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.

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.