Differences Of Odor Characteristics And Influencing Factors Between White Croaker And Small Yellow Croaker

White croaker(Argyrosomus argentatus)and small yellow croaker(Larimichthys polyactis)are two economically important marine fishes distributed widely in the East China Sea.They are often difficult to distinguish in appearance.White croaker are often pretended to be small yellow croaker after dyed by many illegal peddlers.Even so,there is a great difference in economic value between them,especially in odor characteristics.However,researches on Sciaceae fish were mainly focused on the field of molecular systematics.There is few relevant reports on the odor characteristics of the two species at present.Do white croaker and small yellow croaker really differ in odor characteristics? What are the key compounds that make the differences? Are the factors related to odor formation also different between the two kinds of fish? In order to explore the above problems,this study mainly focused on two aspects: the differences of volatile compounds between white croaker and small yellow croaker and the differences of main factors affecting the odor formation.The main results were as follows:1.White croaker(Argyrosomus argentatus)and small yellow croaker(Larimichthys polyactis)from Zhoushan Islands both belong to the family Sciaenidae,but the economic value and odor characteristics are quite different.Electronic nose(E-nose)and gas chromatography-mass spectrometry(GC-MS)were utilized to investigate the differences in volatile compounds of the two fish species.The results showed that samples could be better divided between fresh and different refrigerated periods by strong discrimination ability sensors combined with volatiles with ROAV?1 using Alphasoft V12.44.The results also showed that the differences of odor between two kinds of fish was obvious in fresh state.While through the load diagram,the volatile constituents from fresh white croaker were more complex than small yellow croaker.However,the situation was opposite in the later stage of refrigeration.But the difference was not as big as in the fresh state.Therefore,it was necessary to identify the different odor components in fresh state.The content of trimethylamine,indole and3-methyl-butanol were higher from white croaker of fresh and during prophase of cold storage than small yellow croaker.However,these substances were higher in small yellow croaker in the later stage.According to the results,it was suggested that the microbial diversity of the two kinds of fish might have some influence on them.2.After comparing the solid-phase micro extraction fiber with the mono-trap,the results reflected that the amount of volatile species adsorbed by SPME was limited.While,the the amount and types of volatiles were large than SPME.Thus it was more suitable for the extraction of volatile components from fish.Thus Monolithic Material Sorptive Extraction(MMSE)combined with Gas Chromatography-Mass SpectrometryOlfatometry(GC-MS-O)was used to identify the key odor substances of fresh white croaker and small yellow croaker.The top 10 important volatiles of two species of fish were preliminarily screened out through the Calibration Frequency Method(MF).Among them,the top two MF compounds of white croaker and small yellow croaker were trimethylamine,1-Octene-3-ol and trimethylamine,unknown barbecue odor compounds respectively.Another compound called 6-Methyl-5-Heptane-2-one,which was smelled metallic or bloody were detected from both of the two species fish.But according to MF ranking,this compound had a greater effect on white croaker.3.PTV injection was used to amplify the response of volatiles,and the response of most of the volatiles was significantly higher than that of the non shunting model.21 compounds were detected by the sniffing of white croaker and small yellow croaker respectively.Therefore,the dilution method(AEDA)is more accurate in this mode.Eleven key odors were identified from two kinds of fish by AEDA method.The compounds with Flavor Dilution Factor(FD)of 80 were 1-Octene-3-ol,6-Methyl-5-Heptene-2-one and unknown metallic compound(olfactory time was36.86min)from white croaker,and trimethylamine and 1-Octene-3-ol from small yellow croaker.Compounds such as Hexanal,Heptanal and Nonanal,which are straight chain aldehydes,also have an important contribution to the odor.In comparison,the fishy odor characteristics of small yellow croaker tends to ammonia,and white croakertends to bloody or metallic.The volatiles of fresh white croaker and small yellow croaker were further identified by GC×GC-TOFMS.Compounds such as 2-Methyl-butanal,3-Methyl-butanal,3-Methyl-butanol,2-Methyl-butanol,(Z)-2-Penten-1-ol,(E)-2-Penten-1-ol,2,3-Butanedione and 3-Pentanone could be separated on the polar column in two dimensions,which were difficult to be qualitatively determined by one-dimensional gas chromatography mass spectrometry.In addition,2-Hexenal(grass flavor),cis-4-Decenal(mushroom or fatty flavor),Methanethiol(rotten flavor),2-Heptanol(mushroom flavor),3-Octanol(mushroom flavor),2-Heptanone(milk flavor),1-Octene-3-one(mushroom or metallic flavor),3-Octene-2-one(grassy flavor),(E,E)-3,5-Octadien-2-one(metallic,flavor)and 2-Undecanone(tallow or mildew flavor)were identified from white