| Sour fish is one of the unique traditional food culture representatives of Dong nationality.Industrial production and promotion is an important direction for the future development of sour fish.The promotion and industrial production of sour fish require stable commodity quality and outstanding aroma characteristics.In order to realize industrial production,three dominant strains were screened from sour fish in southwest China in the early stage: Lactobacillus plantarum,Staphylococcus xylosus and Saccharomyces cerevisiae.Inoculating the three dominant strains into the fermentation system can significantly shorten the fermentation time,improve the safety of sour fish,and lay a foundation for the industrial production of sour fish.In order to make the aroma characteristics of sour fish more prominent,the laboratory first studied the volatile compounds of sour fish,and found that they were mainly esters,aldehydes,alcohols,acids,ketones and other substances.It was clear that esters played an important role in the flavor composition of sour fish.Although the flavor composition of sour fish has been preliminarily understood,the flavor profile of sour fish and the specific esters that have an important impact on the aroma of sour fish are not clear,and it is difficult to provide theoretical guidance for the enhancement of flavor characteristics.Secondly,it was found that yeast,like Saccharomyces cerevisiae,played an important role in the aroma of sour fish,especially the formation of esters.Therefore,the purpose of strengthening the flavor properties could be achieved by replacing Saccharomyces cerevisiae with aroma-producing yeast.The screening of the original strains is mainly based on the southwest region,and there are geographical limitations.The application of commercial aroma-producing yeast can get rid of this limitation.Therefore,this paper first identified the key aroma active substances in sour fish,explored the effect of the introduction of commercial aroma-producing yeast strains on the aroma of sour fish,and explored the relationship between aroma-producing yeast and flavor improvement.This study is of great significance for revealing the relationship between the key aroma active substances of sour fish and the aroma-producing yeast and the flavor improvement of sour fish and provides a basis for targeted regulation and guidance of sour fish flavor.The main research contents were as follows :(1)The volatile compounds of fermented sour fish were identified by Headspace SolidPhase Microextraction-Gas Chromatograph-Mass Spectrometer(HS-SPME-GC × GC-MS).The odor active value(OAV)was calculated to screen out the key odor active compounds.The key characteristic aroma substances of fermented sour fish were determined from the key odor active compounds by constructing a flavor omission and flavor recombination model.The results showed that 88 volatile compounds were identified by GC-MS,and 16 key odor compounds(OAV > 1)were screened out by OAV.Through quantitative description analysis,the sensory attributes of sour fish were determined to be fruity,floral,sweet,mushroom,grassy,fatty,meaty and soy sauce-liked.The recombinant model constructed by water and the recombinant model constructed by odorless matrix were analyzed by electronic nose.The results showed that the odorless matrix model was closer to the original sample.Thus,the odorless matrix was used to construct the aroma recombination model in subsequent experiments.The sensory score of the recombinant model constructed by the flavor substance standard with odor activity value greater than 1 was 4.5(full score 5),which could better simulate the aroma of the sample.The flavor substances with odor activity values greater than1 were removed one by one or in groups to construct an aroma omission model.The results showed that there was no significant difference between the omission model of ethyl heptanoate,ethyl palmitate,ethyl valerate,1-octen-3-ol,hexanol and isoamyl alcohol and the flavor recombination model(P > 0.05).There was a significant difference between the omission model of ethyl acetate,ethyl hexanoate,isoamyl acetate,ethyl butyrate,hexanal,hexadecanal and 2-pentylfuran and the flavor recombination model(P < 0.05),indicating that the above seven substances were the key characteristic aroma substances of sour fish.