Effect of low-temperature vacuum cooking (sous vide, SV) on the nutritional and flavor quality of small yellow croaker (Larimichthys polyactis) soup

Abstract

This study investigated the effects of low-temperature vacuum cooking (sous vide, SV) on the nutritional and flavor quality of small yellow croaker soup (SYCS) Initially increasing the temperature and extending the duration can improve sensory scores, but heating at 90 °C for 300 min significantly degrades sensory quality. Regarding color parameters, b^⁎ shows a decrease, whereas the a^⁎ value exhibits a trend of initial decrease followed by an increase (p < 0.05). Temperature and extended time cause soluble proteins to increase initially and then decrease, evidenced by increased total volatile bases nitrogen (TVB-N) and thiobarbituric acid reactive substances (TBARS) values, indicating lipid oxidation and protein breakdown. The most balanced and intense umami perception, along with high nucleotide content, was achieved at 80 °C for 180 min. Although short-time treatment at 100 °C 20 min best preserved amino acids and polyunsaturated fatty acids (PUFAs), the concurrent release of bitter compounds likely compromised its overall sensory umami. Volatile organic compounds (VOCs) analysis revealed temperature-dependent flavor profiles, 70 °C 240 min enhanced roasted notes, while 80 °C imparted a fresh aroma. GC-IMS elucidated formation mechanisms, indicating that flavor derives from degradation and co-oxidation of key components. These findings offer a foundational reference for understanding and regulating quality attributes in small yellow croaker soup, with implications that may extend to other aquatic products following further validation.

Highlights

• •Heating at 75–80 °C for 180 min optimizes flavor, nutrition, and safety.
• •Heating at 70 °C for 240 min reduces nucleotides and weakens umami.
• •Free and essential amino acids increased with heating temperature.
• •Heating at 100 °C for 20 min reduces volatiles, yielding a relatively monotonous flavor profile.

1. Introduction

The small yellow croaker (Larimichthys polyactis) is a marine fish of significant economic value. It is renowned for its tender flesh and pleasant flavor, which makes it highly popular among consumers (Zhong et al., 2025). Its exquisite flavor makes it a crucial ingredient in soup-based dishes, particularly in seafood broths, where it offers irreplaceable advantages. Therefore, selecting appropriate raw materials and employing suitable processing techniques to preserve its unique flavor and nutritional benefits are critical for enhancing the quality of soup products. The flesh of small yellow croaker, characterized by its tenderness and delightful taste, primarily relies on its natural flavor compounds, such as amino acids, peptides, fatty acids, minerals, and volatile aroma substances (Huangfu et al., 2024; Lv et al., 2025). Notably, elements such as calcium, phosphorus, and iron are essential for bone health, blood function, and the normal operation of the nervous system. Furthermore, small yellow croaker soup (SYCS) is rich in Omega-3 fatty acids (n-3 FAs), particularly eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), which positively influence cardiovascular and brain health. These unsaturated fatty acids help regulate blood lipids, lower cholesterol levels, and reduce the risk of heart disease. Additionally, the water-soluble vitamins (e.g., B-group vitamins) and fat-soluble vitamin D in fish soup play a key role in energy metabolism, skin health, and immune function (Lin, Tao, Su, Zhang, & Zhong, 2020). During the cooking process, these compounds are effectively released into the broth, creating a rich seafood flavor that enhances the taste and complexity of the soup.

However, small yellow croaker presents certain challenges due to its delicate flesh, which may lose its desirable taste when subjected to high temperatures during cooking. Moreover, because of its high proportion of unsaturated fatty acids, improper processing may result in fishy odors or an unpleasant taste (He et al., 2022). Additionally, the natural nutrients in small yellow croaker are susceptible to degradation during improper heating, which can affect the overall quality of the soup (Tian et al., 2024). To overcome these challenges, low-temperature vacuum cooking (sous vide, SV) has emerged as an ideal processing method. SV allows for the processing of ingredients at lower temperatures, maximizing the retention of the fish's nutritional components and flavor compounds (Yang et al., 2024). Compared to traditional high-temperature boiling methods, SV not only reduces the degradation of proteins, fatty acids, and volatile compounds but also effectively diminishes the fishy taste, preserving a fresher and more appealing flavor (Shen et al., 2022). For water-soluble vitamins and other sensitive nutrients, SV can effectively maintain their activity, avoiding the nutrient loss or flavor degradation typically associated with high-temperature processing. Moreover, the vacuum environment during the SV process helps minimize oxidation reactions, extending the shelf life and stability of the soup. Therefore, using small yellow croaker as a base ingredient for soups, combined with SV cooking, not only preserves its natural flavor and nutritional content to the greatest extent, but also enhances the sensory quality and consumer experience of the soup. As consumer demand for healthy and natural foods continues to rise, SV offers vast potential for innovation in SYCS, becoming a key method to improve its quality, extend shelf life, and meet the needs of modern consumers.

2. Materials and methods

2.1. Materials and reagents

Small yellow croaker fish (approximately 60.00 g ± 5.00 g) were purchased from Zhoushan International Fisheries City (Zhoushan, China). Fresh, evenly sized fish samples were placed in a foam box containing ice packs and transported to the laboratory within 30 min.

Upon arrival, all fish were subjected to strict sensory evaluation to ensure consistent and high initial freshness. Only fish meeting the following criteria were selected for the experiment: bright and clear eyes, bright red gills without mucus, shiny and tightly adherent scales, firm elastic flesh that springs back upon pressing, and a clean, fresh sea odor without any off-flavors. This screening process was conducted in accordance with the sensory criteria for fresh marine fish as outlined in the Chinese National Standard for Fresh and Frozen Aquatic Products (GB 2733–2015). By implementing this standardized sensory screening, we aimed to minimize variations in the initial quality of the raw materials, thereby ensuring that the observed effects in the study could be more confidently attributed to the experimental cooking parameters rather than to pre-existing differences in fish freshness.

The Coomassie Brilliant Blue assay kit was purchased from Nanjing Jiancheng Technology Co., Ltd. (Wuhan, China). Sodium hydrogen phosphate, anhydrous ethanol, magnesium oxide, boric acid, sodium chloride, sulfuric acid, sodium hydroxide, petroleum ether, diethyl ether, n-hexane, boron trifluoride methanol, indole-3-acetic acid, potassium hydroxide, hydrochloric acid, and dichloromethane were all purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China).

2.2. Experimental methods

The fresh small yellow croaker fish were headless, tailless, finless, and gutted. After thorough washing, surface moisture was removed using absorbent paper. The fish were then minced using a meat grinder to obtain fish. Each 20.00 g portion of fish was vacuum-sealed and evacuated for 45 s to a vacuum level of −0.1 MPa (gauge pressure). The vacuum-sealed fish was first subjected to a mild preheating at 45 °C for 25 min. This step aimed to promote initial enzymatic activity and protein denaturation, facilitating the subsequent release of flavor compounds. It was then followed by the main vacuum low-temperature cooking process in a water bath at 50 °C, 60 °C, 70 °C, 80 °C, and 90 °C for 60 min, 120 min, 180 min, 240 min, and 300 min, respectively. After heating, the package was carefully opened, and the exuded liquid released from the fish matrix was collected and defined as the SYCS for subsequent analysis. A control group was prepared by heating vacuum-sealed fish at 100 °C for 20 min, after which basic parameter measurements were conducted.

The SV SYCS was prepared using the method described above. Samples of the soup heated at 70 °C for 240 min, 80 °C for 180 min, and 100 °C for 20 min were analyzed for amino acids, nucleotides, fatty acids, and volatile organic compounds (VOCs).

All sample measurements were completed within 24 h. The raw materials were from the same batch to ensure consistency and comparability of the experimental data.

