Study On The Aroma Active Compounds And Aroma Improvement During The Submerged Fermentation Of Tuber Melanosporum

The unique aroma is the most important characteristics of Tuber melanosporum fermentation system.It is made up by a series of volatile organic compounds(VOCs),the one,which be impotent to forming the aroma,is called aroma active compounds(AACs).In order to illuminating the aroma-producing process of Tuber melanosporum,it was necessary to have a research on the AACs and aroma improvement during the submerged fermentation.Therefore,gas chromatography-olfactometry(GC-O)was used to screen AACs on the combination of AEDA and OAVs.Recombination models and omission experiments were also performed to find out how these specific AACs affected the original aroma.Then,measures were tried to improve the aroma by adding the metabolic precursors of AACs to the initial fermentation medium and repeated freeze-thaweded treatment.The last,the VOCs profile of improved aroma was analysis to find out the most discriminative compounds,which leaded to the change of aroma.Combination with the analysis of amino acid,it can reveal the impotent role of key AACs on the formation of aroma.The research achievement was as following:Firstly,aroma active compounds were screened from VOCs.A total of 18 VOCs were identified by headspace solid phase microextraction combined with GC-MS technology.Then,the fermentation sample was diluted by twofold dilution levels and following analyzed with GC-O.According to the scale of FD>4 and OAV>1,five VOCs were characterized as aroma active compounds,namely 2,5-dimethyl-pyrazine,phenylethyl alcohol,3-ethyl-2,5-dimethyl-pyrazine,3-methyl-1-butanol and 3-butyl-2,5-dimethyl-pyrazine.Secondly,the quality improvement of fermentation aroma was conducted.(a)It mainly presented five aroma attributes of fermentation aroma,namely ethanol,flowery,fruity,green and sweet.In consideration of the concentration of metabolic precursor of the key AACs,different concentration of methionine,isoleucine and linoleic acid were added to the initial fermentation medium expecting to introduce sulfurous,earthy and mushroom to fermentation aroma.While,it turned out that the addition of metabolic precursor could not improve the aroma quality significance.(b)Taking metabolic reaction of the key AACs into account,repeated freeze-thaweded treatment was utilized,by which compartmentalization would be destroyed during loss of cell wall/membrane integrity thus allowing for mixing of enzymes with their substrates or with the factors that might activate the enzymes.Results indicated that the fermentation sample with 10 cycles of repeated freeze-thaweded treatment was improved significantly,accompanying with the enrichment of fermentation aroma with sulfurous,earthy and mushroom.Thirdly,the research focused on the material foundation of the aroma improvement.Repeated freeze-thaweded treatment led to the change of metabolic reaction of key AACs,then reflected in the quality improvement of fermentation aroma.A total of 40 VOCs were identified by SPME-GC-MS technology.Afterwards,6 VOCs were screened to be the most discriminative VOCs by principal component analysis and Kruskal–Wallis non-parametric N-Way ANOVA.Those were 2-methyl-1-propanol,1-octen-3-ol,3-methyl-butanal,styrene,2,5-dimethyl-pyrazine and limonene.And then metabolic network of amino acids and key VOCs was contributed based on KEGG and NIST.Four amino acids,Val,Leu,Phe and Thr,were selected to be the impotent metabolites to form these key VOCs,and then influence the fermentation aroma.