The Effects Of Different Spreading Time On The Quality Of Tieguanyin Based On The Metabolomics Analysis Of Tea Processing

Tieguanyin,a premium variety of oolong tea,is famous for unique flower-fruit fragrance and 'Yinyun'all around the world.The lightly baked Tieguanyin,a large category in the national standard classification for Tieguanyin tea production,is divided into Zheng Wei,XiaoQing and TuoSuan according to different ZuoQing processes.However,there are few studies regarding the dynamic changes of metabolites during Tieguanyin processing.In addition,information is limited on the differences among different varieties of lightly baked Tieguanyin.This study comprehensively investigated the changes of aroma and flavor metabolites during Tieguanyin manufacture.Moreover,the effect of the Tanqing time on the quality of Tieguanyin raw tea was studied.The results may provide a theoretical basis for the standardization and improvement of the Tieguanyin manufacturing technology,and also provide supporting data for the differentiation and quality identification for different varieties of Tieguanyin.The results are as follows:1.The dynamic changes of major metabolites during Tieguanyin manufactureAt the end of each step during tea processing,a total of 13 samples were collected,including Post-Harvest,Withering,YaoQingl,TanQingl,YaoQing2,TanQing2,YaoQing3,TanQing3,TanQing4,TanQing5,Fixation,Rolling and Raw tea.Non-targeted analysis by UPLC-Q-TOF MS revealed a large variation in multiple classes of flavor-related metabolites.Principle component analysis(PCA)showed that the tea metabolome changed in a step-wise manner and were mainly divided into four groups:(1)samples prior to processing,(2)samples undergoing Yaoqing,(3)samples undergoing Tanqing and(4)samples after fixation.After screening,a total of 25 differential metabolites were identified,including flavan-3-ols,flavonol glycosides,theaflavins,amino acids,nucleosides,phenolic acids,hydrolyzable tannins,aurone glycosides,naphthopyranones,anthocyanin,and proanthocyanidins.The results showed that the contents of theaflavins increased significantly after Tanqing3(p<0.0.1).During the first two Tanqing steps,the contents of three proanthocyanidins(prodelphinidin B,EC-GC dimer and delphinidin 3-(3 "_p-coumaroylglucoside)increased.Most flavonol glycosides first decreased and then increased during the Zuoqing process.They decreased again after fixation.Amino acids,catechins and caffeine in tea samples were quantified by UPLC-TQS MS.The results showed that the total catechin content decreased from 168.95 to 118.42 mg/g(DW).The contents of EGCG,EGC and EGCG3"Me increased after Yaoqing,and decreased after Tanqing.The contents of C,EC,ECG and GC showed a downward trend.The contents of free amino acids such as theanine,glutamic acid,glutamine and aspartic acid in tea leaves tended to decrease,especially after fixation.Most of the free amino acids with low concentrations showed an upward trend with the increase of fermentation.The organic acids and carbohydrates in tea were detected by GC-TOF MS.The results showed that contents of citric acid and shikimic acid in tea showed a downward trend throughout the manufacture.After Tanqing3,the contents of fumaric acid,lactic acid,pyruvic acid,succinic acid,cellobiose,fructose 6-phosphate,glucose6-phosphate and maltose increased while the contents of fructose,glucose,galactose,raffinose,sucrose and xylose gradually decreased.After fixation,the contents of oxalic acid,malic acid,linolenic acid and linoleic acid decreased.2.Effects of the duration of Tanqing on the quality of Tieguanyin leavesHow duration of the TanQing step would affect the tea metabolome was further explored by UPLC-QTOF MS.After the end of Yaoqing3,samples were taken every three hours before fixation,and a total of seven samples were obtained.The results showed that differential metabolites included flavan-3-ol,proanthocyanidins,flavonol glycosides,theaflavins,organic acids and alkaloids.The contents of flavonol glycosides,theogallin and chlorogenic acid first decreased and then increased during the overnight Tanqing process.The changes of theaflavins,caffeine,and camellianin B demonstrated a double-humped curve that increased first and then decreased,followed by a second increase during the overnight Tanqing process.Quantification results showed that the contents of the total amino acids and the total polyphenols dropped during the overnight Tanqing by 30.01%and 13.78%,respectively.During this process,the contents of theanine,glutamine,alanine and phenylalanine decreased gradually while most of the low-level amino acids gradually accumulated.Meanwhile,glucose,galactose,fructose,raffinose succinic acid,malic acid,linolenic acid,linoleic acid and stearic acid decreased whereas maltose,lactose,rhamnose,cellobiose,fructose 6-phosphate,malonic acid,oxalic acid and lactic acid increased.3.Comparative studies of quality-related taste and aroma metabolites in three varities of Tieguanyin raw teaAfter 10,16 and 22 hours of overnight Tanqing,three different varieties of Tieguanyin raw tea including Zheng Wei,XiaoQing and TuoSuan Tieguanyin were produced.Their tastes and aromas were compared.With the increased duration of Tanqing,the contents of total polyphenols(p<0.05)and total amino acids(p<0.01)increased significantly.The ZhengWei Tieguanyin contained higher indole,jasmonide and methyl salicylate,which showed floral fragrances.The XiaoQing Tieguanyin mainly contained ?-farnesene,?-ocimene and jasmonide,showing special floral and fruity notes.Phenylethyl alcohol,(Z)-3-hexen-l-ol and most of the esters were characteristic aromas in the TuoSuan Tieguanyin,which exhibited fruity and strong green-grassy odors.In summary,the contents of amino acids,organic acids,tea polyphenols,sugars,flavonoid glycosides,and theaflavins changed significantly during the whole manufacture process of Tieguanyin,and the overnight Tanqing and the fixation steps have significant effects Tieguanyin tea metabolome transformations.In addition,the extension of the overnight Tanqing time has greatly changed the content of total amino acids,total polyphenols,sugars,organic acids,flavonoid glycosides,theaflavins and procyanidins,three raw teas with the increased duration of Tanqing have their own characteristic profiles related aromas and tastes.Therefore,the overnight Tanqing stage provides suitable temperature and time for the transformation of characteristic tastes and aromas in Tieguanyin raw tea,which is extremely important for the quality formation in Tieguanyin.