loss (CL), WHC, Ca²⁺‐ATPase activity, total sulfhydryl content, carbonyl content, surface hydrophobicity (H ₀), peroxide value (POV), and TBA‐reactive substances were calculated for WIF‐treated frozen L. vannamei (Fig. 6) to elucidate interrelationships among these indices.

Figure 6.

Pearson correlation analysis, employed to quantify the relationship between protein oxidative denaturation and lipid oxidation in WIF‐treated frozen Litopenaeus vannamei. TL, thawing loss; CL, cooking loss; WHC, water holding capacity; H0, surface hydrophobicity; POV, peroxide value; TBA, thiobarbituric acid.

Regarding correlations among lipid oxidation indices, POV and TBA exhibited a strong positive correlation (r = 0.98, P < 0.05). For myofibrillar‐protein oxidation indices, oxidative damage to the tertiary structure led to exposure of buried hydrophobic residues and consequently elevated surface hydrophobicity. ⁶¹ Thus, carbonyl content was positively correlated with surface hydrophobicity (r = 0.98, P < 0.05). Carbonyl content was strongly and negatively correlated with sulfhydryl content (r = −0.91, P < 0.05), reflecting the simultaneous increase in oxidative carbonylation and the oxidation of sulfhydryl (—SH) groups to disulfide (—S—S—) bonds during protein oxidation. ⁶² With respect to the relationship between lipid and myofibrillar‐protein oxidation, POV was positively correlated with surface hydrophobicity, carbonyl content and TBA, yielding correlation coefficients of 0.98, 0.97, and 0.98, respectively. Conversely, POV exhibited negative correlations with sulfhydryl content, WHC, and Ca²⁺‐ATPase activity (r = −0.93, –0.68, and–0.89, respectively). POV was strongly and positively correlated with carbonyl content (r = 0.97, P < 0.05). Oxidative damage to the protein structure exposed buried aromatic residues to the molecular surface, amplifying hydrophobic interactions; concurrently, it triggered the release of additional oxidants within muscle cells, accelerating lipid oxidation and ultimately compromising seafood quality. ⁶³ These findings indicate that lipid and protein oxidation are mutually reinforcing rather than independent processes.

CONCLUSIONS

This study confirmed that low‐molecular‐weight fucoidan (WIF) exhibits significant freeze‐protective and water‐retaining effects on frozen L. vannamei. Following degradation by 50 kGy γ‐ray irradiation, the molecular weight of fucoidan decreased significantly and the glycosidic bonds were altered, resulting in the extensive exposure of active sites such as sulfate groups and hydroxyl groups. These structural changes allow WIF to penetrate more easily into the interstices of myofibrils. During the frozen storage of shrimp meat, WIF forms dual bonds with polar residues within protein molecules and water molecules through its exposed active groups, effectively stabilizing protein structures and inhibiting ice crystal growth, thereby reducing cellular structural damage and juice loss; simultaneously, its abundant hydroxyl groups act as hydrogen donors to effectively block free radical generation, significantly inhibiting lipid oxidation. Consequently, WIF demonstrates superior performance to undegraded high‐molecular‐weight fucoidan (F) in both WHC and resistance to protein and lipid oxidation. Moreover, its protective efficacy is positively correlated with treatment concentration, conferring superior low‐temperature protection properties to WIF. However, the use of WIF alone in practical applications remains limited by economic costs and its single‐function nature. Future research should focus on developing WIF‐based composite anti‐freezing and water‐retaining agents and exploring their synergistic application with physical preservation technologies. Additionally, a systematic evaluation is needed to determine the structure–activity relationship between the degree of fucoidan degradation (molecular weight variation) under different irradiation doses and its freeze‐protection properties, with the aim of optimizing the preservation of aquatic product quality during low‐temperature storage.

CONFLICT OF INTEREST

The authors declare no conflicts of interest.

DATA AVAILABILITY STATEMENT

Data sharing not applicable to this article as no datasets were generated or analysed during the current study.