New insights into the role of lipids in aroma formation during black tea processing revealed by integrated lipidomics and volatolomics

Lipids are important tea aroma precursors. Due to the complexity of black tea processing involving both enzymatic and thermal reactions, the role of lipids in black tea aroma formation remained unclear. Herein, the dynamic changes of lipids and volatiles during black tea processing were simultaneously analyzed by lipidomics and volatolomics using ultra-high-performance liquid chromatography coupled to Q-Exactive Orbitrap mass spectrometry (UHPLC-Q-Exactive) and gas chromatography-tandem mass spectrometry (GC-MS/MS). The lipidomics method was validated in linearity, reproducibility, and recovery, which showed a high reliability. A total of 374 lipids and 88 volatiles were detected. Among them, 362 lipids and 29 fatty acid-derived volatiles (FADVs) were significantly altered depending on different processing stages. During the enzyme-driven stages of black tea processing (withering, rolling and fermentation), monogalactosyldiacylglycerol (MGDG), phosphatidylcholine (PC), and phosphatidylethanolamine (PE) were largely downregulated (<0.33 folds). Instead, in the non-enzymatic drying steps of black tea processing, triacylglycerol (TG), diacylglycerol (DG), and phosphatidic acid (PA) were mainly degraded (<0.24 folds). MS/MS fragmentation revealed that these most prominently degraded lipids were structurally enriched with fatty acyl (FA) 18:2 and 18:3 residues, such as MGDG (18:2/18:3), PC (18:2/18:2), PE (18:1/18:2), TG (18:3/18:3/18:3), DG (18:3/18:3), PA (18:3/18:3). Correlation analysis showed significant negative correlation between these lipids and FADVs such as aliphatic aldehydes, alcohols, ketones, and esters, etc. These most prominently degraded lipids were highlighted as the key potential aroma precursors during black tea processing, which were possibly oxidized and degraded into volatiles through enzyme- and thermal-driven pathways at different processing stages.