2.3. Sensory evaluation

The sensory evaluation was conducted following the method described by Dong et al., 2024, with slight modifications. The sensory evaluation panel consisted of 10 assessors, aged 20–25 years, with an equal distribution of 5 males and 5 females. The panel members underwent a 2-week training on seafood sensory evaluation standards (GB/T 37062–2018). Sensory assessments, including color, odor, mouthfeel, taste, organization, and acceptability, were performed at room temperature (25 ± 1 °C). The average score of the sensory evaluation was calculated. A 10-point scale was used, with sensory attributes rated as excellent, good, fair, or poor, corresponding to scores of 9–10, 6–8, 4–5, and 1–3, respectively, to evaluate the sensory characteristics of SV SYCS under different conditions. The sensory standards are outlined in Table S1.

This study design does not require ethical approval. The privacy rights of all human subjects participating in sensory evaluations were respected, and informed consent was obtained prior to the experiment.

2.4. Color

The color values of the samples, including L^⁎ (lightness), a^⁎ (red/green), and b^⁎ (yellow/blue), were measured at room temperature using a colorimeter (CR-10 Plus, Konica Corporation, Tokyo, Japan).

2.5. pH

1.00 g of SV SYCS was added to 10 mL of distilled water and homogenized for 5 min at 10,000 rpm using a homogenizer (FSH-2, Shanghai LeiQi Instrument Equipment Co., Ltd., Shanghai, China). The pH was then measured using a pH meter (FE-28, Mettler-Toledo International Co., Ltd., Columbus, Ohio).

2.6. Soluble protein content

The soluble protein content in SV SYCS was determined using the Coomassie Brilliant Blue assay kit (Nanjing Jiancheng Bioengineering Institute, Nanjing, China).

2.7. Total volatile bases nitrogen (TVB-N)

The TVB-N concentration was determined using a Kjeldahl nitrogen analyzer (K9860, Haineng Instrument Co., Ltd., Jinan, China). A 5.00 g sample of SV SYCS was weighed and mixed with 75 mL of distilled water. Subsequently, 2.00 g of magnesium oxide (MgO) was added, and the mixture was subjected to distillation. The liberated ammonia vapors were absorbed in a boric acid solution and then titrated with standardized 0.1 mol/L hydrochloric acid. Based on the acid-base neutralization reaction, the TVB-N concentration was accurately calculated.

2.8. Thiobarbituric acid reactive substances (TBARS)

The TBARS value was determined according to the Chinese National Standard GB 5009.181—2016 (National food safety standard—Determination of malondialdehyde in foods).

2.9. Determination of free amino acids

The method described by Li et al. (2025) was followed with slight modifications. Free amino acids in the SV SYCS were determined using a high-performance amino acid analyzer (LA80080, Hitachi Ltd., Tokyo, Japan). A 5.00 g sample of SV SYCS was mixed with 5 mL of 10% perchloric acid and stirred for 5 min. The mixture was then homogenized and centrifuged at 10,000 xg for 10 min. The supernatant was filtered through a 0.22 μm membrane, and 1 mL of the filtrate was transferred to a sample vial for analysis.

2.10. Determination of nucleotide

The nucleotide content in SV SYCS was determined using a slightly modified method based on Liu et al. (2024). Precisely 5.00 g of the sample was weighed and mixed with 10 mL of 10% perchloric acid solution. After centrifugation (H1850, Rongbi Instrument Technology Co., Ltd., Shenzhen, China) at 10,000 rpm for 15 min, the supernatant was collected. The residue was washed twice with 5% perchloric acid and centrifuged again each time. The combined supernatants were adjusted to pH 6.5 using potassium hydroxide (KOH) solution, allowed to stand for 30 min, and then diluted to a final volume of 50 mL. The solution was filtered through a 0.45 μm membrane and stored at 0–4 °C until analysis.

Analysis was performed using an Agilent 1260 high-performance liquid chromatography (HPLC) system (Agilent Technologies, Santa Clara, USA) under the following conditions: flow rate of 1 mL/min, column temperature of 28 °C, and injection volume of 10 μL. The mobile phase consisted of a mixture of 0.05 mol/L potassium phosphate buffer (K₂HPO₄/KH₂PO₄, phase A) and methanol (phase B) in a volume ratio of 95:5 (A:B). Detection was carried out at a wavelength of 254 nm.

2.11. Determination of fatty acids

A 5 g sample, mixed with pyrogallic acid, pumice, and ethanol, was hydrolyzed with HCl at 70–80 °C for 40 min. After cooling, the hydrolyzate was extracted three times with a diethyl ether-petroleum ether mixture. The combined ether extract was evaporated and dried. Prior to methylation, an internal standard (C17:0 methyl ester, 0.5 mg/mL in hexane, Sigma-Aldrich) was accurately added to the fat residue for quantification. The fat residue was methylated with 2% NaOH in methanol at 85 °C for 30 min, followed by 14% BF₃ in methanol for another 30 min. After cooling, n-hexane was added, and the upper layer was collected. A 100 μL aliquot was diluted to 1 mL with n-hexane, filtered, and analyzed by GC-FID (Agilent 7890 A, Agilent Technologies, Santa Clara, USA) (TG-FAME column; 50 m × 0.25 mm × 0.20 μm). The detailed GC conditions were as follows: carrier gas (nitrogen) at a constant flow rate of 0.63 mL/min; injector temperature, 250 °C; detector (FID) temperature, 260 °C; hydrogen flow, 40 mL/min; air flow, 400 mL/min; split ratio, 50:1; injection volume, 1 μL. The temperature program was: initial temperature 140 °C, held for 5 min; increased to 200 °C at 4 °C/min and held for 1 min; then increased to 230 °C at 3 °C/min and held for 15 min. Quantification was achieved using a 37-component FAME mix standard (Supelco) to establish calibration curves (R² > 0.995 for all quantified acids). using a programmed temperature gradient and nitrogen carrier gas at 0.63 mL/min.

2.12. Determination of VOCs

According to the method outlined by Chu, Wang, and Xie (2024), 1.00 g of SV SYCS was weighed and placed in a 20 mL headspace vial for analysis using the FlavourSpec® flavor analyzer. The analysis duration was set for 30 min. A MXT-WAX column (30 m in length, 0.53 mm inner diameter, and 1 μm film thickness) was employed. The carrier/drift gas was high-purity N₂ (99.999%) with the following optimized flow settings: carrier gas flow was ramped from an initial 2 mL/min for 2 min to a final 15 mL/min; drift gas flow was maintained at 150 mL/min. The column temperature was maintained at 60 °C (isothermal, suitable for the rapid separation on this system), and the drift gas used was N₂. The IMS (Ion Mobility Spectrometry) was operated with a radioactive tritium (³H) ionization source. The drift tube voltage was 400 V, corresponding to an electric field strength of 500 V/cm. The IMS temperature was set to 45 °C. A sample volume of 500 μL was injected, with an incubation time of 30 min at 40 °C. The injection needle temperature was maintained at 85 °C. Compound identification was performed by comparing both the GC retention index (RI, calibrated with a C4-C9 n-ketone mixture) and the IMS drift time (Dt) with those of authentic standards (Sigma-Aldrich) and the built-in NIST/IMS and G.A.S. libraries. The data were processed using the Laboratory Analytical Viewer (LAV) and GCxIMS Library Search software.

2.13. Data analysis

For each unique temperature-time combination, three independent batches of soup were prepared as biological replicates (n = 3). All parameters were measured in triplicate, and the average values were used for subsequent analysis. The experimental data were analyzed for significance using SPSS (version 25, IBM, USA). Significant differences among groups were evaluated using one-way ANOVA followed by Duncan's multiple range test for post-hoc comparisons. Differences are indicated by different superscript letters (a, b, c…), where values sharing the same letter within a row are not significantly different (p ≥ 0.05). Data processing and visualization were performed using Origin (2021, OriginLab, USA). The experimental results are presented as “mean ± standard deviation.”

