Study On Quality Formation Mechanism Of Qing Zhuan Tea Pile Fermentation Based On Microbiome And Metabolomics

The Qingzhuan tea is a kind of post-fermented pressed dark tea which is fermented by the Qingzhuan semifinished tea.The pile-fermentation process is the key stage to form its quality.Based on high-throughput sequencing technology and metabolomics,this paper studies the relationship between fungal microorganisms and flavor compounds in the pile-fermentation process of Qingzhuan tea.In order to explore the quality formation mechanism of Qingzhuan tea pile-fermentation.This research provides guidance on improving the quality of Qingzhuan tea.The main results were listed as follows:1.Through Illumina sequencing analysis,the fungal microbial community of the pile-fermentation process of Qingzhuan tea had a total of 78 OTUs,which consisted of 1phyla,9classes,15 orders,26 families,37 genera,and Ascomycota was dominant phyla.There were 11 important genera(relative abundance >1%),and the Aspergillus was the absolutely dominant strain in the pile-fermentation process of Qingzhuan tea,with relative abundance is 38.1%~75.1%.Penicillium,Cyberlindnera,Debaryomyces and Candida were found in the early stage of pile-fermentation.A large number of thermophilic genera such as Thermomyces,Rasamsonia and Thermoascus appeared in the middle stage of fermentation.Byssochlamys was the dominant genera in the middle-late stages,with the relative abundance of 57.2 % at the end of fermentation.In addition,there were two unclassified genera,Sordariomycetes_unclassified and Eurotiales_unclassified,with a relative abundance of about 1%.2.A total of 29 non-volatile compounds were identified by UPLC-QTOF-MS technology,including 3 alkaloids(caffeine,etc.),7 catechins(EGC,EGCG,ECG,etc.),9amino acids(theanine,glutamic acid,etc.),5 organic acids(gallic acid,quinic acid,etc.),5 flavones and flavone glycosides(myricein-3-o-galactosine,kaempferol,etc.),among which caffeine,gallic acid and theanine accounted for 97%,80% and 72.3% of the components respectively.Except for gallic acids and amino acids which tend to increase volatility,non-volatile compounds showed a decreasing trend in the process of pile-fermentation.At the end of fermentation,alkaloid content was the highest,amino acid content was the second highest,and organic acid content was the lowest.The decrease of theine,catechin,flavone and flavone glycosides was related to the quality of Qingzhuan tea.3.A total of 46 volatile compounds were identified by GC-MS,including 17 alcohols,9 ketones and 8 alkenes,5 aldehydes,4 esters,1 heterocyclic oxygen,1 alkane,and 1pyranoid.The change of volatile compounds have been reduced to rise trend.At the end of fermentation,the contents of volatile compounds were ketones(40.3%),alcohols(21.2%),aldehydes(12.9%),alkenes(11.5%),heterocyclic oxygen and other(7.5 %),esters(6.4 %).At the end of fermentation,the main aroma compounds of Qingzhuan tea are violet ketone series,linalool and its oxides,?-cyclocitral,safanal,?-terpinene,limonene,trans-2-hexenyl hexanoate,1,2-dimethoxybenzene,rose oxide and other compounds.4.Through PLS-DA model,VIP>1,P<0.05,20 non-volatile and 17 volatile differential metabolites were selected from the pile-fermentation process of Qingzhuan tea,among caffeine,EGC,EGCG,L-theanine,kaempferitrin,L-phenylalanine,gallic acid,myricetin 3-O-galactoside and other compounds may be important factors for the mellow taste of Qingzhuan tea.Selecting of volatile compounds in combination with the change of aroma components,the characteristics aroma components of Qingzhuan tea were mainly the woody and floral aroma of ionone,linalool and its oxides,the woody,herbal and spice aroma of safanal,the fruity aroma of ?-cyclocitral,?-terpinene,limonene,the fragrance aroma of trans-2-hexenyl hexanoate and the rose aroma of roses oxide,etc.These aroma components may be directly related to the characteristics of wood and herbal of Qingzhuan tea.5.Pearson correlation analysis shows that the flavor and aroma compounds produced a series of physical and chemical reactions such as oxidation,hydrolysis,reconstruction and polymerization under the action of high humidity and heat during the pile-fermentation process of Qingzhuan tea.Meanwhile,a large number of microorganisms,including Aspergillus,Penicillium,Yeast and Thermophilic,participate in the secretion of cellulose,hemicellulose,lignase,protease,lipase,?-glucosidase,catalase,decomposition of phenol oxygenase and a series of rich enzyme systems,directly or indirectly involved in the formation and transformation of flavor and aroma compounds,the two complement each other to form the unique quality of Qingzhuan tea.