Tea is a typical processed beverage from the fresh leaves of Camellia sinensis [Camellia sinensis (L.) O. Kuntze] or Camellia assamica [Camellia sinensis var. assamica (Mast.) Kitamura] through different manufacturing techniques. The secondary metabolites of fresh tea leaves are mainly flavan-3-ols, phenolic acids, purine alkaloids, condensed tannins, hydrolysable tannins, saponins, flavonols, and their glycoside forms. During the processing, tea leaves go through several steps, such as withering, rolling, fermentation, postfermentation, and roasting (drying) to produce different types of tea. After processing, theaflavins, thearubigins, and flavan-3-ols derivatives emerge as the newly formed compounds with a corresponding decrease in concentrations of catechins. Each type of tea has its own critical process and presents unique chemical composition and flavor. The components among different teas also cause significant changes in their biological activities both in vitro and in vivo. In the present review, the progress of tea chemistry and the effects of individual unit operation on components were comprehensively described. The health benefits of tea were also reviewed based on the human epidemiological and clinical studies. Although there have been multiple studies about the tea chemistry and biological activities, most of existing results are related to tea polyphenols, especially (-)-epigallocatechin gallate. Other compounds, including the novel compounds, as well as isomers of amino acids and catechins, have not been explored in depth.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/1541-4337.12479

Chemistry and Biological Activities of Processed Camellia sinensis Teas: A Comprehensive Review.

Large-leaf yellow tea (LYT) is made from mature tea leaves with stems and has unique sensory characteristics different from other teas. To study the chemical changes of LYT during processing, samples were collected from each step for quantitative and qualitative analyses by high-performance liquid chromatography and liquid chromatography-mass spectrometry (LC-MS). LC-MS-based nontargeted and targeted metabolomics analyses revealed that the tea sample after roasting was markedly different from samples before roasting, with the levels of epicatechins and free amino acids significantly decreased, but the epimerized catechins increased dramatically. After accounting for common compounds in tea, N-ethyl-2-pyrrolidinone-substituted flavan-3-ols were found to be the marker compounds responsible for the classification of all samples, as they rapidly rose with increasing processing temperature. These findings suggested that the predominant changes in the tea constituents during large-leaf yellow tea roasting were the thermally induced degradation and epimerization of catechins and the formation of N-ethyl-2-pyrrolidinone-substituted flavan-3-ols from l-theanine.

LC-MS-Based Metabolomics Reveals the Chemical Changes of Polyphenols during High-Temperature Roasting of Large-Leaf Yellow Tea.

Study on the effects of rapid aging technology on the aroma quality of white tea using GC-MS combined with chemometrics: In comparison with natural aged and fresh white tea.

Non-targeted metabolomics analysis reveals dynamic changes of volatile and non-volatile metabolites during oolong tea manufacture.

Roasting, an important process to refine Wuyi Rock tea, could impart different types of aroma to the final products. This study focuses on the differences in aroma characteristics among three kinds of refined teas, named light fire (LF), moderate fire (MF), and high fire (HF). A combination of solid phase microextraction (SPME) and a switchable system between GC-O-MS and GC × GC-O-MS was utilized to identify the odorants. In total, 97 aroma-active compounds could be smelled at the sniffing port, comprising alcohols, aldehydes, ketones, esters, heterocycles, and terpenes. However, only 52 obtained r-OAV >1. Significant differences were uncovered by the application of principal component analysis (PCA) and partial least squares regression (PLSR). Thereby, MF and HF had a more similar aroma profile, while in LF samples, alcohols, aliphatic aldehydes and some ketones were responsible for the aroma profile, such as (E,E)-2,4-hexadienal, octanal, hexanal, (E,Z)-2,6-nonadienal, (E)-β-ionone, 3-octen-2-one etc. Strecker aldehydes had a great impact on the aroma of MF, including 2-methylpropanal, 2-methylbutanal, 3-methylbutanal etc. Some N-heterocyclic compounds also affected the overall aroma, for instance, 6-methyl-2-ethylpyrazine. In HF, the majority of aroma compounds increased with increasing roasting temperature, especially N-heterocyclic compounds as well as furfural and 5-methyl-2-furancarboxaldehyde, which are all closely related to the Maillard reaction. Besides, 5-methyl-2-(1-methylethenyl)-4-hexen-1-ol, trans-linalooloxide and 2-nonanone also remarkably influenced the aroma of HF. In addition, it was supposed that most amino acids that participated in the Maillard reaction during roasting were decomposed from the compounds that combined with tea polyphenols and amino acids.

Differences of characteristic aroma compounds in Rougui tea leaves with different roasting temperatures analyzed by switchable GC-O-MS and GC × GC-O-MS and sensory evaluation.

Study of the aroma formation and transformation during the manufacturing process of oolong tea by solid-phase micro-extraction and gas chromatography-mass spectrometry combined with chemometrics

Wenwen Wang

Papers

Study of the aroma formation and transformation during the manufacturing process of oolong tea by solid-phase micro-extraction and gas chromatography-mass spectrometry combined with chemometrics