不同茶类中挥发性萜类化合物的对映异构体

【目的】挥发性萜类化合物是茶叶香气的重要成分,大多数具有手性结构,存在香气特性完全不同的对映异构体。查明不同茶类中挥发性萜类化合物的立体构型分布特征,为深入研究茶叶香气的形成机理及提高茶叶的香气品质提供科学的理论依据。【方法】采用顶空固相微萃取法结合手性气相色谱-质谱联用技术,以Supelcoβ-DEX110为手性气相色谱柱,以萜类化合物的不同手性异构体标准品为定性和定量分析的依据,建立茶叶中挥发性萜类化合物的对映异构体分析方法,并对代表性绿茶、红茶、乌龙茶、白茶及黑茶样品中萜类化合物的对映异构体组成及含量进行分析。【结果】建立了9种萜类化合物的对映异构体的检测方法,发现所有茶样中均能检测到芳樟醇、芳樟醇氧化物B的对映异构体:大部分茶样中芳樟醇的主导构型为S型,但英德红茶、云南滇红和印尼白茶中以罕见的R-(-)-芳樟醇为主;芳樟醇氧化物B在绿茶、红茶、白茶及黑茶中的构型完全一致,而部分乌龙茶(岭头单枞、文山包种)中该物质的主导构型与之相反。芳樟醇氧化物A在绿茶及黑茶中主导构型一致,而在祁门红茶、印度大吉岭红茶、水仙乌岽单枞及铁观音中构型相反;芳樟醇氧化物C和D在所有茶样中的构型均保持一致,且大部分仅检测到单一构型。其他重要萜类化合物在茶样中分布情况各异:α-蒎烯仅在正山小种红茶中以高比例的S-(-)-α-蒎烯被检测出;α-松油醇的主导构型及对映异构体过量值在不同种类的茶叶中各不相同;4-萜品醇在所有可测得的茶样中均以R构型为主导,且除六堡茶(黑茶)外,其他茶样中均未检测到S-(+)-4-萜品醇;α-紫罗兰酮在除了绿茶以外的茶样中均能被检测到,且均以单一的R构型存在。定量分析结果表明,芳樟醇在大部分茶样中都具有最高的含量,平均含量白茶红茶乌龙茶绿茶黑茶;芳樟醇氧化物B的含量在茶叶中往往仅次于芳樟醇,呈现红茶白茶乌龙茶黑茶绿茶的规律性;而其他萜类化合物在茶样中的含量普遍低于100ng·g~(-1),且在不同茶类中含量分布迥异。【结论】明确了不同种类茶叶中9种重要挥发性萜类化合物对映异构体的变化情况,有助于从更深层次上了解茶叶香气品质的化学实质,为今后茶叶香气品质调控、茶叶品种、产地判别以及质量认证等提供了新思路。

Enantiomeric Analysis of Volatile Terpenoids in Different Teas

【Objective】 Volatile terpenoids are important components of tea aroma, and most of them contain chiral carbon structure which lead to the existence of corresponding enantiomers with different aroma characteristics. Identifying the terpenoid enantiomers among different teas can result in a better understanding of the mechanisms underlying tea aroma formation for tea quality improvement. 【Method】 An analytical approach for enantiomeric analysis of volatile terpenoids in tea was firstly established by headspace solid phase microextraction combined with chiral gas chromatography-mass spectrometry. The chiral capillary column of Supelco β-DEX110 was used for terpenoid enantiomer analyses with corresponding authentic standards. And the compositions of enantiomers of terpenoids in representative green teas, black teas, oolong teas, white teas and dark teas were then investigated. 【Result】 Nine pairs of terpenoids enantiomers were successfully separated. The data of experiment indicated that linalool and linalool oxide B enantiomers were detected in all tea samples with high contents: S-linalool was considered as the major configuration of linalool in most kinds of teas, except for Yingde and Yunnan black teas and Indonesia white tea, which contained R-(-)-linalool as the major configuration of linalool; linalool oxide B presented the same stereo configuration in green tea, black tea, white tea and dark tea, whereas in some oolong teas (Lingtou Dancong and Wenshan Baozhong) a higher proportion of opposite configuration existed. Linalool oxide A was detected in green and dark teas with the same major stereo configuration, while it presented reverse major configuration in some black teas (Keemun and Darjeeling black teas) and oolong teas (Shuixianwudong Dancong and Tieguanyin). The single and consistent configurations of linalool oxides C and D were observed in most tea samples. The other important terpenoids distributed in various situations among different tea samples: α-pinene was only detected in the Lapsang Souchong tea, and the major configuration was S. different configurations and various enantiomeric excess values of α-terpineol were observed in different teas. R-(-)-4-terpineol was the major configurations of 4-terpineol in all tea samples, and S-(+)-4-terpineol could only be detected in the Liu Pao tea (dark tea), and a single R-configuration of α-ionone could be detected in most teas (except green teas). The quantitative results indicated that the highest contents in tea aroma of most tea samples were contributed by linalool, and the ranking order of mean contents of linalool was white tea>black tea>oolong tea>green tea>dark tea. the contents of linalool oxide B frequently second to those of linalool, and the ranking order of mean contents was black tea>white tea>oolong tea>dark tea>green tea. the contents of other terpenoids were generally lower than 100 ng·g^-1, and the content distributions were widely different among various tea samples. 【Conclusion】 This study investigated the content variations of the nine pairs of important terpenoid enantiomers in different kinds of tea for the first time. The present research result could contribute to get a deeper understanding of the chemical substances of tea aroma, and provide a new thought for the quality control of tea aroma, the discrimination of tea cultivars and origins and quality certification of tea.