Effects Of Baking On The Volatile Compounds Of Oolong Tea

Oolong tea, the unique tea to china is one of China’s major tea, loved by consumers for its lofty fragrance aroma, mellow taste. Aroma is the soul of the tea, and it is also the most important indicator to evaluate the quality of Oolong tea. Tea breed is the foundation of Oolong tea’s aroma, withering process, Zuoqing process, baking process and other process generated new aroma compounds and developed the original aroma. Baking is the final step among all Oolong tea processing, and baking is also the critical step to form good quality of Oolong tea.The Tieguanyin and Dahongpao are the best in southern Fujian Oolong tea and north Fujian Oolong tea. So we choose Tieguanyin and Dahongpao as our test materials to study the influence of different baking conditions on Oolong tea aroma compounds. Using solid phase micro extraction(SPME) to extract volatile compounds from Oolong tea and the volatile compounds were analyzed by GC-MS. The Maillard reaction is one of the most important chemical reactions that occur in the tea roasting process. Sioulate the major Maillard reaction occurred during baking process of Oolong tea, to explore the contribution of Maillard reaction of Oolong tea volatile compounds, and achieved the following results:(1)56volatiles are identified in the16Tieguanyin samples after baking processing under different baking condition, including11alcohols,17esters,6ketones,6aldehydes,5olefin and11heterocyclic compounds. Judging from the category, nitrogen-containing heterocyclic, alcohols and esters is the major volatiles of Oolong tea. Geranyl nitrile,6-methyl-5-heptene-2-one, Linalool oxide I, Cis-3-hexenyl isobutyrate, benzaldehyde, β-linalool,3,7-dimethyl-1,5,7-oct-triene-3-ol, Benzeneacetaldehyde,(Z)-Hexanoic acid,3-hexenyl ester,β-famesene, a-Bergamotene, a-farnesene, methyl salicylate,2-phenylethyl alcohol, benzyl nitrile, cis-jasmone,(E)-nerolidol and indole are the main volatiles of Tieguanyin. Differences exist on the composition and content of volatiles between different Tieguanyin samples under different baking conditions.(2) Multivariate statistical analysis was performed on the volatile compounds of Tieguanyin samples using PCA, according to PCA we found out the first8principal explained94.91%of the total variation. The8principal were represented by the following volatile compounds:β-linalool, methyl salicylate, benzaldehyde, benzyl nitrile, cis-jasmone,2H-Pyran-2-one, tetrahydro-6-pentyl-,β-farnesene, N-ethylpyrrole, furfural,1-ethyl-2-formyl pyrrole,2(3H)-Furanone,5-ethyldihydro-,3,7-dimethyl-1,5,7-octtriene-3-ol, indole, palmitic acid methyl ester.(3) Compare all16Tieguanyin samples, we come to the result that sample J and sample L gets the highest score on the principal2, corresponding to pyrrole, pyrazine and furans which presented roasty aroma, it can be concluded that baking with high temperature contributed to roasty aroma. Sample A is characterized by PC1and PC2negative values and received the lowest PC3and PC4values, indicated that the relative content of volatile compounds influenced4PCs of sample A is low, sample A doesn’t have obvious aroma characteristics. High similarity of all the PCs is observed between the original sample X and sample O, resulting that after baking process the volatile compounds of sample O have no difference with the original sample X, so as their aroma.(4)Evaluate all16Tiefuanyin samples with Three-digit random codes sensory evaluate method, and stepwise regression analysis was performed on aroma score, it proved that baking time and baking temperature have a significant impact on the aroma score of Tieguanyin,Tieguanyin samples with high temperature and long time baking received a high aroma score during sensory evaluation.According with the result of PCA, we may come to a conclusion that during oolong tea sensory evaluation,oolong tea sample with prominent roasty aroma will gain a high aroma score.(5) SPME method was used to extract the volatile compounds from3Dahongpao tea samples which were baked with different times.53volatiles were identified by GC-MS including nitrogen-containing heterocyclic, ketones, aldehydes and alcohols. The main compounds of Tieguanyin are1,5,7-Octatrien-3-ol,3,7-dimethyl-,1-ethyl-1H-pyrrole, trans linalool,(E)-nerolidol,6-methyl-5-heptene-2-one, 1-ethyl-1H-pyrrol-2-formaldehyde, hexanal,(+)-beta-linalool. Among them the content of3,7-dimethyl-1,5,7-Octatrien-3-ol is the highest in the three samples,and it is the most important compound of Dahongpao which present orange aroma.(6) With the increase of baking times some compound such as (E)-2-hexenal,(E)-4-heptenal,2-octanone,3-heptene-2-one,α-phellandrene with fruity aroma and2-acety pyrrole with popcorn flavor disappeared. And6-methyl-3,4-dihydro-2H-pyran,2-hexene aldehyde, o-xylene,1,2-Ethanediol,1,2-di-4-pyridinyl-,3-hexenoic acid, Pentanoic acid,3-ethyl-1,4-hexadiene, Cyclohexanone,2,2,6-trimethyl,4-acetyl-3-vinyl and Ethyl4-(ethyloxy)-2-oxobut-3-enoate increased with baking times. The content of Maillard reaction production with baking aroma like1-ethyl-lH-pyrrole, furfural,1-2-furyl-ethanone,2,5-dimethyl pyrazine,3-ethyl-1,4-hexadiene,l-ethyl pyrrole-2-aldehydes,2,5-dimethyl-3-ethyl-pyrazine, N-furfuryl pyrrole showed an increasing trend. Other compounds with floral and fruity like6-methyl-5-heptene-2-one,(E,E)-2,4-heptadienal, phenyl acetaldehyde, trans-linalool,(E)-linalool oxide (furanone),(+)8-linalool,3,7-dimethyl-1,5,7-octtriene-3-ol, indole,(E)-nerolidol,4-acetyl-3-ethyleneethyl showed a decreasing trend, changes in the content of these compounds makes the aroma of Dahongpao change to roasty gradually.(7)18Volatiles were characterized from Maillard reaction between L-theanine and D-glucose:3pyrazine,6furans,5pyrrole,2pyridine,1pyrimidine and1pyran. Where these two compounds3,4-dimethyl-3-pyrrolin-2-one,5-methyl-1,2,5,6-tetrahydro-pyridin-2-one were the main products of these reaction.(8) Among56volatiles1-ethyl-pyrrol-2-aldehyde and2,5-dimethyl pyrazin these two compounds is produced by the Maillard reaction between L-theaine and D-glucose. In the Dahongpao samples there are6reaction products:methyl pyrazine,2,5-dimethylpyrazine,2-furan-ethanone,5-methyl-2-furanmethanol,1-ethyl-pyrrol-2-aldehyde,2-acetyl pyrrole,2-acetylpyrrole. All these volatiles given the Dahongpao and the Tieguanyin caramel and baking fragrant.