The Effects Of Freezing,Thawing On The Quality Of Spanish Mackerel And Storage Temperature On The Quality Of Pacific Saury

There are many species and abundant resources of marine fish in China.Due to the seasonal nature of fishing,it needs to be preserved for later sale or processing after fishing.With unique character of protein,it is particularly important to protect marine fishes from spoilage and maintain good freshness.At present,frozen storage was the most commonly and cost-effective method of long-term storage of marine fish.Low temperature could effectively inhibit not only the growth of microorganisms but also the activity of various endogenous enzymes during frozen storage.The formation of ice crystals and the condensation of cell fluids could lead to the degeneration of fish protein and fat,and also lead the quality of frozen products to deteriorate,such as flavors and color.It would produce a serious impact on the later processing performance.The quality of frozen marine fishes could be affected by freezing method,thawing and storage Conditions.Spanish mackerel and Pacific saury are important offshore and pelagic catches in China,and they were used as experimental materials in this paper.To investigate the effects of different freezing methods,thawing methods and storage conditions of Spanish mackerel or Pacific saury,the total number of colonies and the physicochemical indexes including freezing or thawing loss rate,cooking loss rate,water holding capacity,p H value,TVB-N value,TBA value,the content of myofibrillar protein and active sulfhydryl were measured.The sensory feature including volatile odor and the whiteness value of samples were also performed.It has important academic significance for improving the quality of frozen mackerel and saury,and it also provides a basic theoretical basis for the preservation of frozen fish.The main results of this thesis were showed as follows:1.Effect of different freezing methods on the quality of mackerel: the results showed that there was a significant difference in evaluation indexes of mackerel quality after different freezing methods(p<0.05).The quality of mackerel frozen by-90 ? liquid nitrogen was the best.The quality of mackerel frozen by-50 ? refrigerator was a little bit different from the mackerel frozen by-30 ? plate freezer.They were slightly worse than group of-90 ? liquid nitrogen quick freezing,which were better than freezing in-30 ? fridge and-18 ? cold storage.The results of sensory evaluation were basically consistent with that.PCA analysis method in electronic nose can effectively distinguish the odor difference caused by freezing mode.Rapid freezing of liquid nitrogen and freezing of refrigerators at-50 ? are costly,so they could be used to freezing the marine fishes of high value in practical production,such as tuna and salmon.For ordinary economic fish,-30 ? flat-freezing is the best way for factory production,such as mackerel,hairtails.2.Effect of different thawing methods on the quality of mackerel: the results showed that there was a significant difference in evaluation indexes of mackerel quality after different thawing methods(P<0.05).The time of running water thawing method was the shortest and its thawing rate was the largest in this five methods,which lead to lose some sap and nutrient.The cooking loss rate(22.79 %)of running water thawing method was also significantly higher than other groups(P<0.05).Because of lower ambient temperature and thicker fish body,refrigeration thawing methods spend a long time(1194 min)to thawing that aggravated oxidation decomposition of protein and fat,which reduced the quality of mackerel.Although the indexes value of mackerel thawed by air thawing were batter,the thawing time was as long as 939 minutes with a higher thawing loss rate than 3 %,resulting in large losses and poor economic returns.The indexes value of mackerel thawing in seawater and hydrostatic with a suitable thawing time were better than groups of thawing in running water and refrigeration.However,because the microbial content in seawater was significantly higher than that of tap water,the total number of colonies in this group(3.72 log(cfu/g))was significantly higher than the hydrostatic thawing group(2.83 log(cfu/g)),but it did not exceed 4.00log(cfu/g)and still has a good freshness quality.Both of them are ideal thawing methods,and a more appropriate thawing method can be selected according to the actual situation.3.Changes in quality of mackerels frozen in different freezing methods during the storage: the results showed that with the prolongation of storage time,the quality of mackerels frozen in-90 ? liquid nitrogen was the best,and the changes of quality indexes were the smallest.Followed by mackerels frozen in-30 ? plate freezer.The quality of mackerels frozen in-18 ? cold storage was the worst and the changes of quality indexes were largest.In the storage period of 12 weeks,the TVB-N values of each group mackerels did not exceed 23 mg/100 g,and the TBA values did not exceed1.5 mg/kg.All mackerels were maintained in a good fresh quality by this three different freezing methods.That means this three different freezing methods could meet storage requirements.For marine fishes with high economic value and nutritional value,liquid nitrogen can be selected for quick freezing them.While flat quick freezing can be used to freezing ordinary economic fishes,and for a short-term frozen storage,low-value fishes could be frozen directly in cold storage.4.Effect of different storage temperature on the quality of frozen saury: the results showed that the quality of saury,stored separately in-18 ?,-30 ?,-50 ?refrigerators,decreased slowly during storage period from 60 d to 195 d.After this period,the quality of each group saury decreased faster.On the 240 th day,microorganisms began to grow and multiply,which leaded to the saury accelerate the spoilage.In the first six months of storage,this three different storage temperatures can keep saurys in good quality.With the storage time prolonging,the storage temperature at-18 ? cannot meet the quality requirements gradually,which TBA values has already exceeded 8 mg/kg on the 240 th day,while advantages of storing at-50 ? and-30 ? were gradually revealed.So we could choose the most economical and effective storage temperature according to actual production demand.