Effects Of Freezing Methods And Lightly Salting On The Quality Of Channel Catfish (Ictalurus Punctatus) Fillets

Channel catfish(Ictalurus punctatus)is one of the main farmed freshwater aquaculture species in China,and widely consumed due to its distinctive taste,texture and high nutritional value,but without intramuscular bones.Frozen fillets is the dominating industrial processed products and export of channel catfish in China.However,due to the high water content of freshwater fish and its poor frost resistance,quality deterioration often occurs in different degrees during the process of frozen storage,which will seriously affect the quality and value of fish.In addition,the freezing rate of the existing frozen processing of fillets is slow,and the quality of product is not high.At present,studies on fish freezing mainly focus on the physical and chemical properties and texture characteristics of fish during frozen storage.Changes in ice crystals,water,and flavor components during frozen storage caused by different freezing methods and freezing rates are still not fully clear,and there is still lack of effective control technology for the quality deterioration of frozen freshwater fish.Therefore,clarifying the effect of different freezing methods on the quality of fish,studying and developing more rapid and effective freezing and quality control technologies have become one of the key issues that need to be solved urgently in the aquatic product processing industry.This paper systematically studied the changes of the quality of channel catfish fillets treated by different freezing methods(static air freezing,air-blast freezing,liquid nitrogen immersion freezing)during frozen storage at-18°C,and clarified the freezing rate on the texture characteristics,microstructure,water distribution and taste active compounds.And the effect of quality improver on the quality of frozen channel catfish fillets was studied.The quality of frozen fillets were improved by adding with sodium chloride and compound phosphate.This study has important guiding significance for solving the problem of quality deterioration in conventional quick freezing processing and storage and improving the quality and commodity value of frozen fillets.Firstly,by the determination of physical and chemical properties,texture properties,microstructure,water distribution and taste active compounds,the quality changes of frozen fillets treated by different freezing methods(static air freezing,air-blast freezing,liquid nitrogen immersion freezing)during frozen storage at-18°C were systematically compared,and explained the differences in the quality of fillets caused by different freezing methods.The results showed that the freezing time for the center temperature reaching-18°C was 461 min,60 min,and 1.83 min for static air freezing,air-blast freezing and liquid nitrogen immersion freezing,respectively.With increasing storage time,the quality of fillets was declined to different extents.After 16 weeks of frozen storage,the hardness decreased by 68.0%(static air freezing),63.8%(air-blast freezing),and 58.1%(liquid nitrogen immersion freezing)respectively.Compared with static air freezing and air-blast freezing,the microstructure of the samples frozen by liquid nitrogen had more compact fibre arrangements and smaller gap,and the samples frozen by static air freezing showed the largest intermyofibers space and more myofibers were broken.Fast freezing could form many small ice crystals that were evenly distributed in the cell from the micrographs of frozen channel catfish fillets tissues.Liquid nitrogen immersion freezing with rapid freezing rate could effectively slow down the migration of water from the myofibers into the extracellular space,and alleviate the loss of taste active compounds,and thus could be an effective method to maintain the quality of frozen channel catfish fillets compared with static air freezing and air-blast freezing.Secondly,through the determination of texture properties,water holding capacity,water distribution and microstructure,the quality of frozen fillets that low salted was investigated.After 4 weeks of frozen storage at-18°C,the water holding capacity of fillet that was improved and the drip loss and cooking loss were decreased compared with the control group.Among them,the quality of sample that treated by 2% sodium chloride was the best,and the hardness and shearing force of the fillets were higher than those of the control group by 18.2% and 18.5%,respectively.Lightly salting has a significant effect on improving the quality of water holding capacity and texture of channel catfish fillets.Finally,in order to further improve the quality of frozen channel catfish fillets during storage,the fillet was added with sodium chloride and compound phosphate,and the effect of quality improver on water holding capacity,water distribution and microstructure was studied.The results showed that the drip loss of the fillet treated by 2% compound phosphate was 1.3 times that of the fillet was added with 2% sodium chloride and 2% compound phosphate.The drip loss of fillets that treated by sodium chloride combined with compound phosphate decreased.The quality deterioration was significantly alleviated of the fillet treated by 2% sodium chloride combined with 2% compound phosphate.The effect of sodium chloride combined with compound phosphate was significantly better than that of single quality improver.