Multi Omics Analysis On The Dynamic Changes Of Taste Substances In Black Tea Fermentation

Black tea is one of the six major types of tea,and is favored by consumers for its unique flavor quality and special health benefits.According to different processing techniques and quality characteristics,black tea can be divided into Souchong black tea,broken black tea,and Congou black tea.All types of black tea need to go through the important process of fermentation.Fermentation is very important for the formation of taste and aroma quality of black tea.At present,the research on black tea fermentation mostly focuses on the change of fermentation essence,taste quality components and process optimization,and pays less attention to the subtle change and mechanism of taste quality components in the process of black tea fermentation.This study combined sensory evaluation,physicochemical analysis and multi-omics analysis to reveal the dynamic changes of taste and quality components of black tea during fermentation.The main results are as follows:1.With the progress of fermentation,the sensory quality and the content of fermented leaves changed significantly.The color changes from yellow-green to yellow-red,the color of the tea soup changes from orange-yellow to bright orange-red,and the taste changes from green and astringent to fresh and sweet,thus forming a fresh,sweet and mellow taste;the aroma changes from green to sweet and floral;Green to red evenly soft bright.From 0 h to 4 h,the total catechin content was 7.80%(0 h)～ 2.15%(4 h),and the total catechin content decreased by 272%.The contents of TF,TFDG,TG and T’G reached the highest at 2 h of fermentation,and all four components decreased to different degrees after 4 h of fermentation.The content of tea polyphenols decreased significantly in the 0 ～ 2 h fermentation period,while the content decreased slowly during the 2 ～ 4 h fermentation period.The content of soluble sugar decreased slowly during 2 h to 4 h fermentation.The content of amino acids and total flavonoids did not change significantly.The changes of caffeine and theobromine in the process of black tea fermentation were roughly the same.The former had different changes during 2 h to 4 h,while the latter appeared during 0 h to 4 h.2.Metabolomics-based liquid chromatography-mass spectrometry(LC-MS/MS)was used to detect samples at different fermentation stages,and a total of 554 non-volatile metabolites were identified,including 80 amino acids,peptides and their analogs,nucleosides,48 kinds of nucleotides and their analogs,106 kinds of flavonoids and flavonols(glycosides),17 kinds of flavanols and dimer flavanols,21 kinds of alkaloids and derivatives,and 46 kinds of carbohydrates and carbohydrate conjugates,9 kinds of vitamins,92 kinds of organic acids and their derivatives,135 kinds of lipids and lipid-like molecules.The results of differential metabolite analysis showed that the content of lipids and lipids changed drastically during the fermentation process with FJ2 h as the node;most of the organic acids showed an upward trend,especially oxalic acid,citric acid,succinic acid,malic acid and other components.A total of 19 kinds of carbohydrates with different contents were identified,and their content changes showed two different trends,one was significantly reduced after 2 h of fermentation,and the other was significantly increased after 2 h of fermentation;The contents of catechins and procyanidins showed a significant downward trend,while the content of theaflavins increased significantly(p<0.05);flavonoids and flavonols(glycosides)showed irregular changes;Significantly increased,and the content of a large number of nucleotides and their derivatives also changed significantly.Combined with the changes in sensory quality and the results of metabolome testing,differences in lipids,organic acid content,flavonoids,amino acids,sugars and other substances may affect the quality of black tea.In addition,the material changes in the pre-fermentation stage were obvious,which suggested that the pre-fermentation stage may be the key stage in the formation of black tea quality.3.Based on the results of proteomic detection,there were differences in protein expression patterns at different stages during the fermentation process,and the protein changes of FJ2 h ～FJ4h were more severe than those of FJ0 h ～ FJ2 h.The expression levels of differential proteins involved in the biosynthesis of flavonoids decreased significantly,while the expression levels of oxidoreductases such as peroxidase and polyphenol oxidase were significantly up-regulated;the expression levels of differential proteins related to lipid metabolism were up-regulated after2 h of fermentation;Most proteins involved in amino acid metabolism were down-regulated;more differential proteins related to carbohydrate metabolism were enriched,and important enzymes involved in carbohydrate decomposition pathways,and their expression increased from 0 h of fermentation to 2 h after fermentation;only screening four differential proteins related to organic acid metabolism were found,among which the enzymes regulating malate metabolism began to be up-regulated in the early stage of fermentation.4.The samples at three fermentation time stages of 0 h,2 h and 4 h were analyzed by transcriptome technology,and the number of differential genes showed a decreasing trend with the extension of fermentation time.In FJ0 h vs FJ2 h,2416 differential genes were screened,of which 1036 genes were up-regulated and 1380 were down-regulated;1863 differential genes were screened in FJ2 h vs FJ4 h,436 genes were up-regulated and 1427 genes were up-regulated.because of down-regulated expression.The number of differential genes in the FJ0 h vs FJ4 h group was 8305,and the number of differential genes decreased with the extension of fermentation time.5.Combined with multi-omics analysis,the content of flavonoids decreased significantly during the fermentation process.On the one hand,the synthesis was blocked due to the decrease in the expression of related proteins,and on the other hand,due to the significant increase in the expression of peroxidase and other proteins,flavonoids oxidation of the substance results in a decrease in the content.Carbohydrates are regulated by proteases in the fermentation process,and hydrolase metabolism is active,resulting in the hydrolysis of macromolecular substances into soluble monosaccharides or disaccharides,which accumulate.The expression of enzymes regulating the metabolism of organic acids is relatively stable.The expression of most of the proteins involved in amino acid metabolism decreased,suggesting that the anabolism of amino acids in black tea was reduced during fermentation,and amino acids could not generate new amino acids through biosynthesis.Only a small number of differential proteins and genes related to lipid metabolism were detected,and metabolome detection showed that their metabolism was active,suggesting that the mechanism of lipid changes during black tea fermentation is complex.