This study was executed in order to investigate quality deterioration caused by lipid oxidation during the frozen storage of Acipenser sinensis. Fish pieces were stored at -18 ℃ for 24 weeks after being frozen using a freezer (-20 or -50 ℃) or liquid nitrogen (-80 or -110 ℃). The central temperature, fat and free fatty acid (FFA) contents, fatty acid composition, peroxide value (POV), thiobarbituric reactive substance (TBARS) value, and fluorescent compound content were measured and the microstructure of muscle fibers was also observed during the storage period for the purpose of evaluating the influence of freezing methods on lipid oxidation and muscle microstructure in Acipenser sinensis during frozen storage. The results showed that with increasing storage time, the fat and polyunsaturated fatty acid (PUFA) contents of the four fish samples decreased, while POV, TBARS value, and the contents of FFAs, saturated fatty acids (SFAs), monounsaturated fatty acids (MUFAs) and fluorescent compounds increased. The gap between muscle fibers increased, which was accompanied by breaking of myofibrils. In addition, the contents of fat and PUFAs in liquid nitrogen frozen samples were significantly higher than those in freezer frozen ones (at the end of storage, the freezer frozen sample frozen at -20 ℃ contained 7.89% of fat compared to 9.13% for the liquid nitrogen frozen one at -80 ℃), while POV, TBARS value, and the contents of FFAs, SFAs, MUFAs and fluorescent compounds were significantly lower than those in freezer frozen samples (P < 0.05), and the muscle fiber structure was more complete. The above results indicated that liquid nitrogen frozen alleviated the degree of myofibrillar damage and lipid hydrolysis in Acipenser sinensis during frozen storage, thereby reducing the speed of lipid oxidation.

Phyllostachys praecox shoots were frozen using liquid nitrogen at –60, –90 or –120 ℃ to an internal temperature of –18 ℃, or frozen at –90 ℃ to an internal temperature of –6, –12 or –18 ℃, vacuum-packed and stored in a freezer at –18 ℃ for 24 weeks. In order to analyze the effect of liquid nitrogen freezing on physiological and biochemical characteristics of P. praecox shoots during frozen storage, L-phenylalanine ammonialyase (PAL) and peroxidase (POD) activities, total phenolic content, relative electrical conductivity, and water state were measured and ice crystal structure and cell morphology were observed. The results showed that with increasing freezing time, the PAL and POD activities, total phenolic content, and peak area of free water in all six groups decreased significantly (P < 0.05), while the relative electrical conductivity increased significantly (P < 0.05), with the ice crystals and cells being deformed and damaged to varying degrees. Lower freezing temperature led to smaller ice crystals, lower PAL and POD activities and relative electrical conductivity, higher total phenolic content, and better maintenance of cell morphology, but there was no significant difference in physiological and biochemical properties between P. praecox shoots frozen at –90 and –120 ℃ (P > 0.05). The PAL and POD activities and relative conductivity of P. praecox shoots frozen at –6 ℃ were higher than those frozen at –12 and –18 ℃, and the size of ice crystals was smaller and the degree of cell damage was greater in P. praecox shoots frozen at –6 ℃ than at –12 and –18 ℃. The difference between P. praecox shoots frozen at –12 and –18 ℃ was not significant (P > 0.05). Collectively, these findings indicated that the most suitable liquid nitrogen freezing conditions of P. praecox shoots are –90 and –12 ℃ for freezing and internal temperature, respectively.