Most of what herbs and spices taste of is really smell. A few families of volatile molecules, chiefly terpenes and phenylpropanoids, account for the characteristic aroma of nearly every spice.
Taste versus smell¶
The tongue detects only five basic tastes — sweet, sour, salty, bitter and umami. Almost everything that distinguishes cumin from coriander or thyme from rosemary is smell, detected by olfactory receptors at the top of the nasal cavity. Aroma molecules reach them two ways: through the nostrils when sniffing (orthonasal smell), and up the back of the throat while chewing and swallowing (retronasal smell). Retronasal smell is what most people mean by "flavour", which is why food tastes of little when the nose is blocked.
The molecules responsible are volatile: small and light enough to evaporate at room or mouth temperature. In a plant they are usually stored in its essential oil, in glands, ducts or hairs. A dried spice typically contains somewhere between a fraction of a per cent and several per cent of essential oil; cloves are unusually rich, at around 15–20%.
Humans have around 400 kinds of functional olfactory receptor, each responding to a range of molecules. Genetic variation in these receptors is why people disagree about some smells — the best known case being coriander leaf.
The main families¶
Terpenes¶
Terpenes are built from five-carbon units. Monoterpenes (10 carbons) are the lightest and most volatile, giving the bright top notes; sesquiterpenes (15 carbons) are heavier and woodier.
| Compound | Smells of | Found in |
|---|---|---|
| Linalool | Floral, citrusy | coriander-seed, basil, lavender |
| Limonene | Orange peel | Citrus, dill, caraway, sichuan-pepper |
| α- and β-pinene | Pine | rosemary, juniper, black-pepper |
| 1,8-cineole | Eucalyptus, camphor | Bay, green-cardamom, rosemary |
| Thymol | Thyme, medicinal | thyme, ajwain |
| Carvacrol | Oregano | oregano, savory |
| Menthol | Mint, cooling | Peppermint (see menthol-and-cooling) |
| Citral | Lemon | lemongrass, lemon-verbena, lemon-balm |
| β-caryophyllene | Woody, peppery | black-pepper, cloves, basil |
| Zingiberene | Ginger | ginger |
| Rotundone | Peppery, woody | black-pepper |
Phenylpropanoids¶
Built around a benzene ring, these are often sweet, warm and pungent — the smells of the classic "sweet spices".
| Compound | Smells of | Found in |
|---|---|---|
| Eugenol | Cloves | cloves, allspice, basil, cinnamon leaf |
| Cinnamaldehyde | Cinnamon | cinnamon, cassia |
| Anethole | Aniseed, liquorice | aniseed, star-anise, fennel-seed |
| Estragole | Anise, green | tarragon, basil |
| Myristicin | Warm, woody | nutmeg, mace, parsley |
| Vanillin | Vanilla | vanilla |
Others¶
- Aldehydes such as cuminaldehyde give cumin its sweaty, earthy note; long-chain aldehydes such as (E)-2-decenal give coriander-leaf its leafy — or soapy — smell.
- Sulphur compounds dominate garlic (allicin and its breakdown products) and asafoetida.
- Sotolon, a lactone, gives fenugreek its maple-syrup, curry-powder smell; it is potent enough to scent the sweat of people who have eaten a lot of fenugreek.
- Safranal is the main aroma of saffron, formed from the bitter compound picrocrocin as the stigmas dry.
🔬 One molecule, two mirror images
Carvone exists as two mirror-image forms, identical except for handedness. (R)-carvone smells of spearmint; (S)-carvone smells of caraway and dill-seed. Olfactory receptors are themselves chiral, so they can tell the two apart — one of the neatest demonstrations that smell depends on molecular shape.
Aroma made in processing¶
Some aromas are barely present in the fresh plant and are created afterwards.
- Curing. Green vanilla pods contain vanillin locked up as glucovanillin; weeks of sweating and drying release it.
- Drying. Saffron's safranal develops as the stigmas dry.
- Toasting. Dry heat drives off some volatiles but creates new ones through the Maillard reaction and caramelisation, including nutty, roasted pyrazines. That is why toasted cumin smells different from raw.
Why aroma fades¶
Volatile molecules are, by definition, on their way out. Four things speed their loss:
- Grinding multiplies the surface area. A ground spice loses aroma many times faster than whole seed (see whole-vs-ground-spices).
- Heat increases volatility. Long cooking drives off the lightest top notes first, which is why many cuisines add some spices early and others, or fresh herbs, at the end (see when-to-add-herbs-and-spices).
- Air oxidises many terpenes, turning bright citrusy notes stale or turpentine-like.
- Light breaks down some compounds and colours.
💡 Test by nose
If a jar of spice smells of little when rubbed between the fingers, it will taste of little in the pot. See how-long-spices-last and storing-spices.
Solubility¶
Most aroma compounds are fat-soluble and only slightly water-soluble, which is why frying spices in oil at the start of a dish carries flavour so effectively. See fat-and-flavour and blooming-spices-in-oil.