Effects Of Liquid Nitrogen Quick Freezing On The Qualities Of Tonguefish And Swimming Crab

Swimming crab was treated by liquid nitrogen spray freezer,plate freezing and a common refrigerator.Tonguefish was pretreated by liquid nitrogen spray freezer and plate freezing before being stored at-18?.TBA value,total volatile basic nitrogen(TVBN),sensory attributes,salt soluble protein content and pH were investigated.And the microstructure of tonguefish was observed.The cause of quality change of them was investigated.The results were as following:(1)TBA and TVBN values of tonguefish treated by those two pre-freezing methods increased over time.TVBN and TBA values of the sample by plate freezing increased quicker than those by liquid nitrogen spray freezer.TVBN of the sample by liquid nitrogen freezer after being stored for 105 days was lower than 13.0 mg/100g with TBA values only 0.29mg/100g,which indicated that freshness of samples were still of first grade with better quality than plate freezing.The increase of fat oxidation may be due to the damaged area exposed to the air.The ice crystal caused by different plate freezing rates may bring different mechanical damages.(2)Salt soluble protein content,retention ability,Ca2+-ATPase enzyme activity,sensory attributes of tonguefish treated by those two methods reduced over time.Plate freezing presented quicker changes than liquid nitrogen freezer,which showed that liquid nitrogen freezer could delay the reduction of salt soluble protein and inhibit the degeneration of muscle fiber protein.The muscle protein had slow degeneration rates under liquid nitrogen freezer with slow ice crystals by quick freezing speeds.(3)The results of scanning electron microscope showed that myofibril of samples by liquid nitrogen was neat and smooth,which may possibly because of the small ice crystals formed by quick frozen method with small mechanical damages.(4)pH,K value,TBA,TVB-N,sensory attributes and salt soluble protein content were taken to study the effects of liquid nitrogen freezing,refrigerator freezing and plate freezing on the swimming crab quality.Results indicated that the pH had a trendency of decrease first before increase with the extension of storage time.The slowest pH change was found in the lowest freezing temperature.Quality change of the samples by liquid nitrogen increased slower than the others,which indicated that liquid nitrogen freezing helps keep the quality of the sample and extended their shelf life.(5)The crab was frozen by liquid nitrogen freezing under the quick freezing procedure of the chamber ambient temperature until the central temperature of the crab to be-40 ?within 15-30min.The ice crystal formation zone was through in several minutes.The crack rates of crab reduced efficiently by precooling below 13?.The crab had a lower cracking rate with excellent quality under the quick freezing procedure of the chamber ambient temperature decreased to-20 ? in 2 min,further to-40 ? in 2 min,then to-80 ? in 3 min until the central temperature to be-40 ?.