Effects of Cryogenic Freezing by Liquid Nitrogen on the Quality and Microstructure of Silver Pomfret (Pampus argenteus)
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Abstract:
Silver pomfret (Pampus argenteus) was pretreated with liquid nitrogen (LN2) by spray freezing (central point temperature of -40°C in 20 min), followed by freezing storage at -18°C. For the control, samples with similar sizes in the same batch were pretreated by plate freezing (central point temperature of -20°C in 6 h) or frozen directly using a freezer (central point temperature of -18°C in 20 h). Control samples were subsequently stored at -18°C, similar to LN2 samples. The results showed that the pH values of the LN2-frozen samples, plate-frozen samples, and freezer-frozen samples reached minima at 6.42, 6.35, and 6.11, respectively, on the 105th day; the salt-soluble protein contents were 6.71, 3.24, and 1.38 mg/g, respectively, and the Ca2+-ATPase activities were 1.73 × 10-3, 0.76 × 10-3, and 0.56 × 10-3 μmol/(min?mg), respectively. The increases in K values and total volatile base nitrogen were limited with lower prefreezing temperatures. Texture profile analysis (TPA) showed that the hardness, springiness, and resilience of frozen fish meat decreased significantly (P < 0.05) with prolonged storage time. In contrast, a low prefreezing temperature may result in slower changes in these indices. Microstructure observations showed that LN2-frozen samples appeared the most similar to the fresh material, with a narrow gap between the myofibrils and excellent cell integration after storage for 150 days. The results indicated that the LN2 spray method was the best of the three tested methods for maintenance of the quality of silver pomfret.