3. Results and discussion

3.1. Sensory analysis

Sensory evaluation is the most direct indicator for assessing the quality of processed products. As shown in Fig. 1, with the increase in heating temperature and extension of heating time, the sensory scores for SV SYCS, including color, odor, mouthfeel, taste, organization, and acceptability, show an upward trend (p < 0.05). This change is closely related to the protein and fat content in the soup. Prolonged cooking promotes the interaction between proteins, phospholipids, and fat particles, thereby enhancing the soup's color. The release of fat particles results in an increasing fat content in the soup, which further improves the mouthfeel and flavor of the SV SYCS. As the cooking time extends, more proteins, fats, nucleotides, and umami amino acids dissolve into the soup, enriching its taste and intensifying its aroma. The SV SYCS heated at 70 °C for 240 min and at 80 °C for 180 min exhibited the highest sensory scores. Statistical analysis revealed that the SV SYCS heated at 70 °C for 240 min and at 80 °C for 180 min exhibited the highest sensory scores, which were significantly higher (p < 0.05) than those of groups with shorter heating times or lower temperatures. This suggests that these heating conditions favor the retention of flavor compounds, facilitate protein hydrolysis, further break down muscle fibers, and release more flavor compounds, thereby enhancing the overall flavor of the soup (Yue et al., 2025). However, as the heating temperature and time continued to increase beyond these optimal points, the sensory scores of the SV SYCS began to decline significantly (p < 0.05). Notably, the sensory score of the group heated at 90 °C for 300 min was significantly lower (p < 0.05) than all other groups, which may be due to the increased content of bitter amino acids, leading to a pronounced decline in the soup's taste. Overall, higher heating temperatures and longer cooking times exacerbate the Maillard reaction, resulting in an increase in floating oil in the soup, thereby affecting its sensory evaluation. This is consistent with the findings of Yan et al. (2024), which suggest that high temperatures contribute to the formation of more colorless intermediates and dark-colored substances, increase the consumption of glucose and proteins, and promote the occurrence of the Maillard reaction.

Fig. 1.

Sensory evaluation of SYCS under different SV conditions.

Note: A: SV heating temperature is 50 °C, B: SV heating temperature is 60 °C, C: SV heating temperature is 70 °C, D: SV heating temperature is 80 °C, E: SV heating temperature is 90 °C, F: SV heating temperature is 100 °C.

3.2. Color analysis

As shown in Fig. 2, this study investigated the effects of cooking temperature and duration on the lightness (L^⁎), yellowness (b^⁎), and redness (a^⁎) of SV SYCS. The L^⁎ value of the soup gradually decreased with increasing cooking temperature and time. This phenomenon may be attributed to the elevated fat content in the broth. During the cooking process, fragments of fish skin, bones, and muscle tissue were released into the soup, leading to the continuous dissolution and dispersion of proteins, lipids, and nucleic acids. These compositional changes ultimately reduced the light transmittance of the soup. Meanwhile, the collagen in fish flesh undergoes thermal degradation into gelatin molecules during heating. These gelatin components interact with emulsifying proteins (Li et al., 2022), contributing to the whitish appearance of the soup. In SV SYCS, the b^⁎ value exhibited a positive correlation with increasing cooking temperature and duration. This chromatic shift primarily originates from the oxidation of unsaturated fatty acids and the Maillard reaction. Oxidation-derived reactive species interact with amino groups from proteins and peptides, facilitating the formation of yellowish pigments. Concurrently, the a^⁎ value showed a non-linear trend, initially decreasing slightly before a gradual increase. This pattern is likely governed by the competitive pathways of thermal degradation of initial color compounds and the formation of reddish-brown polymers via advanced Maillard reactions and lipid-protein interactions. Prolonged heating thus promotes these cumulative browning reactions, leading to a shift towards darker yellow-brown hues, as reflected in the increased b^⁎ value and the dynamic change in a^⁎.

Fig. 2.

Changes in L^⁎ value, b^⁎ value, and a^⁎ value of SYCS under different SV conditions.

Note: Different uppercase letters indicate significant differences between different temperatures at the same time (p < 0.05); different lowercase letters indicate significant differences between different times at the same temperature (p < 0.05).

3.3. pH value analysis

As illustrated in Fig. 3, SV SYCS exhibits acidic characteristics with increasing cooking temperature and duration, manifested by an initial pH decline followed by a subsequent recovery. This phenomenon is likely attributable to the increased leaching of soluble compounds during thermal processing. As cooking temperature and duration increase, a greater proportion of constituents—including acidic compounds such as acidic amino acids and organic acids—dissolve into the soup. The release of these substances induces a progressive decline in pH, particularly during prolonged heating. Concurrently, the gradual depletion of dissolved oxygen promotes glycogen breakdown in the fish flesh, generating lactic acid that further elevates broth acidity and amplifies the pH reduction. This trend aligns with the findings of Lekjing, Venkatachalam, and Wangbenmad (2021), who reported analogous pH dynamics in sea bass soup subjected to extended boiling. With prolonged cooking time and elevated temperature, the release of acidic compounds in SV SYCS stabilizes, which may contribute to the subsequent pH recovery. Prolonged cooking not only accelerates the release of certain acidic compounds but may also induce their transformation or stabilization. These processes inhibit further dissolution of acidic substances, thereby facilitating pH recovery. Conversely, elevated temperatures during cooking may alter the molecular structure or reaction kinetics of specific components, consequently modifying their solubility and the release patterns of acidic compounds. At shorter durations and lower temperatures, acidic compound release predominates, leading to pH reduction. However, with extended heating at higher temperatures, the release of acidic substances stabilizes or diminishes, resulting in pH rebound (Li et al., 2025). This phenomenon underscores the intricate interplay among temperature, duration, and chemical reactions in the soup, while providing a theoretical foundation for optimizing SV SYCS processing parameters.

Fig. 3.

Change in pH value of SYCS under different SV conditions.

Note: Different uppercase letters indicate significant differences between different temperatures at the same time (p < 0.05); different lowercase letters indicate significant differences between different times at the same temperature (p < 0.05).

3.4. Soluble protein content analysis

As shown in Fig. 4, during the initial stage of steaming, the soluble protein content increased gradually. This trend can be attributed to enhanced molecular motion and accelerated kinetics, which promoted the release of proteins from SV SYCS. At this stage, the fish muscle retained a tightly packed structure. As steaming temperature increased and heating duration extended, the protein structure became disrupted, leading to progressive loosening of the muscle tissue. This structural breakdown facilitated the dissolution of nitrogenous compounds and soluble substances, resulting in a significant rise in crude protein content in the SV SYCS (Chen et al., 2024). However, when the steaming temperature reached 90 °C and was maintained for 300 min, a pronounced decline in soluble protein content occurred. This reduction likely stems from protein degradation under prolonged high-temperature and high-pressure conditions. Excessive thermal processing induced protein denaturation and degradation in the small yellow croaker, thereby reducing crude protein content. Furthermore, hydrophobic interactions triggered protein aggregation and precipitation, further diminishing soluble protein levels (Wu et al., 2024). This trend aligns with the findings of (Nie et al., 2025), who observed a similar pattern of initial increase followed by subsequent decrease in soluble protein content during pressurized steam cooking of silver carp soup. In summary, the effects of temperature and duration during the steaming process on soluble protein content demonstrated a complex trend, exhibiting both promotive and inhibitory effects that were likely associated with physicochemical transformations of the proteins.

Fig. 4.

Change in soluble protein content of SYCS under different SV conditions.

