Effect of freezing methods on quality changes of grass carp during frozen storage

The effects of freezing method on ice crystal, distribution of water and freshness properties of grass carp during frozen storage was investigated. The freezing methods contained air-blast freezing (AF), immersion freezing (IF) and liquid nitrogen freezing (LNF). Results showed that the freezing speed of IF samples was 4.22 times that of AF samples. Ice crystals of IF samples were smaller than that of AF samples (p< .05). Consequently, there was less damage to muscle fiber, lower water mobility and high retention of texture in IF samples during frozen storage. Meanwhile, equivalent diameter and area ratio of ice crystals increased by 101.2% and 38.8% (IF), 396.8% and 211.3% (LNF) after 5 months of frozen storage. Compared with LNF samples, IF samples had the lower growth rate of ice crystals, almost the same thawing loss and muscle tissue state. Principal component analysis showed that IF was similar to LNF in maintaining quality characteristics of fish. Overall, IF was a promising way to delay the degradation of fish during frozen storage. Practical Applications Liquid nitrogen freezing technology is one of the mature technologies on the market with many advantages because of its fast freezing speed; however, it is prone to cause low-temperature fracture and whitening of products, affecting sensory quality of products. Immersion freezing technology and the coolants have been improved in recent years. The coolants used in immersion freezing can be recycled for many times and reduce costs of freezing. Once the effect of immersion freezing technology on quality of food is similar to that of liquid nitrogen freezing and the cost is relatively low, it is expected that the immersion freezing technology will be further promoted by industry processors.