To enhance the quality of liquid nitrogen-frozen fresh Monopterus albus fillets and reduce energy consumption costs, this study established a freezing rate versus temperature curve for liquid nitrogen freezing at various temperature gradients (–50 to –120 ℃), and investigated the effects of varying freezing rates on quality parameters of fresh M. albus, including water-holding capacity (moisture content, water-holding rate, and thawing loss), water state, textural properties, ice crystal morphology, and muscle tissue microstructure. On this basis, a “freezing-point precooling followed by variable-rate liquid nitrogen freezing” protocol (PC-V-LNF, precooling at -1.36 ℃ and then freezing at varying rates from 9 to 4 ℃/min) was developed, and its energy consumption characteristics were evaluated. The results demonstrated a linear relationship between liquid nitrogen freezing temperature and freezing rate (y = -3.6686x - 43.082, R² = 0.981). Compared with samples frozen at 6 ℃/min, the proportion of bound water (P₂₁) in samples frozen at 9 ℃/min decreased by 19.00%, and the proportion of immobilized water (P₂₂) increased 1.40%; the hardness and chewiness increased by 39.44% and 264.90%, respectively (P < 0.05), accompanied by reduced gaps between ice crystals and more compact arrangement of muscle fibers. As the freezing rate increased from 9 to 12 ℃/min, the chewiness decreased by 21.23% (P < 0.05), and the gaps between ice crystals slightly increased. Relative to freeze-frozen samples (FF, 0.38 ℃/min), the moisture content of samples subjected to constant-rate liquid nitrogen freezing (C-LNF, 9 ℃/min), variable-rate liquid nitrogen freezing (V-LNF, 9 → 4 ℃/min), or PC-V-LNF increased by 7.87%–10.90%, the water-holding rate increased by 11.36%–13.39%, and the thawing loss decreased by 27.60%–35.43% (P < 0.05). Notably, PC-V-LNF resulted in no significant differences in water-holding capacity, water state, hardness, springiness, chewiness, resilience, or muscle fiber integrity compared with C-LNF and V-LNF (P > 0.05). The evaluation results of energy consumption characteristics revealed that the liquid nitrogen consumption of C-LNF, V-LNF, and PC-V-LNF were 7920, 6712, and 5233 L per ton of samples, respectively, with electricity consumption of 16.8, 14.4, and 2.52 kW·h, respectively. Compared with C-LNF and V-LNF, PC-V-LNF reduced liquid nitrogen consumption by 33.93% and 22.04% and lowered comprehensive cost by 34.02% and 22.15%, respectively. In conclusion, PC-V-LNF enhances freezing quality in fresh M. albus while significantly reducing energy costs, providing theoretical and technical support for the efficient liquid nitrogen quick-freezing preservation of fresh M. albus products.

To investigate the effects of varying durations of high-intensity ultrasound-assisted extraction on the quality of Monopterus albus bone broth, the fish bone broth, prepared at atmospheric pressure, was subjected to low-frequency and high-intensity ultrasound (power 360 W, and frequency 20 kHz) treatment for 0, 1, 2, 3, 4, 5, and 6 min. Subsequently, color, microscopic distribution, zeta potential, particle size, water-soluble protein, soluble solids, and mineral contents (e.g., Mg, K, Ca, and Na) were measured. Results indicated that as ultrasonic treatment time increased, the absolute value of the ζ-potential first rose (up to 9.28 mV) and then declined (to 7.39 mV) (P < 0.05), and the average particle size plummeted to 97 μm before rebounding to 119 μm (P < 0.05), both peaking at 4 min of ultrasonic treatment. Water-soluble protein content increased initially and then decreased (P < 0.05), peaking at 1.06 mg/mL at 5 min, which was 45.20% higher than the level before treatment. Soluble solids content peaked at 0.31 g/100 mL at 6 min, and then remained unchanged significantly (P > 0.05). Compared with those before treatment, the contents of Mg, K, Ca, and Na increased by 10.20%, 37.18%, 5.82%, and 28.75%, respectively, after 4 min of treatment. Electronic tongue analysis indicated that the response values for bitter taste and aftertaste initially increased and then decreased, reaching peak values of 8.56 and 1.53 at 1 min of ultrasonic treatment, respectively. Additionally, the response values for bitter taste and bitter aftertaste were reduced at durations equal to or more than 3 min. Gas chromatography-ion mobility spectrometry (GC-IMS) analysis demonstrated that ultrasonic treatment significantly enhanced the concentrations of fruity, oily, meaty, and nutty flavor compounds (e.g., methyl heptanoate, butyl butyrate, β-cyclocitral, n-octanal, 2-methoxy-3-sec-butylpyrazine, and 2-ethyl-5-methylpyrazine), while effectively reducing the concentrations of pungent odor substances such as hexanal and butyric acid. The sensory score of the fish bone broth was the highest at 4 minutes of ultrasound treatment. In conclusion, high-intensity ultrasonic treatment can significantly improve the quality of M. albus bone broth, providing a theoretical foundation for its efficient and high-quality processing.