Note: Different uppercase letters indicate significant differences between different temperatures at the same time (p < 0.05); different lowercase letters indicate significant differences between different times at the same temperature (p < 0.05).

3.5. TVB-N value analysis

TVB-N refers to volatile basic nitrogenous compounds (including ammonia, amines, and other alkaline nitrogen-containing substances) produced during protein decomposition in animal products. This biochemical process occurs through the action of enzymes and spoilage bacteria, which break down proteins via extracellular enzymatic activity. As an objective indicator of bacterial proliferation and spoilage progression, TVB-N content serves as a widely adopted metric for evaluating the quality and safety of aquatic products and other animal-derived foods. According to national food safety standards, the maximum permissible TVB-N content is 300 mg/kg to ensure product freshness and consumer safety.

As illustrated in Fig. 5, TVB-N values demonstrated a progressive increase with elevated steaming temperatures and extended processing durations. This trend results from thermally accelerated protein degradation in fish tissue, leading to enhanced release of volatile nitrogenous compounds. Notably, more pronounced TVB-N increases were observed in SV SYCS subjected to higher temperatures and longer cooking times. These findings suggest that prolonged thermal processing, particularly under high-temperature conditions, accelerates protein breakdown through intensified bacterial activity and enzymatic reactions (Lee et al., 2019). The thermal denaturation of proteins and disruption of cellular structures in small yellow croaker further contribute to TVB-N liberation. As proteolysis advances, accumulating concentrations of ammonia and amine compounds drive the continuous rise in TVB-N content (Lee, Tsai, Hwang, Lin, & Huang, 2022). This phenomenon underscores the delicate balance between thermal processing parameters and food quality preservation.

Fig. 5.

Change in TVB-N value of SYCS under different SV conditions.

Note: Different uppercase letters indicate significant differences between different temperatures at the same time (p < 0.05); different lowercase letters indicate significant differences between different times at the same temperature (p < 0.05).

3.6. TBARS value analysis

Malondialdehyde (MDA), a secondary product generated during the oxidation of polyunsaturated fatty acids, primarily forms through the decomposition of unstable hydroperoxides. As a crucial biomarker of lipid oxidation, MDA content is typically quantified using the TBARS assay. The TBARS value serves as a reliable indicator of fat oxidation levels in food products and is therefore widely employed for assessing food quality deterioration. According to Chinese national standards, the MDA content in aquatic products and their derivatives must not exceed 30 mg/kg to ensure food safety and nutritional value.

Our investigation revealed significant variations in TBARS values of SV processed SV SYCS under different temperature-time combinations. As demonstrated in Fig. 6, elevated heating temperatures accelerated lipid oxidation rates, while prolonged heating durations progressively increased oxidation kinetics, resulting in enhanced MDA generation and consequently higher TBARS values (Hu, Zhang, & Mujumdar, 2022). These findings clearly indicate that cooking parameters substantially influence lipid oxidation processes in croaker soup. The TBARS progression exhibited strong correlation with quality deterioration in SV SYCS. Maximum TBARS levels were recorded at the extreme processing condition (90 °C for 300 min), a result consistent with sensory evaluation outcomes indicating poorest consumer acceptability at these parameters (Das et al., 2023). This thermal extreme not only promoted excessive lipid oxidation but also induced detrimental modifications in soup flavor profiles and textural characteristics, ultimately compromising sensory appeal.

Fig. 6.

Change in TBARS of SYCS under different SV conditions.

Note: Different uppercase letters indicate significant differences between different temperatures at the same time (p < 0.05); different lowercase letters indicate significant differences between different times at the same temperature (p < 0.05).

3.7. Analysis of free amino acids in SV SYCS

As essential flavor compounds and precursors in aquatic products, amino acids play a crucial role in the development of the taste of SV SYCS (Zhu et al., 2021). As shown in Table 1, the changes in free amino acid content in SV SYCS under different temperature and time conditions were analyzed. The results indicate that the free amino acid content gradually changes with increasing heating temperature and duration. These alterations may be attributed to multiple factors, including the thermal stability of proteins, the degradation and transformation of amino acids, and the influence of different treatment methods on the flavor compounds in the SV SYCS.

Table 1.

Free amino acid content of SV SYCS (mg/100 g) Data are presented as mean ± standard deviation (n = 3). Values within the same row with different superscript letters are significantly different (p < 0.05) as determined by one-way ANOVA followed by Duncan's multiple range test. Same below.

Free amino acid	Contents
	70 °C 240 min	80 °C 180 min	100 °C 20 min
P-Ser	15.18 ± 0.57b	18.57 ± 0.61a	20.29 ± 1.35a
Tau	16.01 ± 0.23c	18.13 ± 0.78b	20.12 ± 1.14a
※Asp	5.95 ± 0.14b	6.69 ± 0.27a	7.88 ± 0.58a
*Thr	5.04 ± 0.14c	7.66 ± 0.14b	8.65 ± 0.33a
Ser	4.34 ± 0.12c	6.43 ± 0.16b	7.34 ± 0.51a
※Glu	4.89 ± 0.22b	7.94 ± 0.79a	8.59 ± 0.23a
Sar	12.93 ± 0.14b	15.00 ± 0.71a	15.00 ± 0.77a
Gly	4.18 ± 0.45b	5.70 ± 0.19a	5.86 ± 0.12a
Ala	5.90 ± 0.1b	6.62 ± 0.03a	6.64 ± 0.04a
*Val	12.88 ± 0.20b	14.04 ± 0.24a	13.98 ± 0.21a
Cys	18.23 ± 0.20b	18.66 ± 0.41b	56.31 ± 0.17a
*Met	12.27 ± 0.03c	13.01 ± 0.09b	19.73 ± 0.26a
*Ile	19.40 ± 0.46c	22.13 ± 1.07b	24.29 ± 0.2a
*Leu	19.33 ± 0.22c	21.77 ± 0.99b	23.66 ± 0.17a
Tyr	14.32 ± 0.10c	15.74 ± 0.83b	22.85 ± 0.62a
*Phe	19.62 ± 0.23b	25.12 ± 1.17a	17.86 ± 0.76a
ΣTAA	194.60 ± 2.03c	223.23 ± 7.39b	248.16 ± 4.73a
ΣEAA	66.4 ± 0.90c	103.74 ± 3.22b	118.86 ± 0.79a
ΣNEAA	66.75 ± 1.11c	104.48 ± 3.14b	114.30 ± 3.26a
ΣDAA	8.13 ± 0.09c	14.63 ± 1.02b	16.47 ± 0.73a
ΣDAA/ΣTAA	0.040 ± 0.29a	0.06 ± 0.00a	0.06 ± 0.00a
ΣEAA/ΣTAA	0.34 ± 0.00b	0.45 ± 0.00b	0.46 ± 0.06a
ΣEAA/ΣNEAA	0.99 ± 0.00b	1.00 ± 0.00b	1.04 ± 0.23a

Note: *Essential amino acids, ※Flavor amino acids.

