Effect of Freezing-Point Precooling Followed by Variable-Rate Liquid Nitrogen Freezing on the Quality of Fresh Monopterus albus Slices and Evaluation of Freezing Energy Consumption Characteristics

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.