To enhance the liquid nitrogen freezing efficiency and improve the quality of frozen prepared Monopterus albus, the effect of variable temperature liquid nitrogen quick-freezing combined with composite cryoprotectant treatment on the quality of prepared M. albus was investigated. Deep-fried cured M. albus was treated with a novel composite cryoprotectant consisting of 4% xylooligosaccharides, 4% sorbitol and 0.3% sodium bicarbonate before precooling followed by liquid nitrogen freezing at variable temperatures (from -80 to -50 ℃). Internal temperature, moisture content, thawing loss, centrifugal loss, moisture state, thiobarbituric reactive substances (TBARS) value, acid value (AV), fluorescent compound content, and total volatile basic nitrogen (TVB-N) content were measured, and ice crystal morphology and microstructure were observed. The results indicated significantly lower thawing loss (3.80%), centrifugal loss (11.6%) and free water percentage (24.4%) under variable temperature liquid nitrogen quick-freezing compared with freezer freezing (P < 0.05), and significantly lower thawing loss under variable temperature liquid nitrogen quick-freezing compared with constant temperature liquid nitrogen quick-freezing (P < 0.05). After freezing for 24 weeks, prepared M. albus treated with the cryoprotectant exhibited significantly lower ice crystal size, thawing loss, centrifugal loss, TBARS value, AV, fluorescent compound content, and TVB-N content as well as more complete microstructure of muscle fibers, and its quality did not significantly differ from that of M. albus treated with a commercial cryoprotectant (P > 0.05). These findings suggested that pre-cooling followed by variable temperature liquid nitrogen quick-freezing combined with the novel composite cryoprotectant could enhance the quality of prepared M. albus during frozen storage, which will offer theoretical support for the development of highly efficient freezing and preservation technology for prepared M. albus products.

To investigate the microbial succession and the spoilage potential of specific spoilage organisms (SSOs) in refrigerated Monopterus albus at 4 ℃, Illumina high-throughput sequencing was employed to analyze the diversity and dynamic shifts in microbial communities. Traditional selective culture methods were utilized to isolate and identify SSOs, and their spoilage potential was assessed by comparing the yield factor of spoilage metabolites (Y_(TVB-N/CFU)). The results indicated that before storage (day 0), the microbial richness and diversity were the highest, with the relative abundance of Streptococcus (31.56%), Gemella (11.69%), Salinivibrio (7.5%), and Burkholderia-Caballeronia-Paraburkholderia (5.46%) being greater than 5%. During the middle and late stages of storage (≥ 15 days), Vagococcus, Acinetobacter, Burkholderia-Caballeronia-Paraburkholderia, and Psychrobacter emerged as the dominant genera. Linear discriminant analysis effect size (LEfSe) analysis revealed that the marker genera showing significant differences between before storage and after storage for ≥ 15 days were primarily Streptococcus, Gemella, and Salinivibrio, Vagococcus, Acinetobacter, and Psychrobacter. Six SSOs were isolated from the 30-day stored sample, namely Pseudomonas fluorescens, Bacillus cereus, P. putida, Acinetobacter johnsonii, Lactococcus raffinolactis, and Carnobacterium divergens. The decreasing order of the Y_(TVB-N/CFU) values of these SSOs was P. fluorescens (41.18 mg/CFU) > B. cereus (29.70 mg/CFU) > L. raffinolactis (15.07 mg/CFU) > A. johnsonii (14.59 mg/CFU) > P. putida (8.22 mg/CFU) > C. divergens (4.61 mg/CFU). This study provides a theoretical foundation for the development of efficient low-temperature preservation technologies for M. albus.

Our aim was to select the most suitable thawing and reheating methods for frozen prepared Monopterus albus. Four thawing methods (refrigerated, running water, microwave, and ultrasonic static water) were used to thaw frozen samples. Thawing time, moisture content, thawing loss, thiobarbituric acid reactive substances (TBARS) value, peroxide value, acid value (AV), total volatile basic nitrogen (TVB-N) content, and total bacterial count were measured. After thawing by the most suitable thawing method (ultrasonic static water), three methods (steam, microwave, and water bath heating) were used to reheat prepared Monopterus albus, which was then evaluated for textural properties and sensory quality. Furthermore, the flavor quality was analyzed using an electronic nose (E-nose), an electronic tongue (E-tongue), and gas chromatography-ion mobility spectrometry (GC-IMS). The results showed that ultrasonic static water thawing resulted in the lowest thawing loss (3.60%), TBARS value (0.32 mg/kg), AV (0.47 mg/g), and TVB-N content (1.99 mg/100 g). Furthermore, steam reheating resulted in the lowest hardness (1099 N), chewiness (627 N), and shear force (7.9 N) indicating excellent textural properties, as well as the highest organoleptic score. Linear discriminant analysis (LDA) of the E-nose sensor response showed a clear discrimination between the different reheated samples (LD1 and LD2 explained 94.3% of the total variance). E-tongue analysis showed that steam and water bath reheating significantly reduced the bitter taste and bitter aftertaste of the product while enhancing the salty taste. GC-IMS analysis revealed that steam reheating led to higher concentrations of hexanal and (E,E)-2,4-nonadienal and consequently enhanced fatty and pine-like aromas, and effectively reduced the concentrations of compounds with an irritating odor such as 2-hexanone and dimethylamine. In conclusion, the combination of ultrasonic static water thawing and steam reheating could effectively improve the eating quality of frozen prepared Monopterus albus, providing scientific guidance for the high-quality thawing and reheating of frozen prepared freshwater fish products.