Roasting-induced transformations in Longjing tea: Sensory, chemical, and molecular insights into roasted aroma profiles

Roasting is a pivotal step in Longjing tea processing that shapes its sensory quality, yet the molecular basis of the characteristic roasted aroma remains underexplored. In this study, we systematically investigated the impact of roasting durations (10–30 min) and temperatures (210–230 °C) on the sensory attributes, biochemical composition, and volatile organic compounds (VOCs) of Longjing tea, while elucidating aroma perception through molecular docking. Sensory evaluation revealed enhanced “roasted” and “burnt” notes, alongside increased astringency and bitterness, with intensified roasting treatment. These sensory shifts coincided with substantial degradation of umami-related free amino acids and soluble sugars, as well as oxidation and epimerization of catechins. Using headspace solid-phase microextraction-gas chromatography–mass spectrometry (HS-SPME-GC–MS/MS), we identified 1286 VOCs. Heterocyclic compounds, particularly pyrazines, were markedly enriched and closely correlated with the roasted aroma. Relative odor activity value (ROAV) analysis pinpointed 27 key aroma-active compounds, including 2-ethyl-3,5-dimethylpyrazine, 2,3-diethyl-5-methylpyrazine, 2,3,5-trimethyl-6-ethylpyrazine, 2-methoxy-3,5-dimethylpyrazine, and 2,6-diethylpyrazine. Molecular docking simulations further demonstrated that pyrazines bind stably to olfactory receptors OR1A1 and OR1D2, primarily through cooperative hydrophobic, hydrogen-bonding, and π–π stacking interactions. These findings elucidate the molecular basis of the roasted aroma in Longjing tea and provide scientific guidance for optimizing the roasting process.