Study On The Interaction Between Myoglobin Oxidation And Lipid Oxidation In Frigate Mackerel Muscle

Frigate mackerel is one of the important high yield fishes in China, which is rich in myoglobin in muscle and contains high proportion of unsaturated fatty acids in lipid. So the discoloration and lipid oxidation of Frigate mackerel are accelerated during the storage and handling, which lowered the quality of related products. Relevant researches show that discoloration correlates closely with lipid oxidation. This thesis is aimed at the study of the changes of pigments and lipid oxidation of ordinary and dark muscles of Frigate mackerel during refrigerated storage and the interaction between myoglobin oxidation and lipid oxidation in Frigate mackerel muscle. The main results were showed as follows:Firstly, pigments and lipid oxidation variation of ordinary and dark muscles of Frigate mackerel stored at4℃were investigated. It showed that the content of mygolobin in dark muscle was significantly higher than ordinary muscle during storage, and the original content of mygololin was27789.4mg/Kg and8652.6mg/Kg separately (P<0.05); the content of total extractable pigments and myoglobin, and redness index, are gradually decreasing, and MetMb%was increasing during storage; the color of ordinary muscle occurred to be browning and discoloration in the later period of storage (day9-15). The content of crude lipid of dark muscle (46.76%) is higher than ordinary muscle (33.61%), so as the percentages of saturated fatty acids. Moreover the degree of lipid oxidation in dark muscle is much greater than ordinary muscle. Then, the combination sampling and analyzing method (SPME-GC-MS) was applied to study volatile compounds generated from lipid oxidation. The results suggest that the percentage of ketones and aldehydes are reduced, while the percentage of alcohols and furans are accentuated during15days storage,1-octen-3-ol,2-pentyl furan, nonanal, hexanal and2-nonanone were the major components in later storage, which possibly contributed to the strong intensities of fishy and rancid off odors in Frigate mackerel muscle.Secondly, exaction and purification of myoglobin from ordinary and dark muscle and the properties of purified myoglobin were studied. The results showed that the purity of mygolobin in ammonium sulfate precipitated fraction extracted from dark muscle (35.12%) was much higher than it from ordinary muscle (13.92%), so choose dark muscle as the source of myoglobin. The purity of mygolobin in ion-exchange chromatography fraction reached91.3%, and the molecule weight of myoglobin was determined to be16kDa. The results of the study on thermal and acid-base stability of myoglobin suggested that both oxymyoglobin and metmyoglobin denatured rapidly at70℃water bath and kept stable under20℃. MetMb%content was decreasing and OxyMb%content was increasing, as the rise of pH value, in both oxymyoglobin and metmyoglobin solutions. Besides, oxymyoglobin is much more stable at neutral to alkaline pH, while metmyoglobin kept its stability at acidic to neutral pH.In order to investigate the pro-oxidation characteristics of myoglobin and compare the activities of the pro-oxidation between heme iron and non-heme iron, the model system of washed Frigate mackerel ordinary muscle mice was carried out. The results indicated that myoglon is an effective promoter of lipid oxidation, and it engaged in the oxidation as participator but not catalyst. The pro-oxidation activity of oxymyoglobin was enhanced at low pH. Moreover, EDTA had no effect on the pro-oxidation activities of myoglobin and Hemin, which indicated that the released iron from myoglobin and Hemin has no effect on promoting lipid oxidation. The pro-oxidation activities were in the order:MetMb>OxyMb> Hemin>FeCl2(addition was40μmol/Kg), which indicated that metmygolobin was a greater promoter than oxymygolobin and heme iron is a greater promoter than non-heme iron towards lipid oxidation. Moreover, Fe2+and Fe3+showed pro-oxidation activity at high concentration, and Fe2+was a greater promoter than Fe3+.According to the previous analysis of volatiles by GC-MS, seven kinds of volatile oxidation products (hexanal, nonanal,2-nonanone, benzaldehyde,2-ethyl funan,(E, E)-2,4-heptadienal and1-octen-3-ol) were chosen to investigate their effect on oxymyoglobin oxidation. The results showed that except for the low autoxidation rate of oxymyoglobin in2℃,(E, E)-2,4-heptadienal and2-ethyl funan were still effective promoters towards mygolobin oxidation. So we should pay attention to the oxidation products-mediated oxymyoglobin oxidation besides freshness during low temperature storage. In addition, the activities of oxidation products were much greater at acidic pH (5.7) compared with neutral pH. The pro-oxidation activities of these compounds were in the order:(E, E)-2,4-heptadienal>2-ethyl funan> benzaldehyde> nonanal> hexanal,2-nonanone>1-octen-3-ol.