croaker.However,3-Hexenal(fishy odor)and(E,E)-2,4-Octadienal were identified from small yellow croaker.In addition,metallic odor compounds such as(Z)-2-Penten-1-ol and1-Octanol were not identified by EI,however,which were the focus of this paper.The metallic substance with a sniffing time of 37 min and 38 min might be(E,E)-3,5-Octadiene-2-one or 1-Octanol.The cucumber flavor substance with a sniffing time of 34 min might be(E,Z)-2,6-Nonadienal.4.The odor characteristics of white croaker and small yellow croaker mainly included fishy,grassy,earthy,fatty,metallic and mushroom flavor using MF score,PTV and GC×GC methods.Causes related to odor formation were measured and the results show that there was no significant difference in fat,protein,water and free amino acids contents between white croaker and small yellow croaker.However,there was a significant difference in the composition of fatty acids between the two kinds of fish.Combined with the important odor components identified in the two kinds of fish,a summary of the relationships that could be established with precursors is as follows:The FD value of 1-Octene-3-ol(mushroom flavor)was 40 in both kinds of fish.The results might be consistent with that there was no significant difference in the content of linoleic acid in the two kinds of fish.The 1-Octene-3-one(mushroom or metallic flavor)identified by GC×GC was only identified in white croaker.It was speculated that the result had a certain correlation with high content of N-6 fatty acids in white croaker.Fatty compounds such as Nonanal contributed much more to the odor of small yellowcroaker than that of white croaker(FD = 40).This was related to the fact that the precursor,oleic acid,was significantly higher in small yellow croaker than in white croaker.The content of Hexanal(grassy)FD was 40 in two kinds of fish,which was consistent with the content of linoleic acid in two kinds of fish.In addition,the content of 1-Penten-3-ol(grassy)in white croaker was significantly higher than that in yellow croaker under PTV mode,which was related to the higher content of EPA in white croaker.The 2-Hexenal(grassy)identified by GC×GC was only identified in white croaker.It is speculated that there is a certain correlation between the results and higher n-3 fatty acids content in white croaker.1-Octene-3-one was identified only in white croaker by GC×GC,which may also be related to higher content of n-6 polyunsaturated fatty acids in it.Secondly,the lipoxygenase of white croaker were taken as the research object,the preliminary analysis of its enzymatic properties showed that the optimal temperature of LOX was 20?,the substrate concentration was 10mmol/L,and the p H value was 4.4.Under this condition,the LOX enzyme activity in small yellow croaker was significantly higher.The main product of arachidonic acid oxidation catalyzed by LOX in the two kinds of fish was 15-HETE.Although the content of 15-LOX in white croaker was higher,there was no significant difference between them.5.After extracting the bacterial DNA of white croaker and small yellow croaker during the period of fresh and cold storage,there was differences between the two kinds of fish in the perspective of bacterial diversity at the early stage of cold storage.With the extension of cold storage time,the difference is more obvious.And the results tended to be consistent with the results of odor identification.The results showed that although there was no significant difference in species composition between the two kinds of fish,the proportion of several species of shewanella bacteria,such as OTU113(unclassified?g?Shewanella),OTU23(Shewanella?baltica?OS678),OTU177(Shewanella?glacialipiscicola),was higher than that of small yellow croaker in the early stage of cold storage,which was related to the higher content of trimethylamine that could be detected in white croaker in the early stage of refrigeration.However,the proportion of OTU39(Psychrobacter cibarius)and OTU5(unclassified?g?Psychrobacter)in small yellow croaker was higher than that in whitecroaker,which might be related to the increase of trimethylamine content in the later stage of small yellow croaker.The results of environmental factor analysis showed that OTU73(g?Flavobacterium)was negatively correlated with trimethylamine and3-methylbutanol.The abundance of OTU73 in fresh small yellow croaker was higher than that in white croaker,which might inhibit the production of putrefactive odor substances.In addition,there was a significant negative correlation between OTU124(Flavobacterium?oncorhynchi)and indole.6.After steaming,most of the odor components in the small yellow croaker were much more abundant.Although the absolute peak area of the important volatiles after steaming was significantly higher than that of the fresh samples,the odor active substances remained the same or even decreased,which showed that the odor characteristics of the two kinds of fish after steaming were not as obvious as when they were fresh.However,the fatty acid content of white croaker was significantly higher than that of small yellow croaker after steaming,which was contrary to the o difference of volatiles content between them.It was speculated that this situation might be related to the higher lipoxygenase activity in small yellow croaker.