Keemun black tea, produced from the Camellia sinensis cultivar Qimen in Anhui province, is prized for its pronounced floral, fruity and sweet notes, a profile that has earned it a place among the most famous black teas on Earth. Its manufacture follows a strict sequence of withering, rolling, fermentation and drying. While withering and fermentation have attracted considerable research attention, the drying step, despite being the final thermal process that locks in color, taste and aroma, has remained surprisingly underexplored. Previous studies largely focused on drying temperature or variable temperature profiles, leaving the role of drying time itself largely unquantified.

To close that gap, the research team, led by Erdenechimeg Otgonbat and colleagues, sampled tea directly from a commercial-scale hot-air dryer operating at 110 degrees Celsius. Subsamples of roughly 100 grams were pulled from three separate grids of the drying bed before drying and after 10, 15, 20, 25 and 30 minutes, providing biological replicates that eliminated micro-environmental gradients inside the machine. Crucially, every sample was immediately plunged into liquid nitrogen and freeze-dried. This lyophilization step served two purposes: it standardized all samples to a uniform dry weight, because headspace partitioning of volatiles is exquisitely sensitive to moisture, and it halted residual enzymatic activity and thermal reactions, preserving a genuine chemical snapshot of each moment.

The volatiles were then captured using headspace solid-phase microextraction, a solvent-free technique, followed by gas chromatography-mass spectrometry. In total, the team identified 104 volatile compounds spanning alcohols, aldehydes, esters, alkenes, ketones, alkanes and heterocyclics. Alcohols dominated the profile, accounting for between 66 and 71 percent of total volatiles, with aldehydes close behind. The total volatile content followed a strikingly non-linear trajectory: it fell from 18,148 micrograms per kilogram before drying to a low of 14,623 at 15 minutes, then surged to 16,856 at 20 minutes before settling near 15,679 at the end of the process.

That transient spike at 20 minutes is one of the study’s most intriguing observations. The researchers hypothesize that it reflects a dynamic equilibrium between physical evaporation, which strips volatiles away in the hot-air stream, and thermal chemical generation, which creates new aroma molecules. As heat accumulates, non-enzymatic cascades such as the Maillard reaction and Strecker degradation of amino acids churn out fresh aldehydes and heterocycles, while thermal hydrolysis cleaves non-volatile glycosidically bound precursors, liberating a wave of free linalool and geraniol derivatives. For a brief window, the rate of generation outpaces the rate of loss. Beyond 20 minutes, precursors become depleted and the balance tips back toward evaporation. The authors caution that this mechanism remains a working hypothesis requiring isotope-labeling studies to confirm.

Sensory evaluation, conducted under China’s national standard for tea assessment with a panel of seven trained assessors, corroborated the chemistry. As drying progressed, the infusion color deepened from bright orange-yellow to darker orange-red, while green and astringent notes faded completely between 20 and 30 minutes. By the end of drying, the tea reached its peak sensory quality, displaying the classic high sweet and floral Keemun aroma alongside a bright red liquor. The phenotypic trajectory of the panel aligned neatly with the molecular data, giving the chemical findings a human anchor.

To identify which compounds actually mattered, the team deployed partial least squares discriminant analysis, validated with a 999-permutation test to rule out overfitting. This chemometric screen flagged 20 differential volatile markers that reliably distinguished the drying intervals. A subsequent odor activity value analysis, which compares each compound’s concentration against its odor threshold in water, narrowed the field to 39 aroma-active compounds, of which six emerged as the primary drivers of Keemun’s signature scent: trans-beta-ionone, linalool, geraniol, benzeneacetaldehyde, nonanal and theaspirane.

The dynamics of these key odorants reveal two competing chemical stories. Trans-beta-ionone and related C13-norisoprenoids surged during the 20-to-30-minute window, direct evidence of thermal oxidative cleavage of carotenoid pigments such as beta-carotene and neoxanthin, which fragment into sweet-floral and sweet-woody molecules once thermal energy overcomes the activation barrier under low-moisture conditions. Meanwhile, the monoterpene alcohols linalool and geraniol fluctuated as residual glycoside-hydrolyzing enzymes released them early in drying, only for desiccation to inactivate those enzymes past 20 minutes, letting volatilization win. Most tellingly, benzeneacetaldehyde, the honey-like note, accumulated late in drying through Strecker degradation of the amino acid phenylalanine within the Maillard reaction cascade, confirming that drying acts not merely as dehydration but as a thermal-chemical reactor.

The practical implications extend beyond academic curiosity. Because the most critical accumulation of Keemun’s key odorants occurs in the middle-to-late drying stages, manufacturers now have a quantitative basis for tuning drying time as a precision parameter alongside temperature, potentially enabling smarter, aroma-targeted control in tea production. The authors acknowledge limitations, noting that absolute quantification could be cross-validated with solvent-assisted flavor evaporation and that future multi-omics work should trace precursor degradation kinetics. Still, the study delivers something rare in food science: a minute-by-minute molecular biography of a flavor in the making, showing exactly when a leaf stops smelling green and starts smelling like Keemun.

Subject of Research: Time-dependent changes in volatile and aroma compounds during the hot-air drying of Keemun black tea

Article Title: Time-dependent changes of aroma and volatile compounds during drying of Keemun black tea

Article References: Otgonbat, E., Cui, J., Luo, Z., Song, Y., Gao, T., & Song, C. (2026). Time-dependent changes of aroma and volatile compounds during drying of Keemun black tea. Food Chemistry: X, Article 104531. https://doi.org/10.1016/j.fochx.2026.104531

Image Credits: AI Generated

DOI: 10.1016/j.fochx.2026.104531

Keywords: Keemun black tea, tea drying, aroma chemistry, volatile compounds, Maillard reaction, Strecker degradation, HS-SPME-GC/MS, linalool, trans-beta-ionone, odor activity value, food chemistry, sensory evaluation