An automatic thermal desorption-gas chromatography-mass spectrometry (ATD-GC-MS) method was developed, optimized and applied in combination with chemometrics for the detection of the aroma components of three different grades (superfine, grade A and grade B) of Rougui oolong tea. In automatic thermal desorption, the operating parameters adsorption temperature, adsorption time and cold trap temperature were optimized by single factor experiments and response surface methodology. The optimal parameters were determined as follows: adsorption temperature of 55 ℃, adsorption time of 37 min and cold trap temperature of -29 ℃. A total of 173 aroma components were identified, 90 of which were common to all the three grades. In addition, a partial least squares discriminant analysis (PLS-DA) model to distinguish the different grades of Rougui oolong tea was established based on the peak areas of the aroma components of Rougui oolong tea. and the distribution of 47 characteristic aroma components in the three different grades of Rougui oolong tea was discussed by hierarchical cluster analysis. The results of this study could provide a theoretical basis for the identification of the aroma quality and grade of Rougui oolong tea.

The sensory quality of six brands of strong and light aroma-type Anxi Tieguanyin tea (three batches each) was evaluated by sensory testing, and their chemical quality was analyzed by gas chromatography-mass spectrometry (GC-MS) combined with multivariate statistical analyses including principal component analysis (PCA), partial least square (PLS) and hierarchical cluster analysis (HCA). The results showed that there were differences in sensory quality among the six brands. Moisture, free amino acids and flavonoids were the key components contributing to the differences in taste among tea products. Light aroma-type tea products A and B had high contents of moisture and free amino acids, and tasted clean, fresh and mellow. The moisture content, free amino acids and flavonoids of strong aroma-type tea products C and D were in the middle and they tasted mellow and relatively thick. Strong aroma-type tea products E and F had high content of flavonoids, and tasted thick, heavy and mellow. These differences in flavor components could mainly be affected by the baking process. Indole, jasmine lactone and cis-3-hexenyl caproate were the key components that determined the aroma characteristics of different tea products. The relative contents of indole, jasmine lactone and cis-3-hexenyl caproate in light aroma-type products A and B were higher, and they had a clean and high aroma. Product A had more esters with a green odor than product B. The contents of indole and jasmine lactone in strong aroma-type products C and D were the lowest, and their aroma was relatively strong. The contents of indole, jasmine lactone and cis-3-hexenyl caproate in product D were slightly higher than those in product C, so the aroma was better. The contents of indole and jasmine lactone aroma components in strong aroma-type products E and F were in the middle, and the baking degree was slightly higher, so the aroma was strong and rich with flowery and fruity fragrance, and product F had more aldehydes which may be responsible for the fragrant aroma with fruity and flowery notes of product F. The difference in aroma components could mainly be affected by raw materials and the baking process. This study is expected to provide some theoretical basis for the fine production, quality management and flavor innovation of Anxi Tieguanyin tea products.

Automatic thermal desorption-gas chromatography-mass spectrometry (ATD-GC-MS) was used to capture and detect environmental volatile components in Tieguanyin tea at the key stages of the fine manipulation process, and short time series expression miner (STEM) and partial least squares-discriminant analysis (PLS-DA) were used to analyze the volatile components. The results showed that a total of 122 volatile components belonging to eight chemical classes, i.e., alcohols, esters, olefins, alkanes, aldehydes, aromatic hydrocarbons, ketones and other compounds were identified, esters being the major ones. The STEM results showed that the detected volatile components could be divided into 19 trends, and there was significant enrichment in two trend models. Among them, the levels of 2-butanol, ethyl acetate, isovaleraldehyde, 3-methyl-1-butanol, 1-hexanol, 2-heptanol, methyl hexanoate, 6-methyl-5-hepten-2-one, and octyl isobutyrate, which have a special aroma, increased significantly with increasing number of fine manipulation cycles. PLS-DA could effectively distinguish between tossing and sun withering, and seven characteristic volatile components (variable importance in the projection (VIP) > 1) were common to the two stages including (Z)-4-hexen-1-yl acetate, (E)-butanoic acid, 3-hexenyl ester, (E)-3-hexen-1-ol, 2-hexenal, 3-methyl-butanenitrile, (E)-4,8-dimethylnona-1,3,7-triene and (Z)-3,7-dimethyl-1,3,6-octatriene. The proportion and composition of environmental volatile components in Tieguanyin tea changed during the fine manipulation process. Most of the volatile components showed obvious and regular changes. The seven shared volatile components could be considered as aroma indexes for judging intelligent fine manipulation.