First, the total free amino acid content in the SV SYCS showed a gradual increase with rising heating temperature and duration. Specifically, in the control group heated at 100 °C for 20 min, the total amino acid content (ΣTAA) reached 248.16 mg/100 g, which was significantly higher than that in the 70 °C 240 min group (194.60 mg/100 g) and the 80 °C 120 min group (223.23 mg/100 g). This phenomenon may be attributed to the more thorough hydrolysis of proteins in the SV SYCS under high temperature, leading to the release of more free amino acids. Hydrolysis refers to the process where proteins break down into smaller amino acid molecules under high-temperature conditions. Generally, as temperature increases, the rate of hydrolysis accelerates, resulting in a greater release of free amino acids (Nie et al., 2025). In contrast, although low-temperature sous-vide cooking at 70 °C and 80 °C helps preserve certain flavor compounds and nutritional components, the relatively lower heating temperatures result in slower amino acid hydrolysis and fewer free amino acids released. Particularly under the 70 °C 240 min condition, the total free amino acid content remained relatively low (194.60 mg/100 g), and changes in some individual free amino acids were moderate. These results indicate that higher temperatures are not always more favorable for the release of all amino acids. Excessively low temperatures may fail to fully hydrolyze proteins, thereby affecting both the amount of amino acids released and the composition of the amino acid profile. Moreover, notable differences were observed in the content of certain essential amino acids. For instance, the total essential amino acids (ΣEAA) in the 70 °C 240 min group was 66.4 mg/100 g, while that in the 100 °C 20 min group was 118.86 mg/100 g. This discrepancy suggests that high-temperature treatment significantly enhances the release of essential amino acids. This is closely related to the increased degree of protein hydrolysis, where elevated temperatures promote the cleavage of peptide bonds in proteins, releasing more free amino acid molecules—particularly essential amino acids (Chen et al., 2024).

Secondly, the changes in individual free amino acids revealed that the contents of certain essential and non-essential amino acids varied significantly under different heating conditions. For instance, in the sample heated at 100 °C for 20 min, the levels of amino acids such as taurine (Tau) and phosphoserine (P-Ser) were notably higher. This suggests that high-temperature heating not only promotes protein hydrolysis but may also facilitate the synthesis or transformation of specific amino acids. Such transformations could involve the activity of endogenous enzymes in the fish, which may participate in specific amino acid synthesis or metabolic pathways under appropriately elevated temperatures.

When analyzing changes in umami amino acids, the experimental results indicated that the total content of umami amino acids (ΣDAA) also differed among treatment groups, with the proportion increasing alongside temperature. The 100 °C for 20 min group exhibited the highest umami amino acid content (16.47 mg/100 g), which is associated with the release of flavor compounds in the SV SYCS under high-temperature conditions. High-temperature heating likely accelerates the formation or release of certain umami substances, thereby enhancing the overall flavor of the broth (Tu et al., 2025). Heat treatment significantly altered the balance characteristics of the free amino acid composition. Notably, the essential amino acid to non-essential amino acid ratio (ΣEAA/ΣNEAA) in the 100 °C short-time (20 min) treatment group (1.04 mg/100 g) was particularly prominent, demonstrating a significant advantage over other temperature groups. This difference fully reflects the regulatory effect of heating conditions on the composition of protein-derived amino acids.

3.8. Analysis of nucleotides in SV SYCS

As shown in Table 2, the nucleotide content in SV SYCS under different heating conditions was determined. The results indicate that both heating temperature and duration significantly influenced the nucleotide levels. Specifically, the total nucleotide content in the 80 °C 180 min group was markedly higher than that in the other groups, suggesting that elevated temperatures promote an increase in nucleotide concentration. Nucleotides are degradation products of proteins and other biomacromolecules. During heating, cells and proteins in the SV SYCS undergo a certain degree of hydrolysis. This hydrolytic process often triggers the breakdown of RNA and DNA, leading to the release of nucleotides. Higher temperatures accelerate protein degradation, resulting in more pronounced nucleotide release at 80 °C and 100 °C. Similar findings have been reported in studies on duck soup (Wang et al., 2024). In contrast, although prolonged heating at 70 °C for 240 min may facilitate hydrolysis to some extent, the relatively low temperature results in a milder hydrolytic effect compared to the 80 °C treatment. Consequently, the nucleotide content released under this condition was comparatively lower. It is worth noting that when adenosine monophosphate (5′-AMP) interacts with glycine (Gly), it can produce a distinctive umami taste and a rich mouthfeel. This observation is consistent with the amino acid measurement results, which showed the lowest Gly content in the fish soup heated at 70 °C for 240 min.

Table 2.

Nucleotide content of SV SYCS (mg/100 g).

Temperature Time	5’-GMP	5’-IMP	5’-AMP	Total Amount
100 °C 20 min	12.0 ± 1.78b	30.08 ± 3.39b	102.1 ± 10.04c	148.29
70 °C 240 min	10.05 ± 0.19b	33.12 ± 5.57b	95.11 ± 4.57b	138.28
80 °C 180 min	17.46 ± 0.55a	50.64 ± 5.91a	92.57 ± 0.56a	160.67

The distribution of nucleotides also varied under different heating conditions. In the control group heated at 100 °C for 20 min, the content of 5′-AMP was the highest (102.1 mg/100 g), while that of guanosine monophosphate (5′-GMP) was relatively low (12.0 mg/100 g). This suggests that higher temperatures may promote the formation of 5′-AMP, likely due to enhanced hydrolysis of ATP under high-temperature conditions. In contrast, the groups treated at 70 °C for 240 min and 80 °C for 180 min showed relatively higher levels of inosine monophosphate (5′-IMP). Specifically, the 80 °C 180 min group exhibited a 5′-IMP content of 50.64 mg/100 g, which was significantly higher than that in the other two groups. This difference may be attributed to the varying effects of temperature on RNA degradation processes. More thorough degradation of RNA under elevated temperatures likely facilitates increased generation of 5′-IMP.

The variation in nucleotide content may also be related to the activity of endogenous enzymes in fish muscle. Fish typically contain certain enzymes, such as adenylate deaminase and guanylate deaminase, which may remain active during heating and further promote the hydrolysis of high-energy compounds like ATP, leading to the generation of various nucleotides. At elevated temperatures, the activity of these enzymes may be enhanced or accelerated, resulting in increased nucleotide release. Changes in nucleotide content are also directly associated with the development of flavor in fish. Nucleotides are key contributors to the “umami” taste in foods, particularly 5′-GMP and 5′-IMP, which significantly enhance the savory flavor (Jiang et al., 2024). In this experiment, the 80 °C 180 min group exhibited the highest nucleotide content, suggesting that it may have an advantage in terms of flavor enhancement. Heating temperature and duration significantly influenced nucleotide release. When the temperature reached 80 °C, the contents of umami nucleotides such as 5′-GMP and 5′-IMP increased markedly. This can be attributed primarily to heat-induced protein denaturation and enhanced RNase activity, which collectively promote the release of nucleotides.

3.9. Analysis of fatty acids in SV SYCS

As shown in Table 3, the content of various fatty acids increased with rising temperature. Statistical analysis indicated that the total saturated fatty acid (ΣSFA) content in the 100 °C 20 min group (2609.73 mg/100 g) was significantly higher (p < 0.05) than those in the 70 °C and 80 °C groups. This phenomenon can be attributed to the rupture of fish cell membranes under high temperature, which facilitates the release of intracellular fatty acids. Additionally, high temperatures may promote the hydrolysis of lipids, leading to an increase in certain saturated fatty acids (SFAs), such as C16:0 and C18:0.

Table 3.

Fatty acid content of SV SYCS (mg/100 g).

