The Odour Detection Threshold Principle
Presence does not equal impact. A compound present at 100 parts per billion may have no perceptible aroma, while another at 0.001 parts per billion is overwhelming. The key metric is the odour activity value (OAV) — the ratio of a compound's concentration to its odour detection threshold. Only compounds with OAV > 1 contribute to perceived aroma; those with OAV >> 1 are the defining "character impact compounds."
In practice, 20–30 compounds typically drive the core aromatic impression of a tea, even though hundreds are analytically detectable. This is why tea from the same garden can smell dramatically different in two consecutive seasons — a shift in the relative concentrations of the dominant 20-30 compounds creates a different OAV hierarchy, changing aroma character entirely even with similar total volatile mass.
🧠 Expert Tip: Sensory Science
Your nose is more sensitive than any analytical instrument for many important tea volatiles. GC-MS detects compounds in parts per trillion; human olfactory receptor neurons can detect some substances at single-digit parts per quadrillion. This extraordinary sensitivity evolved to detect threats (rotten food, toxic plants) but serves tea appreciation equally well.
Major Volatile Classes in Tea
| Compound Class | Key Compounds | Aroma Descriptor | Primary Tea Source |
|---|---|---|---|
| Monoterpene alcohols | Linalool, geraniol, nerol | Floral, rose, lavender | Darjeeling 1st flush, oolongs, greens |
| Sesquiterpenes | Nerolidol, farnesene, cadinene | Woody, floral, earthy | Black teas, pu-erh, aged teas |
| Aldehyde-C6 | (Z)-3-hexenal, (E)-2-hexenal | Fresh green, cut grass | Green teas, white teas (fresh leaf) |
| Aromatic aldehydes | Benzaldehyde, phenylacetaldehyde | Almond, honey, floral | Oxidised teas, black tea |
| Esters | Hexyl acetate, linalyl acetate | Fruity, floral, green | Oolongs, green teas |
| Pyrazines | 2-methylpyrazine, trimethylpyrazine | Roasted, nutty, grain | Hojicha, roasted oolongs, black teas |
| Phenols | Guaiacol, 4-ethylguaiacol | Smoky, medicinal, earthy | Lapsang souchong, roasted teas |
| Ketones | β-ionone, dihydroactinidiolide | Violet, woody, tobacco | Darjeeling 2nd flush, aged teas |
| Furans | 2-acetylfuran, furfural | Caramel, sweet, bready | Roasted teas, black teas |
| Sulfur compounds | Dimethyl sulfide, DMTS | Marine, cooked vegetable | Sencha, gyokuro (positive at trace) |
Type-Defining Volatile Fingerprints
Each major tea category has a characteristic volatile fingerprint — not a single compound but a ratio pattern. Darjeeling first flush is defined by linalool and hotrienol dominance, producing a lavender-floral character. Darjeeling second flush (muscatel) adds high 2,6-DMHP from leafhopper-bitten leaves. Gyokuro and matcha are characterised by elevated dimethyl sulfide and (Z)-3-hexenyl acetate — the marine and fresh-green notes from shading and steaming. Assam CTC shows a different balance dominated by phenylacetaldehyde and benzaldehyde — the malty honey character. Wuyi rock oolongs are defined by nerolidol, indole, and roasting-derived pyrazines.
How Brewing Conditions Change the Volatile Profile in Your Cup
The volatile compounds in brewed tea are a subset of those in the leaf — affected by water temperature, steeping time, pH, and the physical dynamics of the infusion. Water temperature is particularly critical: highly volatile compounds (low boiling point) such as (Z)-3-hexenal (BP 92°C) flash off rapidly above 80°C, while terpene alcohols like linalool (BP 198°C) remain dissolved in the liquor at normal brewing temperatures and reach your olfactory system via the retronasal route.
🧠 Expert Tip: Capturing Aroma
In professional tea tasting (cupping), the curved lid of the cupping bowl is used to trap volatile compounds above the liquor. When the lid is lifted and smelled, the concentrated headspace gives a very different aromatic impression than smelling the open cup — often revealing delicate top notes that disperse instantly. Try this with a mug covered by a saucer.
Pu-erh's Unique Volatile Universe
Aged and fermented pu-erh teas have a volatile profile that differs radically from all other tea categories. Microbially-derived compounds dominate, including: 1-octen-3-ol (mushroom/earthy, from fungi), 2-heptanone and 2-nonanone (fruity-earthy ketones from bacterial metabolism), geosmin (damp earth/petrichor scent, from actinobacteria), and high concentrations of sesquiterpenes including agarospirol and isoshyobunol, which are characteristic of aged wood and are produced by tea-specific fungi. This is why aged pu-erh smells like nothing else in the world of tea — or indeed in food chemistry at large.
Comments