Effect Of Cryogenic Freezing By Liquid Nitrogen On The Quality And Microstructure Of Hairtail And Silver Pomfret

Silver pomfret (Pampus argenteus) and hairtail(Trichiurus haumela) were pretreated by liquid nitrogen (LN2) spray freezer (central point of-40℃in20min), samples were also pretreated by plate freezing (central point of-20℃in6h) or frozen directly by a refrigerator (central point of-18℃in20h) for control, followed by freezing storage at-18℃. All samples were stored at-18℃for150days’storage. Proximate composition and physicochemical properties, such as pH, thawing loss, total volatile basic nitrogen (TVB-N), K value, salt soluble protein content, Ca2+-ATPase activity and sensory attributes were investigated during storage. Texture properties, such as hardness, springiness, gumminess, chewiness and resilience, were determined. Light microscope was used to observe the microstructure of frozen samples. This study aims at fiinding out the internal cause of quality change in silver pomfret and hairtail meat, providing basic theory for different pre-freezing preservation technology of fish.The main results were as follows:1) The proximate composition of fresh silver pomfret which captured in early December in Zhoushan showed to be water of79.42%, crude protein of11.84%, fat of6.42%, ash of1.53%. While for hairtail which captured in mid-November showed to be water of77.60%, crude protein of15.20%, fat of4.78%, ash of1.30%.2) pH, thawing loss, total volatile basic nitrogen (TVB-N), K value were taken as physicochemical index to investigate the effects of three pre-freezing methods (liquid nitrogen spaying, plate freezing and refrigerator freezing) on the freshness quality of silver pomfret and hairtail. Results showed that with the extension of storage time, the pH value decreased first before increased. The lower the prefreezing temperature was, the slowest pH changed. Thawing loss, TVB-N value and K value increased with prolonged storage, which indicated a gradual spoilage in fish. Liquid nitrogen spaying helps keep the freshness of marine fish with inhibiting microbial growth to extended shelf life. 3) The protein properties of silver pomfret and hairtail meat were also evaluated during storage. The results showed that Ca2+-ATPase activity and the total sulphur content of all samples increased first, followed by declining later with the storage time. Salt soluble protein content of silver pomfret declined slowly at the early stage and became faster in the later storage, which is in direct contradiction to that of the hairtail. This is probably caused by the differences between kind and quantity of proteases in fish.From SDS-PAGE, the myosin heavy chain and actin band turned thin with the prolonged storage. Generally, samples frozen by liquid nitrogen showed better protein index compared with that of the control groups, suggeting it was effective in suppression of protein denaturation.4) The sensory attributes and texture profiles analysis of silver pomfret and hairtail were also evaluated during storage. The sensory assessments and texture indicators of all samples decreased significantly with prolonged storage while that of the samples frozen by liquid nitrogen occured slowly with the best sensory assessments. Sensory assessments and texture indicators of fish were in correlation with the regression models as follows:Y1=1.045-0.025X1+8.775X2-0.008X3+0.067X4+9.842X5Y2=0.241X1+0.187X2+1.297X3+0.516X4-0.27X5(Y1-sensory assessments of silver pomfret; Y2-sensory assessments of hairtail; X1-hardness; X2-springiness; X3-gumminess; X4-resilience; X5-chewiness)5) Microstructure observation (LM) showed that fresh fillets had compact texture, intact cells and no destruction by ice crystals which form in freezing and damage the organization during storage. Microstructure of samples frozen by refrigerator changed the most severely while liquid nitrogen frozen samples appeared to be the most similar to that of the fresh material with narrow gap between the myo fibrils and excellent cell integration. Microstructure observation of hairtail showed the similar results that liquid nitrogen freezing could protect the fish microstructures.6) Comparing the physicochemical properties and organizational structures between groups, it is obvious that liquid nitrogen freezing was better fresh-keeping than plate freezing. Freezing directly by refrigerator was proved to be the worst among the tested methods in this study. Cryogenic freezing by liquid nitrogen showed an excellent protective effect on fresh-keeping, protein properties, texture properties and organizational structures of marine foods and it has a great potential to be popularized.