| Silver carp which is rich in high-quality protein,polyunsaturated fatty acids and contains a variety of trace elements as well,is an important species of freshwater fish in China and has a good processing prospects.However,the processing rate of silver carp is low and still be sold as fresh because of its heavily fishy smell,a lot of processing waste and other shortcomings.So the effects of non-thermal physical processing methods such as ultrasound wave(UW)and ultrahigh pressure(UHP)on the smell of silver carp were studied in this paper.The products of fat oxidation is an important source of volatile components of meat,and they are the main reason for the fishy smell in the silver carp muscle as well.Therefore,this study took the silver carp muscle as raw material,the changes of fatty acid composition and volatile matter composition were tested under low temperature storage conditions,and then the conditions of lipoxygenase(LOX)extraction were optimized,the effects of UW and UHP treatment on the conformation and enzyme activity of LOX were studied,and the effects of UW and UHP on lipid oxidation and volatile composition were studied in the fish system as well.In the study of the association between fat oxidation and the formation of fishy matter,the silver carp muscle was stored at 4 °C for 5 days,and the changes of fatty acid composition were detected by gas chromatography(GC),it was found that the content of total unsaturated fatty acids(TUFA)decreased 34.90% and the polyunsaturated fatty acids(PUFA)decreased 34.08% while the POV value,TBARS value increased significantly.The volatile components were detected by gas chromatograph-mass spectrometer(GCMS),and it was found that there were 36 substances in the the volatile components which contended 16 kinds of aldehydes and ketones.the content of E,E-2,4-heptadienal,E,E-2,4-decadienal,E-2-decenal which were contributed mainly to fishy off-odor increased significantly.And the aldehydes and ketones in the volatile components were also the main product of fatty acid oxidation,so it could be concluded that the fatty acid oxidation of silver carp had effect on the composition of volatile compounds.In the study of the enzymatic properties of LOX in silver carp muscle,the optimum extraction conditions of LOX were optimizated by single factors and center combination experiments,the results were as follows: pH 7.8,ionic strength 50 mmol /L,EDTA 2 mmol /L and DTT 2 mmol /L,the enzyme concentration was 1.93 mg / mL and the activity was 238.52 U under that condition.And then the secondary and tertiary structure changes of LOX was detected under different storage time,temperature and pH conditions by circular dichroism and fluorescence spectroscopy while its enzyme activity was measured,the results showed that the LOX of silver carp muscle was stable in 0 ~ 2 days at room temperature,the optimum temperature was 40 ? and the optimal pH was 7.8.In order to inhibit the oxidation of polyunsaturated fatty acids and the formation of fishy substances in muscle of silver carp,the changes of the activity,the secondary conformation and tertiary conformation of LOX under different non-heat treatment conditions were studied.As the UW processing power and time increased,the LOX enzyme activity and the content of ?-helix and ?-sheet decreased,the secondary and tertiary structure of LOX were greatly changed;the optimal UW conditions was 300 W,3 h,and the activity decreased 66.07%.As the UHP processing pressure and time increased,the activity and the content of ?-helix and ?-sheet decreased significantly,the secondary and tertiary structure of LOX were greatly change and the optimum condition of ultrahigh pressure was 300 MPa,20 min,the activity decreased 99.10%.In this paper,the effects of non-heat treatment conditions on fat oxidation and volatile matter composition of silver carp muscle were studied by establishing simulated fish system and complex fish system.In the simulated fish system,E,E-2,4-heptadienal,E,E-2,4-decadienal and E-2-decenal which were the more obvious contribution to the fishy material decreased by 33.2%,64.6% respectively,alcohol content increased by 31.1%,51.6% respectively.The value of POV decreased by 38.0%,72.0% and the TBARS value decreased by 39.0%,60.7% respectively.In the complex fish system,the content of fishy smell of silver carp was decreased lightly after ultrasonic treatment,while that was significantly lower than the control group after ultra-high pressure treatment,the alcohol content increased by 27.9%,41.6% respectively.The values of POV decreased by 21.0%,56.0% and the values of TBARS decreased by 24.8%,57.1% respectively.Compared to the simulated fish system and the complex fish system,it was known that in addition to lipoxygenase,but also contained hemoglobin,metal ions and others could promote fat oxidation reaction,greatly weakened the effect of UW,UHP on fishy material generation and inhibition of fat oxidation in the complex fish system,limited the role of UW in particularly. |
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