Fatty acids	Contents
	70 °C 240 min	80 °C 180 min	100 °C 20 min
C12:0	4.35 ± 1.11a	3.86 ± 0.46a	4.02 ± 0.18a
C13:0	4.35 ± 3.77a	2.07 ± 0.32a	2.02 ± 0.68a
C14:0	245.93 ± 24.21a	237.54 ± 14.81a	268.29 ± 11.83a
C14:1	4.69 ± 0.46a	4.69 ± 0.37a	4.97 ± 0.37a
C15:0	44.69 ± 7.10a	40.46 ± 2.43a	44.19 ± 0.73a
C16:0	1761.36 ± 160.22a	1728.40 ± 89.93a	1855.27 ± 109.24a
C16:1	763.32 ± 71.42a	762.42 ± 45.51a	787.34 ± 50.25a
C17:0	29.95 ± 7.96a	32.91 ± 2.27a	35.19 ± 4.68a
C18:0	305.87 ± 27.94a	299.62 ± 16.79a	332.41 ± 21.00a
C18:1n9c	1599.60 ± 145.82a	1536.04 ± 61.34a	1626.49 ± 28.49a
C18:2n6c	110.13 ± 11.29a	101.39 ± 3.94a	114.83 ± 10.99a
C20:0	29.52 ± 9.91a	23.23 ± 1.45a	25.82 ± 5.33a
C18:3n6	11.18 ± 11.83a	4.58 ± 0.31a	8.62 ± 6.88a
C18:3n3	61.54 ± 5.30a	56.25 ± 10.22a	59.79 ± 8.00a
C20:1	135.81 ± 16.82a	138.65 ± 9.96	140.27 ± 6.62a
C21:0	8.26 ± 5.62a	4.97 ± 0.47a	8.03 ± 3.53a
C20:2	30.71 ± 1.46a	23.20 ± 1.46a	24.91 ± 1.97a
C22:0	17.67 ± 4.98a	17.53 ± 5.96a	16.65 ± 0.95a
C20:3n6	8.61 ± 6.08a	8.73 ± 6.59a	7.09 ± 2.07a
C20:3n3	11.49 ± 3.27a	11.51 ± 1.84a	16.10 ± 4.12a
C20:4n6	256.15 ± 22.49a	251.91 ± 19.91a	253.01 ± 20.42a
C22:1n9	36.45 ± 2.62a	38.25 ± 5.52a	42.19 ± 9.45a
C23:0	4.97 ± 0.46a	5.06 ± 0.30a	5.44 ± 0.25a
C22:2	3.32 ± 5.14a	4.97 ± 4.64a	4.64 ± 3.77a
C24:0	12.17 ± 1.13a	16.52 ± 7.65a	12.40 ± 0.5a
C24:1	66.25 ± 8.83a	61.69 ± 2.31a	65.06 ± 9.56a
C20:5n3(EPA)	425.14 ± 41.70a	424.20 ± 39.12a	481.78 ± 11.95a
C22:6n3(DHA)	966.88 ± 147.72a	994.03 ± 108.44a	1085.20 ± 84.81a
DHA/EPA	2.27 ± 0.29a	2.34 ± 0.81a	2.25 ± 0.12a
EPA + DHA	1392.02 ± 290.56a	1418.23 ± 146.36a	1566.98 ± 96.46a
ΣSFA	2469.09 ± 163.59a	2412.16 ± 109.65a	2609.73 ± 144.71a
ΣMUFA	2606.13 ± 98.79a	2541.74 ± 45.42a	2666.32 ± 77.34a
ΣPUFA	1885.16 ± 235.13a	1880.77 ± 165.04a	2055.98 ± 126.09a
Σn-3 PUFA	1465.05 ± 194.36a	1486.98 ± 138.69a	1642.88 ± 94.27a
Σn-6 PUFA	416.78 ± 51.56a	389.81 ± 25.76a	408.46 ± 29.29a

Note: SFA: saturated fatty acid; MUFA: monounsaturated fatty acid; PUFA: polyunsaturated fatty acid.

Meanwhile, the content of monounsaturated fatty acids (MUFAs) also exhibited a consistent trend across different temperature conditions. Notably, the level of C18:1n9c under 100 °C treatment was 1626.49 mg/100 g, which was significantly higher (p < 0.05) than that in all other groups. The high stability of oleic acid allows it to withstand thermal processing without significant degradation. As an important energy source, oleic acid also plays a crucial role in enhancing the flavor and nutritional value of the soup (Wu et al., 2024). From the perspective of polyunsaturated fatty acids (PUFAs), especially n-3 and Omega-6 fatty acids, their variations under different treatments are of considerable importance. The total n-3 PUFA (Σn-3 PUFA) and total PUFA (ΣPUFA) contents were numerically highest in the 100 °C 20 min group (1642.87 and 2055.98 mg/100 g, respectively), showing a clear preservation trend. However, compared to some other groups (e.g., 80 °C-180 min), these differences did not always reach statistical significance (p ≥ 0.05). The indicating a considerable retention of EPA and DHA naturally abundant in small yellow croaker. Notably, the DHA content after 100 °C treatment was 1085.20 mg/100 g, accounting for a relatively high proportion. This suggests that high-temperature short-time cooking can be effective in preserving these cardiovascular-beneficial fatty acids, although the absolute quantitative advantage over all other conditions requires careful interpretation based on the specific comparison. Furthermore, the DHA/EPA ratio remained relatively stable across treatments, which is beneficial for nutrient absorption and cardiovascular protection. When comparing the overall contents of n-6 and n-3 PUFAs, the total n-6 PUFA (Σn-6 PUFA) values under all four heating conditions were lower than those of Σn-3 PUFA. This result aligns with the innate nutritional composition of small yellow croaker. Moreover, a balanced n-3/n-6 ratio helps alleviate inflammatory responses and enhance immune function (Zhao et al., 2025). In summary, while the 100 °C 20 min treatment consistently yielded the highest numerical values for key fatty acid categories (ΣSFA, C18:1n9c, Σn-3 PUFA, ΣPUFA), with some differences being statistically significant, it provided the most favorable conditions for the preservation of fatty acids overall, supporting the conclusion drawn in the abstract.

3.10. Analysis of variations in volatile organic compound species in SV SYCS

3.10.1. Analysis of VOCs in SV SYCS by GC-IMS

Fig. 7(A) and (B) present the top view and difference map of the sample obtained by GC-IMS, respectively. As shown in Fig. 7(B), using the SV SYCS heated at 70 °C for 240 min as a reference, the red regions indicate higher concentrations of volatile compounds, while the blue areas correspond to lower concentrations. Comparative analysis revealed statistically significant differences in the overall volatile compound profiles among groups. Specifically, the SV SYCS heated at 80 °C for 180 min exhibited a significantly more abundant (p < 0.05) content of a range of volatile flavor compounds compared to that heated at 70 °C for 240 min. These results suggest that the heating condition of 80 °C for 180 min promotes a more effective release of volatile flavor substances in the SV SYCS, thereby enhancing the overall flavor profile of the product.

Fig. 7.

A: GC-IMS spectrum of SV SYCS sample (Top view) B: GC-IMS spectrum of SV SYCS sample (Difference view). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

3.10.2. Analysis of volatile organic compound fingerprints in SV SYCS

As shown in Fig. 8, the VOC fingerprint profiles of the SV SYCS were determined under three heating conditions: 70 °C for 240 min, 80 °C for 180 min, and the control group at 100 °C for 20 min.

Fig. 8.

Fingerprints of VOCs in SV SYCS.

The red-framed region indicates compounds with significantly higher concentrations (p < 0.05) in the sample heated at 70 °C for 240 min. Notably, substances such as 1-propanal, 2-aminopentanol, ethanol, propanol, n-butanol, 1-penten-3-ol, 2-butanone, ethyl acetate, 2-pentanone, trans-3-hexenol, and trans-2-hexenol were significantly more abundant in this group, while their contents were the lowest in the control group heated at 100 °C for 20 min. The prolonged heating at a lower temperature (70 °C) facilitates the gradual accumulation and formation of certain VOCs, whereas high-temperature short-time treatment promotes rapid release and evaporation of volatiles, preventing substantial accumulation. These results suggest that lipid oxidation and amino acid degradation occur more slowly at 70 °C, gradually generating flavor-related compounds such as 1-propanal and 2-aminopentanol. As a product of lipid oxidation, 1-propanal accumulates progressively under extended mild heating but volatilizes quickly under high-temperature conditions, leading to significantly lower final concentrations in the control group (p < 0.05). Furthermore, the decomposition and transformation of fatty acids and esters at 70 °C also contribute to the accumulation of volatile compounds. Alcohols (e.g., ethanol, propanol, n-butanol) and ketones (e.g., 2-butanone, 2-pentanone) derived from fatty acid breakdown are common volatile flavor substances that impart distinct aromatic and taste characteristics to the soup. Compounds such as 1-penten-3-ol and ethyl acetate provide floral, fruity, and spicy notes, enhancing the complexity and layered perception of the flavor profile. Due to the relatively slow conversion of esters, alcohols, and ketones at 70 °C, extended heating promotes the progressive buildup of these compounds, resulting in a more intricate flavor composition. In contrast, although the 100 °C for 20 min treatment involves high temperatures, the short duration limits the accumulation of many volatile substances, which instead undergo rapid volatilization or degradation, ultimately leading to significantly lower concentrations (p < 0.05) of these key flavor compounds.

