Tea is an important economic crop with various health benefits, including delaying aging, regulating glucose and lipid metabolism, and promoting anti-inflammation. However, due to its raw materials and processing techniques, tea also possesses unique aroma characteristics and taste properties. Green tea is a globally popular non-fermented tea with unique sensory qualities, including a green liquor, green leaves, and a mellow aroma.
The taste, color, and aroma of tea directly affect its quality and economic performance. These quality attributes are closely related to non-volatile metabolites (NVMs). Catechins, flavonoids, and flavonoid glycosides are the main components affecting the astringency of tea liquor; caffeine, theobromine, and some diketopiperazines are typical components of bitterness; L-theanine and soluble sugars are important sources of freshness. Fat-soluble pigments, such as chlorophyll and carotenoids, play a role in the color, appearance, and brewing of finished tea, while water-soluble pigments, such as anthocyanins, mainly affect the color of the tea liquor. Furthermore, certain nonvolatile metabolites (NVMs) are precursors to aromatic compounds in tea, such as linolenic acid and linolenic acid in lipids, which, under the action of lipoxygenases, produce aliphatic aromatic compounds, such as pyrazines and furans, which have roasted aromas.
NVMs vary considerably during tea processing, with different processing procedures, methods, and parameters having significantly different effects on these metabolites. Generally, green tea processing includes withering, fixation, shaping, and drying, with fixation being the key stage affecting green tea quality. In previous studies on green tea processing, the research team found that the most intense NVM transformations occur during fixation, especially amino acids, lipids, flavonoid glycosides, and phenolic acids, which are crucial for the formation of green tea's color, aroma, and flavor. However, few studies have investigated the derivation patterns and formation pathways of NVMs during fixation, and the interaction between enzymatic and thermochemical reactions during fixation remains unclear and requires further exploration.
Previous studies have elucidated the effects of fixation methods and parameters on green tea quality. Hot-air drum fixation is beneficial for the production of chestnut-scented green tea, and high-temperature fixation produces a richer aroma. Coupled fixation methods result in higher amino acid and soluble sugar content compared to single fixation methods. However, these studies were conducted on finished tea products, focusing on volatile metabolites (NVMs) and failing to elucidate the sources of substances throughout the fixation process, particularly NVMs.
Therefore, to comprehensively analyze the derivation mechanisms of NVMs during green tea fixation, this study collected fixation tea samples at seven different time points during the fixation process, measuring leaf temperature and moisture content in real time. Instead, the inactivation points of enzymes during the fixation process were investigated by detecting enzyme activity in the tea leaves. A comprehensive analysis of NVMs during green tea fixation was conducted using broad-targeted metabolomics techniques based on ultra-high performance liquid chromatography-tandem mass spectrometry (UHPLC-MS/MS). Least squares discriminant analysis, heatmap clustering, and K-means statistical analysis were used to study changes in NVMs during fixation and to identify inflection points of abrupt NVM changes. These methods were used to identify differential NVMs at different fixation stages.