茶叶焙火过程中形成的儿茶素转化产物及其呈味特性

“焙火”是茶叶加工的一个关键工序，对提升茶叶感官品质具有重要意义。前期研究发现焙火后茶汤涩味减轻、滋味醇和；EGCG明显减少，少量与茶氨酸形成缩合产物，大量EGCG的转化途径未知；焙火茶滋味物质基础也尚不清楚。基于焙火后总氨基酸/可溶性糖同步降低的现象，我们提出科学假设：EGCG的活泼位点捕捉了美拉德反应中间产物（醛、酮、二羰基类），接合特定取代基团，转化为不同呈味特性的衍生物，从而掩蔽或消减了茶汤的苦涩味，形成焙火茶滋味醇和的特点。本项目首先通过LC-MS非靶向和LC-MSn靶向代谢组学发现焙火茶中EGCG衍生物；然后采用滋味导向分离和“EGCG-总氨基酸/可溶性糖”热模拟反应制备目标产物；接着利用色谱和波谱学手段分离、纯化并鉴定其结构；最后通过感官审评探明各物质的滋味属性和呈味特点，揭示焙火过程中EGCG的转化规律及其参与滋味品质形成的化学基础，为茶叶加工和品质化学相关研究开阔新思路。

Roasting is a critical step of tea processing, and has important meaning for improving the sensory quality of tea. In the previous study, we found the tea had mellow taste, less astringency after roasting. Furthermore, the content of (-)-epigallocatechin gallate (EGCG) was significantly decreased, few condensed with theanine, but the transformation pathway of main lost EGCG was still unknown. Furthermore, the chemical basis of taste of roasted tea is still unclear. Because of the concurrent decreasing of free amino acids and soluble sugar of tea during roasting, we propose a scientific hypothesis that the reactive carbons of EGCG may capture many intermediates of Millard reaction (aldehydes, ketones, and dicarbonyls), and subsequently connect specific moieties. These special EGCG derivatives have different taste characteristics, then decrease the astringency and endow the mellow sensory quality of roasted tea. The present project firstly searches for the EGCG derivatives by non-targeted LC-MS and targeted LC-MSn metabolomics. Secondly, the targeted compounds are prepared by sensory-guided separation and simulative thermal reaction of “EGCG-total amino acids/soluble sugar”. Thirdly, EGCG derivatives are separated, purified and identified regarding chemical structures by chromatographic and spectroscopy tools. Fourthly, the taste attributes and characteristics are described by sensory evaluation. Finally, the transformation mechanism of EGCG and its impact on the formation of sensory quality of roasted tea is explained. The obtained findings of this project will provide new minds for the study of tea processing and quality chemistry.