The green-framed region in the figure indicates compounds with significantly higher concentrations (p < 0.05) in the SV SYCS heated at 100 °C for 20 min. Substances such as methylacetyl carbinol, acetaldehyde, propionaldehyde, nonanal, acetic acid, 2-methyl-1-propanol, and ethyl butyrate were significantly more abundant in this group, while their contents were significantly lower (p < 0.05) in the sample heated at 70 °C for 240 min. These results suggest that higher temperatures promote thermal degradation and chemical reactions, leading to increased formation of certain VOCs (e.g., acetaldehyde and propionaldehyde). In contrast, although the low-temperature treatment involved prolonged heating, it was less effective in facilitating the release or generation of these volatiles. Acetaldehyde and propionaldehyde are common aldehydes known for their pungent and fresh aroma qualities. In terms of flavor, acetaldehyde contributes a fresh, green note to the soup, while propionaldehyde is associated with the “cooked” aroma characteristic developed during heating (Zamora, Alcon, & Hidalgo, 2023). The increase in these aldehydes is generally correlated with the decomposition of lipids and proteins, particularly at elevated temperatures, where their formation is enhanced. The higher concentrations observed in the control group (100 °C, 20 min) are likely due to accelerated breakdown reactions under rapid high-temperature processing. Nonanal, a long-chain aldehyde with a distinct “oily” note, possesses strong aromatic and sensory properties. It is a well-known product of lipid oxidation (Zhao et al., 2025). During heating, especially at higher temperatures, lipid oxidation becomes more pronounced, resulting in increased generation of nonanal. Consequently, samples heated at 80 °C or above tend to produce significantly more nonanal (p < 0.05) compared to the 70 °C group, enriching the overall taste profile of the soup. Acetic acid, a typical sour volatile in foods, provides a mildly acidic taste and is often associated with tangy flavor characteristics. Ethyl butyrate, on the other hand, imparts a fruity and sweet aroma (Zhao et al., 2025). The presence of both compounds enhances the complexity and layering of the soup's flavor profile. Their concentrations are generally significantly higher under elevated temperatures (e.g., 80 °C or 100 °C) compared to prolonged low-temperature heating (p < 0.05), contributing to greater flavor diversity and sensory depth.

Through analysis of the VOC fingerprint profiles of SV SYCS under different heating conditions, it was found that both heating temperature and duration significantly influence the release, transformation, and degradation of volatile compounds, with statistically significant differences observed among groups (p < 0.05). In the sample heated at 70 °C for 240 min, higher levels of compounds such as 1-propanal, 2-aminopentanol, and ethanol were observed. This can be attributed to the prolonged heating, which facilitates gradual lipid oxidation and amino acid degradation, thereby enhancing the roasted aroma, umami perception, and overall complexity of the soup. In contrast, samples heated at 80 °C exhibited significantly elevated concentrations (p < 0.05) of substances such as acetaldehyde, propionaldehyde, and nonanal, reflecting intensified thermal degradation and chemical reactions under higher temperatures. These compounds contribute fresh green notes, cooked aroma characteristics, and a richer mouthfeel to the soup. In summary, extended heating at 70 °C promotes the accumulation of flavor compounds, resulting in a more rounded and developed flavor profile. Meanwhile, higher temperatures, such as 80 °C, enhance the formation of a significantly different set of volatile substances (p < 0.05), adding further complexity and sensory depth to the soup.

3.10.3. Analysis of principal component and “nearest neighbor” fingerprint of VOCs in SV SYCS

Principal Component Analysis (PCA) effectively reveals differences and similarities among sample data. As shown in Fig. 9 (A), significant differences are observed between the SV SYCS heated at 70 °C for 240 min and 80 °C for 180 min when compared to the control group heated at 100 °C for 20 min. This indicates that the composition and quantity of VOCs in the soup vary considerably under different heating conditions. Fig. 9 (B) illustrates that under the 70 °C 240 min condition, the relatively low temperature and prolonged heating duration lead to a slower release of VOCs. This may allow more thorough degradation or transformation of certain volatile components, resulting in a chemical fingerprint distinctly different from that of the 100 °C 20 min sample. In contrast, under the 80 °C 180 min condition, the higher temperature facilitates a more rapid release of VOCs, while the relatively shorter heating time helps retain volatile compounds effectively and prevents excessive degradation. This combination of temperature and duration results in a higher concentration of volatile flavor substances in the soup. Consequently, the chemical fingerprint of the 80 °C 180 min sample shows greater similarity to that of the 100 °C 20 min group compared to the 70 °C sample. These findings highlight the critical role of heating temperature and time in the release and preservation of volatile compounds, providing important insights for optimizing the processing conditions of SV SYCS.

Fig. 9.

A: PCA plot of SV SYCS B: Fingerprint analysis of the “nearest neighbor” of the SV SYCS.

3.11. Correlation analysis of free amino acids, nucleotides, fatty acids, and flavor compounds

As shown in Fig. 10 (A), (B) and (C), the volatile flavor profiles of SV SYCS samples—heated at 70 °C for 240 min, 80 °C for 180 min, and 100 °C for 20 min, respectively—were analyzed using GC-IMS technology. Combined with compositional data on amino acids, nucleotides, and fatty acids, the results systematically reveal the chemical basis and interaction mechanisms underlying flavor formation. From a metabolic perspective, the degradation, oxidation, and synergistic reactions of amino acids, nucleotides, and fatty acids serve as key drivers of flavor development in the fish soup. The aldehydes, ketones, alcohols, and esters detected via GC-IMS are closely associated with the transformation of these precursor compounds.

Fig. 10.

A, B, and C respectively represent the correlation analysis heatmaps of free amino acids, nucleotides, fatty acids, and GC-IMS flavor compounds in SV SYCS heated at 70 °C for 240 min, 80 °C for 180 min, and 100 °C for 20 min.

Note: The colors in the heatmap represent the correlation coefficients, with red indicating a positive correlation, blue indicating a negative correlation, and the intensity of the color corresponding to the strength of the correlation. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

At the amino acid level, aspartic acid (Asp) and glutamic acid (Glu), as umami-enhancing amino acids, contribute to the formation of various volatile compounds through Maillard reaction and Strecker degradation pathways. For instance, under high temperatures, Glu reacts with reducing sugars to generate furans such as 2-pentylfuran, which—along with hexanal and 2-heptanone detected via GC-IMS—enhances the roasted aroma of the fish soup. The grassy note of hexanal and the sweet aroma of 2-heptanone may originate from deamination and decarboxylation of Glu, while degradation of Asp can yield propionaldehyde and its derivatives. These correspond to propionaldehyde peaks in the GC-IMS spectrum and contribute pungent olfactory characteristics to the soup. Furthermore, tyrosine (Tyr) undergoes decarboxylation during thermal processing to form p-cresol, which can be further oxidized into phenolic derivatives. These compounds, along with nonanal—generated through fatty acid oxidation—collectively enhance the lipid-like aroma, illustrating the cross-talk between amino acid and fatty acid metabolic pathways (Gui et al., 2026). Leucine (Leu) and isoleucine (Ile), through Strecker degradation, produce 3-methylbutanal and 2-methylbutanal, corresponding to butanal and 2-methyl-1-propanol in GC-IMS profiles. The former imparts a nutty aroma, while the latter, in synergy with ethanol, helps mitigate undesirable fishy odors. The decomposition of threonine (Thr) and P-Ser may indirectly influence acetaldehyde formation through sulfur-containing precursors. The sharp odor of acetaldehyde is reflected in distinct GC-IMS peaks, and its concentration variations across heating conditions likely reflect differences in the degradation rates of specific amino acids (Yang et al., 2024).

