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.