| The production of Zhangping Shuixian tea is mainly according to the canonical processing procedures of southern Fujian oolong tea,and the tea shoots are compressed into square tea cakes using wooden molds.Zhangping Shuixian tea is the only compressed oolong tea,which is famous for its unique flavor with rich aroma,mellow and fresh taste.However,the flavor development of Zhangping Shuixian tea is still unclear,and the formation of premium tea is unstable to date.In this study,metabolomic approaches were used to detect the accumulation of non-volatile and volatile components in tea samples during the withering and turning-over steps of of Zhangping Shuixian tea production.The Etongue and E-nose technology were applied to analyze the dynamic changes of taste and aroma attributes,in order to reveal the flavor development during withering and turning-over during the Zhangping Shuixian tea manufacturing process steps,and provide the foundation for the improvement of Zhangping Shuixian tea processing technique.The major results of this study are as follows:1.The impact of withering procedure on the flavor constitutes of Zhangping Shuixian teaThe widely-targeted metabolomic determination was carried out to identify and quantify the non-volatile components of withered leaves under sunlight(SW),indoor withered leaves(IW)and fresh leaves(FL).The principal components analysis(PCA)and orthogonal projection to latent structures discriminant analysis(OPLS-DA)were performed to compare the significant changed metabolites in FL,IW,and SW samples.These results indicated that the constitutes in the withered tea samples(SW and IW)were dramatically distinct from those in the fresh leaves(FL),the plucked tea leaves suffered from heat,dehydration,or solar irradiation,resulting in the changes in the intracellularly biological processes and metabolites accumulation.The abundance of flavonoids such as apigenin,naringenin,myricetin and luteolin in the withering samples(SW and IW)was significantly lower than that in the fresh leaf samples(FL),which could be ascribed to the oxidation caused by the adverse external environment over withering,contributing to theconsump-tion of flavonoids as antioxidant.The decreasing of flavonoids reduced the bitter taste and improved the quality of Shuixian tea.The contents of organic acids such as jasmonic acid,methyl jasmonate.2-hydroxyisobuty-ric acid and 2-methylglutaric acid in SW and IW were significantly higher than those in FL,and the increase of organic acid content was conducive to the formation and transformation of taste attribute of Zhangping Shuixian tea.GC-MS/MS was used to detect volatile components in SW,IW and FL samples.The result of OPLS-DA showed that there was a significant separation of the sample on the first principal component.The significant different metabolites in SW/IW showed increasing trends,and the hierarchical cluster analysis indicated that ester volatiles are the most abundant compounds in SW/IW,potentially accounting for the floral and fruity aromas.The contents of hexyl acetate,hexyl butyrate and hexyl caproate in SW was higher than that of FL.The accumulation of hexyl esters in tea leaves are probably induced by ultraviolet irradiation over sunlight withering step,contributing to the enrichment of fruity odor in the leaves of SW sample.2.The impact of withering and turning-over on the flavor constitutes of Zhangping Shuixian teaThe non-volatile components of Zhangping Shuixian tea in the withering and turning-over steps were determined by UPLC-MS/MS,and the taste attributes were measured by electronic tongue(E-tongue).A total of 980 metabolites were identified,mainly including phenolic acids,amino acids and their derivatives,organic acids,flavonoids,nucleotides and their derivatives,flavonols,free fatty acids etc.The analysis on these metabolites showed that:(1)the number of significant different metabolites in PL/CK group showed an increasing trend along with the progress of the Zhangping Shuixian tea production;(2)lipid compounds accounted for a large proportion in PL/CK group;(3)the oxidative polymerization of catechins were accelerated during turning-over steps,and resulted in the increasing contents of theaflavins(TFs).The analyses on paried-comparisons showed that the contents of TFs such as theaflavin(TF1),theaflavin-3-gallate(TF-3-G),and theaflavin-3’-gallate(TF-3’-G)over turning-over groups were dramatically higher than the counterparts in control groups,contributing to the flavor improvement of Zhangping Shuixian tea;(4)the result of E-tongue detection indicated that the astringent intensity was alleviated after withering and turningover steps,which was consistent with the results of UPLC-MS/MS analysis.The content of epicatechin-3-O-(3,5-O-dimethyl)gallate was decreased in the BF comparing to that in CK5,leading to lowering of bitter taste intensity of Zhangping Shuixiantea.GC-MS/MS combined with electronic nose(E-nose)determinations were performed to analyzed the dynamic changes of volatile metabolites and flavor of Zhangping Shuixian tea over the withering and turningover process.A total of 157 metabolites were identified,mainly including esters,terpenes,aromatic hydrocarbons,and alcohols etc.The analysis on these metabolites indicated that:(1)the number of significant different volatiles in the paired-comparisons showed an increasing trend along with the progress of Zhangping Shuixian tea manufacturing;(2)the content of trans-nerolidol in the turning-over process was significantly higher than that in withering procedure,which was probably attributed to the continuous mechanical damage and low temperature stress accelerated the accumulation of trans-nerolidol;(3)the intensity of S4 sensor of the E-nose detection was higher than that of other sensors,indicating that the content of alcohols and organic odors were enriched over the withering and turning-over steps,agreed with the variation of alcohols measured by GC-MS/MS. |