Although nucleotides and their derivatives do not directly generate volatile flavor compounds, their metabolic pathways play a non-negligible synergistic role with amino acids and fatty acids in flavor formation. For example, Tau may influence the generation of sulfur-containing compounds—such as methanethiol—through the regulation of sulfur metabolism. Although such compounds were not detected by GC-IMS due to limitations in detection sensitivity, their precursors may interact with oxidation products of acetaldehyde to modulate the intensity of fishy odors. Degradation products of 5′-AMP and related nucleotides, such as hypoxanthine, may indirectly promote the formation of pyrazines by enhancing the efficiency of the Maillard reaction. These compounds are potentially linked to 3-hydroxy-2-butanone (detected via GC-IMS), which exhibits a creamy aroma and may help balance the oily sensation of the fish soup (Guan et al., 2025). Furthermore, the dephosphorylation of P-Ser may release serine, which can subsequently participate in ethanolamine metabolism and influence the formation of 1-pentanol and 1-butanol. Both alcohols show distinct peaks in GC-IMS spectra and contribute mild, wine-like aromas that may mask undesirable odors associated with fish processing.

The oxidation and thermal degradation of fatty acids serve as a core source of flavor diversity in fish soup, with the autoxidation of polyunsaturated fatty acids (PUFAs) significantly influencing the composition of aldehydes and ketones. Specifically, DHA and EPA, as major PUFAs, undergo β-oxidation during heating to form hexanal, trans-2-hexenal, and nonanal. Hexanal contributes a grassy note, while trans-2-hexenal imparts a fruity aroma—both compounds showed high response values in GC-IMS analysis and were more abundant in samples treated at 80 °C, indicating that elevated temperatures accelerate the oxidation of DHA and EPA. The decomposition pathway of hydroperoxides derived from C18:2n6 (linoleic acid) yields 2-pentanone and 1-penten-3-ol. The former exhibits a sweet aroma, and the latter carries a mushroom-like odor. Together with 2-heptanone detected via GC-IMS, these compounds contribute to the complex aromatic profile of the fish soup (Yan et al., 2025). Additionally, the thermal degradation of saturated fatty acids (SFAs) such as behenic acid (C22:0) may generate heptanal and pentanal, which correspond to their respective peaks in GC-IMS and present a blend of oily and green odor characteristics. Their concentrations were higher in samples subjected to prolonged heating at 70 °C, likely due to the gradual release and breakdown of saturated fatty acids under mild thermal conditions (Bi et al., 2025). Moreover, oxidation products of nervonic acid (C24:1) may participate in the formation of ethyl butyrate, which exhibits a fruity aroma in GC-IMS and complements the fresh, sweet notes of the fish soup.

Different heating conditions significantly influenced the correlations described above. Heating at 70 °C for 240 min favored the gradual degradation of amino acids and slow oxidation of fatty acids, leading to the accumulation of compounds such as hexanal, nonanal, and propionaldehyde. This resulted in a mild flavor profile characterized by roasted and lipid-like notes. In contrast, heating at 80 °C for 180 min accelerated the β-oxidation of PUFAs and Strecker degradation, increasing the concentrations of trans-2-hexenal, 2-heptanone, and acetone, and yielding a more intense fruity and sweet aroma. Furthermore, the differences in ethanol and acetic acid levels between the two treatments may be related to microbial metabolism or Maillard reaction byproducts. Variations in their concentrations further influenced the acidity of the fish soup and the stability of its volatile components (Xu, Zheng, Zeng, Tian, & Xu, 2025; Yang et al., 2025). In summary, the flavor characteristics of SV SYCS arise from the synergistic interactions of amino acids, nucleotides, and fatty acids through a complex metabolic network. GC-IMS technology effectively revealed the dynamic relationships between different precursor compounds and volatile flavor substances, providing a theoretical basis for the targeted regulation of flavor in aquatic products.

4. Conclusion

This study investigates the effects of low-temperature vacuum cooking (sous vide, SV) on the nutritional and flavor quality of small yellow croaker soup (SYCS). Results show that heating temperature and duration significantly influence product quality. Heating at 60–80 °C for 120–180 min yielded color, odor, mouthfeel, taste, organization, and acceptability. In contrast, extending heating to 300 min at 90 °C led to notable quality deterioration, as reflected by reduced sensory scores. With increased heating intensity, the soup's brightness (L^⁎ value) decreased, yellowness (b^⁎ value) intensified, and redness (a^⁎ value) decreased initially then increased. Soluble protein content rose with temperature and time but declined after 300 min at 90 °C due to protein degradation. Total volatile bases nitrogen (TVB-N) and thiobarbituric acid reactive substances (TBARS) values increased under more intense heating, peaking in the 90 °C 300 min group, indicating intensified lipid oxidation and protein breakdown. The highest nucleotide content (160.67 mg/100 g) was observed at 80 °C for 180 min, with 5′-IMP reaching 50.64 mg/100 g, significantly enhancing umami. Conversely, while the treatment at 100 °C for 20 min was most effective at preserving total amino acids, essential amino acids, umami amino acids (e.g., glutamic and aspartic acids), and polyunsaturated fatty acids (Σn-3 PUFAs, including EPA and DHA) due to minimal thermal exposure, this group exhibited lower overall sensory acceptability compared to the optimized SV groups (e.g., 80 °C-180 min). This apparent discrepancy highlights the distinct impacts of processing conditions: high-temperature short-time treatment prioritizes nutrient retention but may limit the development of a complex flavor profile, whereas optimized sous-vide conditions strike a balance by facilitating sufficient thermal reactions for flavor formation while still maintaining good nutritional quality. Volatile organic compounds (VOCs) analysis revealed that 70 °C 240 min promoted accumulation of compounds such as propanal and 2-aminopentanol, enhancing roasted and savory notes; 100 °C 20 min caused rapid loss of some volatiles but generated the highest levels of acetaldehyde and propionaldehyde; while 80 °C produced a rich blend of volatiles, including notably higher nonanal, imparting a fresh and rich overall aroma. Flavor formation stemmed from degradation and co-oxidation of amino acids, nucleotides, and fatty acids. GC-IMS analysis elucidated the volatile formation mechanisms, providing a theoretical basis for targeted flavor regulation in aquatic products.

CRediT authorship contribution statement

Shan-Shan Jiang: Writing – original draft, Visualization, Methodology, Investigation. Rui-Yang Shen: Writing – review & editing, Formal analysis. Xiao-qi Liu: Writing – original draft, Formal analysis. Jian-Hua Xie: Methodology, Supervision, Writing – review & editing. Hui-Min Lin: Writing – review & editing, Software, Formal analysis. Soottawat Benjakul: Writing – review & editing, Supervision, Methodology. Bin Zhang: Writing – review & editing, Supervision, Methodology, Funding acquisition, Conceptualization.

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.

Appendix A. Supplementary data

Supplementary material

Table S1. SV sensory scoring criteria for SYCS

Data availability

Data will be made available on request.