In this study, the contents of (Z)-3-hexenal and (E)-2-hexenal during oolong tea processing were measured and four (3Z):(2E)-hexenal isomerase (HI) genes were selected based on transcriptomic data. Meanwhile, the correlation between the changes of (Z)-3-hexenal and (E)-2-hexenal contents and related gene expression was analyzed. The results indicated that during oolong tea processing, one of the two compounds fell, while the other rose. Mechanical damage caused by tossing increased and reduced the contents of (Z)-3-hexenal and (E)-2-hexenal, respectively. Subsequent spreading contributed to the transformation of (Z)-3-hexenal into (E)-2-hexenal, resulting in an increase in the content of (E)-2-hexenal. The four selected genes all responded to mechanical stress and water deficit stress. The constructed phylogenetic tree indicated that CsHI was closely related to many germin-like proteins in plants such as tea (Camellia sinensis) and carrot (Daucus carota). This study provides a reference for clarifying the formation and transformation mechanism of volatile substances during oolong tea processing and improving the quality of oolong tea.

Stir bar sorptive extraction (SBSE) combined with gas chromatography-olfactory-mass spectrometry (GC-O-MS) was used to identify and describe the key aroma components of three representative oolong tea varieties, Huangdan, Tieguanyin and Jinguanyin. Comparative analysis was conducted in terms of odor activity value (OAV), aroma character impact (ACI) value and sensory evaluation. The sensory evaluation showed that each variety showed obvious aroma characteristics. Huangdan oolong tea had an obvious floral aroma as well as a slight milky aroma. Tieguanyin oolong tea had a strong floral aroma. Jinguanyin oolong tea had a sweet fruity aroma as well as a slight woody aroma. According to the results of OAV and GC-O-MS analysis, geraniol, phytol, methyl jasmonate, trans-nerol tertiary alcohol, 2-nonone, and phenyl ethanol were identified as key aroma components in Huangdan oolong tea, which provided it with clean and high floral aroma and obvious milky aroma characteristics. In Tieguanyin oolong tea, linalool, 3,5-octylodiene-2-one, linalool oxide, cis-jasmonone, dehydrolinalool, and α-terpineol showed diverse floral aromas, which were closely related to the characteristic aroma of Tieguanyin oolong tea. The key aroma components identified in Jinguanyin oolong tea included linalool, canalaldehyde, geranyl acetone, cis-jasmonone and isoeugenol, which were responsible for the characteristic sweet floral and woody aromas of Jinguanyin oolong tea.

In order to explore the quality characteristics and components of Jiulong Dabaicha, a type of white tea, we made white tea from six varieties, Jiulong Dabaicha, Fuan Dabaicha, Fuyun 6, Zhenghe Dabaicha, Fuding Dabaicha and Songxi Caicha and comparatively analyzed their quality using sensory evaluation, biochemical analysis, high performance liquid chromatography (HPLC), headspace-solid phase microextraction (HS-SPME) combined with gas chromatography-mass spectrometry (GC-MS). The results showed that each variety of white tea had their own characteristics, showing good sensory quality. The buds of Jiulong Dabaicha were particularly plump and shiny; the color was gray-green with a moist appearance; the infusion had a bright apricot yellow color, a fresh and tender aroma with floral and pekoe notes, and a fresh, mellow, sweet and refreshing taste; the leaf base was soft and tender. Among the six varieties, the contents of total catechin, ester catechin, epicatechin gallate, theanine, glutamic acid and arginine in Jiulong Dabaicha were the highest, 102.52, 96.31, 71.29, 11.00, 0.68 and 1.07 mg/g, respectively. A total of 38 differential aroma compounds between the six varieties were identified. Jiulong Dabaicha had the highest content of alcohol compounds, with significantly higher levels of phenylethyl alcohol, hexanoic acid (both responsible for rosy aroma) and (Z)-3-hexen-1-ol (fresh floral aroma) and significantly lower levels of ethylbenzene (gasoline odor), 2-acetylpyridine (barbecue flavor), and naphthalene (spicy flavor) compared to the other varieties. Our results are helpful to elucidate the flavor and quality characteristics of Jiulong Dabaicha, providing a scientific basis for product development and promotion of Jiulong Dabaicha.