Changes Of Biochemical Properties And Freshness Evaluationin In Some Maricultured Fish-shellfishes Stored At Low Temperature

In recent years,maricultured fisheries have developed rapidly in china,and taking the fresh or live sale as the main way can not adapt to the condition that the yield of fish and shellfishes quickly increase.It is very important to keep the freshness of the maricultured fish molluscs by iced or frozen storage.However,a series of changes which effect seriously the freshness and edible quilities of the maricultured fish molluscs occur at low temperature storage.Using the salt-solubility of actomyosin,-SH content,ATPase activities,the contents of ATP-related compounds, glycogen,glycine betaine,free fatty acids and total volatile basic nitrogen(T-VBN) as assessment indexes,changes of biochemical properties and freshness from Pseudosciaena crocea,Scianeops ocellatus and Mugil cephalus as well as Sinonovacula constricta during low temperature storage were studied respectively. The main research contents and conclusions are as follows:1) The contents of ATP-related compounds in Pseudosciaena crocea meat were determined by HPLC,and the freshness of Pseudosciaena crocea meat during iced storage were assessed using K value,T-VBN level and sensory scores.Results showed that the contents of ATP and ADP decline rapidly on the first day of storage and thereafter change slowly.AMP didn't accumulate and its content was low duing storage.On the other hand,IMP content was present at high level.After the first day of storage,IMP content reached a maximum value of 9.12μmol/g and remained at higher level until the 8th day,and then decreased.The content of HxR increased fastly on the first day of storage,then decreased as storage progressed.Hx content increased slightly until the 8th day and then increased fastly from 0.97μmol/g on the 8th day to 3.10μmol/g on the 13th day.Both K and T-VBN values increased during iced storage. Combined with the sensory scores,Pseudosciaena Crocea meat was in the first grade of freshness until the 4th day of storage,and reached the initial decomposition stage after 11 days storage.The limit time for sensory acceptability of Pseudosciaena Crocea meat stored in ice was 10 or 11 days.2) Changes of biochemical properties from Sciaenops ocellatus during iced and -20℃forzen storage were analyzed by measuring the salt-solubility of myofibrillar proteins,-SH content,ATPase activities and freshness indexes such as pH value,sensory scores and T-VBN.At the same time,the effects of ultra rapid-deep-freezing and antistaling agents on the protecting myofibrillar proteins from denaturation were compared.The results indicated:During iced storage,the salt-solubility of myofibrillar proteins,-SH content and ATPase activities of Sciaenops ocellatus all decreased.IMP content was present at high level,and K values increased slowly as storage progressed.K value got to 22.42%after 13 days of storage,but at the same time the sensory scores indicated that Sciaenops ocellatus meat was in the early corrupt appearance.It was not fit to use the recommended K values to evaluate the fresh-degree in Sciaenops ocellatus.It was suggested that K values of 6%and 12%,or Hx contents of 0.3μmol/g and 0.6μmol/g in the Sciaenops ocellatus meat were boundary of first and second grade freshness. Combined with the T-VBN value and sensory scores,the limit time for the first and second grade freshdegree of Sciaenops ocellatus meat stored in ice were 4～5 and 8 days,respectively.During frozen storage at -20℃,the salt-solubility of myofibrillar proteins,-SH content and Ca²⁺-ATPase activities in Sciaenops ocellatus all decreased.However,the Ca²⁺-Mg²⁺-ATPase and Mg²⁺-EDTA-ATPase activities of Sciaenops ocellatus by ultra rapid-deep-freezing increased at the first two-weeks of storage,then decreased as storage progressed.Although the myofibrillar-protein denaturation in Scianeops ocellatus meat was not stoped by ultra rapid-deep-freezing or antistaling agents,the denaturation could be delayed.Compared with the antistaling agents group,ultra rapid-deep-freezing was more effectual on the reducing myofibrillar protein denaturation and helpful to increase the water binding capacity and sensory scores in the 12 week's frozen storage at -20℃. 3) Changes of biochemical properties from Mugil cephalus at different storage temperature were studied.Meantime the effects of ultra rapid-deep-freezing on the denaturation of myofibrillar protein were analyed.Results showed that:During iced storage,the actomyosin salt-solubility and -SH content of Mugil cephalus decreased.It had good relativity to make use of T-VBN value,sensory scores and K value as freshness evaluation indexes.It was suggested that K values of 20%and 40%in the Mugil cephalus meat were boundary of first and second grade freshdegree.The limit time for the first and second grade freshness of Mugil cephalus meat stored in ice were 5 and 9 days,respectively.The limit time for sensory acceptability of Mugil cephalus meat stored in ice was 11 days.Compared with the direct freezing,ultra rapid-deep-freezing could defer the myofibrillar protein denaturation particularly at -20℃frozen storage.Using the same freezing method,the lower storage temperature,the slower protein denaturation.4) During keeping alive at 0℃,the contents of glycogen,glycine betaine and ATP-related compounds,the composition of free amino acids and sensory assessment of Sinonovacula constricta were determined.Moreover,based on the study of myofibrillar protein denaturation and changes of other related-compounds,the freshness of Sinonovacula constricta during -20℃frozen storage were evaluated.The results showed that:During iced storage,the survival rate of Sinonovacula constricta amounted to 80%.ATP contents reached a maximum value of 1.89μmol/g on the 8th day.The content of AMP was in higher level and changed slowly.On the contrary,IMP content was lower during storage.The original contents of glycogen in Sinonovacula constricta was higher and decreased quickly on the first day of storage,then descended slowly.The free amino acids of Sinonovacula constricta included mainly Ala,Ser and Glu as well as Arg.During iced storage,glycogen,glycine betaine,AMP and free amino acids with sweet-fresh flavor played the important role in the delicious taste of Sinonovacula constricta.During frozen storage at -20℃,the content of AMP was higher obviously in comparison with IMP.The content of HxR and Hx increased on the 14 weeks of storage.To evaluate the freshness of Sinonovacula constricta,the K value was more fitter than the Ki value.After 10 weeks of frozen storage at -20℃,the contents of T-VBN,HxR and Hx as well as free fatty acids increased quickly,but the actomyosin salt-solubility and -SH content decreased rapidly.For the above reasons,the quality of Sinonovacula constricta descended quickly in the late frozen storage.Combined with the results of sensory assessment and T-VBN values,the keeping time of Sinonovacula constricta frozen at -20℃was about 4 mouths.