The Degradation Regularity And Its Influencing Factors Of Grass Carp Lipid

China has the world's richest freshwater fish resources,but its industrialization degree is rather low(less than 10%).In addition,the muscles of freshwater fish are easily corrupted during processing,transportation and sale at the rate of more than 30%,which causes a great waste of resources and environmental pollution.Although many reports are documented on the fresh-keeping preservation of freshwater fish,the effect on shelf-life extension are limited.Hence,in order to put forward effective preservation strategies,the researches to uncover their corruption mechanisms become one of the inevitable ways for further development of freshwater fish industry.In this study,the hydrolysis and oxidation degradation process of grass carp lipid during chilled preservation at 2?4? was studied by determinating chemical indicators related to fat oxidation such as acid value(AV),peroxide value(POV)and sulfur barbituric acid value(TBARS).The technologies such as gas chromatography-mass spectrometry(GC-MS)and transmission electron microscope(TEM)had been used to investigate the compositions,differences and dynamic changing regularity of grass carp muscular fatty acids as well as the dynamic changing of microstructure and fat storage patterns in grass carp muscle tissue during chilled preservation.Then the key fatty acids closely related to corruption will be screened out.The changes of fat oxidation index and fatty acids relative contents in grass carp muscle with light and dark packaging during the chilled storage were compared to explore the influences of illumination on grass carp muscle fat.In order to analysis the catalytic effect of lipase on fish muscular lipid hydrolysis,the dynamic changes of muscular lipase activity were determined during dark chilled storage.Based on the predecessors' studies on plant and mammalian lipoxygenase(LOXs),the properties of grass carp LOXs was figured out and its quantitative detection method had been established.Its catalytic effect on fish muscular lipid oxidative degradation had also been analysed.The research results showed that the grass carp muscular lipid continue to hydrolyze and oxidate during the preservation period.The total fat contents of grass carp muscle were significantly reduced with the extension of preservation time(P<0.05).While the AV and TBARS values of muscular lipid were significantly increased(P<0.05).After 10 days,they reached 20.49 and 4.48 times as much as that of fresh fish.The POV of grass carp muscular lipid increased first and then decreased gradually.It maximized to 12.07 meq/kg in the 8th day.The grass carp muscular fat began to rapidly hydrolyze and oxidate from the 4th day and the 6th day respectively until seriously corrupted.The fatty acid composition of grass carp muscle was constantly changing during the preservation period.The monounsaturated fatty acid(MUFA)and polyunsaturated fatty acid(PUFA)were gradually degradated.The PUFA significantly degradated(P<0.05)with the extension of preservation time,resulting in the significant increase of saturated fatty acids(SFA)(P<0.05).There was a positive correlation between the rate and extent of fatty acid degradation and their extent of unsaturation.Octadecenoic acid,octadecadienoic acid and a-octadecatrienoic acid,which were the most abundant MUFA,?-6 PUFA and co-3 PUFA species in grass carp muscle were easy to be oxidated.As a result,they were identified as key fatty acid species closely related to corruption.The TEM results showed that there were abundant oval and circular intracellular lipid droplets with a diameter ranging from approximately 0.5-1.5?m around the mitochondria between myofibrils in grass carp abdominal muscle.However,no intracellular lipid droplets were observed between myofibrils in grass carp back muscle.More lipid droplets accumulated in peripheral positions of the cytoplasm near the cell membrane in abdominal muscle than back muscle.The myofilaments necrosed and the muscle fiber structure became blurred gradually with the extension of preservation time.Hydropic degeneration,even bulliform cells or deformed cell debris could be found in grass carp muscle in severe cases.At the same time,the intramuscular lipid drops gradually decreased and eventually disappeared.There was significant effect of light on the degradation of grass carp muscular lipid(P<0.05).Though the lipids in grass carp muscle with dark package still hydrolyzed and oxidated during chilled storage but the degradation rate and degree were significantly lower than the light group.Packaging grass carp muscle with light proof materials would slow down the degradation of UFA,especially the oxidation degradation of PUFA such as linoleic acid and alpha linolenic acid(P<0.05).Light displayed a significant effect on the decline of ?-3 PUFA relative content(P<0.05),while no significant effect on the degradation of 9-octadecenoic acid,9-hexadecenoic acid and the total MUFA(P>0.05).During the preservation period,the activity of lipase in grass carp muscle with dark package was significantly increased from the 4th day,and finally reached a maximum of 37.32mg/g in the 10th day.It had a significant catalytic role on the hydrolysis of grass carp muscular fat.A method for quantitatively testing of the activity of lipase had been established based on the properties of grass carp LOXs.During the preservation period,the activity of the grass carp muscular LOXs increased first and then decreased gradually.LOXs had a significant catalytic role on the oxidation of grass carp muscular fat.So we should attach importance to the lipid degradation of freshwater fish products to avoid adverse effects on its products.And effective measures should be taken to control hazards caused by lipid degradation to extend the shelf life of fresh preservated products.