(2)Preliminary screening of commercial aroma-producing yeast was carried out according to the growth and environmental tolerance.The fish simulation system was constructed to simulate the fermentation of the initially screened yeast.The microbial growth during the fermentation process,the total acid content in the system,the amino acid content at the end of the fermentation,and the volatile compound content were used as indicators to screen out the aroma-producing yeast suitable for sour fish fermentation.The results showed that Hansenula anomala(Ha),Pichia kudriavzevii(Pk),Pichia fermentans(Pf),Wickerhamomyces anomalus(Wa)and Candida krusei(Ck)could reach the stationary phase within 24 h.Saccharomyces rouxii(Sr)had a long growth lag period and a low colony number.The first five yeasts were selected for acid and alcohol tolerance experiments.The results showed that when the p H was about 4.0,the growth rate of Ck,Ha,Pf,Pk and Wa was about 90% compared with the blank group(original medium).There was no significant difference in acid tolerance(P > 0.05).Compared with the blank group,the growth rate of Ha and Pf was about 90% when the ethanol concentration was 1%～3%.The growth rate of Ck was about 60% under this condition,and the growth rate of Pk and Wa was lower than 60%.There were significant differences in the number of colonies between Ck,Ha and Pf in the same fermentation time(P < 0.05),but there was no difference in the number of colonies.There was a significant difference in total acid content(P< 0.05).There was no significant difference in total amino acid content(P > 0.05).There was a significant difference in the total content of volatile substances(P < 0.05),with the highest content in Pf group,followed by the Ha group,and finally the Ck group.Indicating that Ha and Pf can better adapt to the sour fish environment and have aroma enhancement potential.In summary,Ha and Pf were preferred aroma-producing yeasts.(3)The screened aroma-producing yeast was inoculated into the sour fish,and the Sc inoculation group was used as the control group.The effects of aroma-producing yeast on the aroma of sour fish were studied by p H,microbial quantity,organic acids,amino acids,volatile aroma substances,sensory analysis and alcohol acyltransferase activity during fermentation.The results showed that the electronic nose could accurately distinguish the difference between Ha group and Pf group and control group(Sc).Gas Chromatography-Ion Mobility Spectrometry(GC-IMS)identified differences in the types and contents of volatile compounds among the three groups.The volatile compounds were extracted by HS-SPME.The results of GC-MS analysis showed that there were 89,97,83 volatile compounds in Pf group,Ha group and Sc group,respectively.The total concentrations of volatile compounds were 19948 μg/kg,8463 μg/kg and 4277 μg/kg,respectively.The total concentrations of esters were 12495 μg/kg,4235 μg/kg and 1812 μg/kg,respectively.The contents of ethyl acetate,isoamyl acetate and ethyl caproate in Pf group were 3-7 times higher than those in Sc.The contents of ethyl acetate,isoamyl acetate and ethyl caproate in the Ha group were 2-5 times higher than those in Sc.Indicating that the introduction of Pf and Ha increased the content of volatile compounds in sour fish in varying degrees.The results of sensory analysis showed that compared with the control group,the sauce aroma in Pf group was more prominent,and the flower and fruity aroma in Ha group were more prominent.During the fermentation process,the number of microorganisms in each group showed a downward trend.The number of the same type of microorganisms in different groups remained at the same order of magnitude at the end of fermentation.The trend of water content in each group was the same.There was no significant difference between the groups at the end of fermentation(P > 0.05).The p H of each group decreased first and then stabilized.At the end of fermentation,the p H of Ha group was higher than that of Sc group and higher than that of Pf group.There were significant differences among the groups(P < 0.05).The trend of organic acid content in each group was the same.At the end of fermentation,there was a significant difference in organic acid content among the groups(P< 0.05).The Pf group was higher than Ha group.The trend of free fatty acids in each group was similar.At the end of fermentation,the Pf group was the highest,followed by the Ha group.There was a significant difference between the groups(P < 0.05).The trend of alcohol acyltransferase activity in each group was the same.At the end of fermentation,there was a significant difference in alcohol acyltransferase activity between the groups(P < 0.05),Pf group was higher than Ha group and Sc group.In summary,the results showed that the inoculation of Ha and Pf could actively improve the aroma of sour fish. |
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