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A pale brown ginger root, a turmeric with an orange core, a length of pink skinned, ringed galangal. Three rhizomes share one basket in the Vietnamese kitchen and one family, Zingiberaceae, and when distilled each gives an oil led by a different main molecule.

Three roots in one basket

Cut across all three and the difference shows before the smell does. Ginger has a pale yellow, fibrous core. Turmeric has a deep orange core, deep enough to stain fingertips and a wooden board yellow. Galangal is the hardest, its core ivory white and tightly grained, the knife needing firm pressure to pass through.

In the kitchen the three rarely stand in for each other. Ginger goes with fish and with dishes that need warmth. Turmeric brings colour and an earthy smell to curry and to bún bò Huế. Galangal goes with meat, with fermented shrimp paste, with dishes that want a pungency with a medicinal edge. Cooks told them apart by tongue and nose long before gas chromatography.

The essential oils of the three roots let that difference be read in another language. Each bottle has a composition table, and each table has a molecule at its head. In ginger it is zingiberene, in turmeric the turmerones, in galangal 1,8-cineole. Three molecules, three directions of scent, on a single botanical frame.

This piece sets the three materials side by side on the blotter, without discussing uses, reading only smell and chemistry. The starting point is the shared frame: the ginger family.

The Zingiberaceae family and the rhizome

The part of ginger, turmeric and galangal usually called the root is not a root. It is a rhizome, a stem that grows sideways under the soil, with nodes and buds, and from those buds new shoots rise. The rings clearly visible on galangal skin are the marks of a stem lying flat in the earth.

The ginger family, botanical name Zingiberaceae, includes many perennial tropical herbs. Besides ginger, turmeric and galangal, it holds cardamom, black cardamom, zedoary and finger root. Many species in the family store their scent in rhizomes or seeds, and many have become spices in Asia.

The three plants here belong to three different genera. Ginger is Zingiber officinale, a perennial herb from Southeast Asia now grown worldwide. Turmeric is Curcuma longa, produced mainly in India, Indonesia, China and Vietnam. Greater galangal is Alpinia galanga, a plant up to 2 metres tall, a familiar spice in Thailand as kha and in Indonesia as laos.

Common names mislead easily. The English word galangal can point to three different plants: greater galangal Alpinia galanga, the smaller lesser galangal Alpinia officinarum, and a third plant in the genus Kaempferia. When reading a galangal oil label, the botanical name matters more than the common one.

Ginger: warm spice and lemon peel

Ginger oil is usually steam distilled from dried rhizomes. The composition table in the library lists zingiberene at 38.1 to 40.2 percent, ar-curcumene at 17.1 percent, β-sesquiphellandrene at around 7.2 percent, β-bisabolene at 5.2 to 6.0 percent. Most of the bottle is sesquiterpenes, 15 carbon molecules heavier and slower to evaporate than the light terpenes.

The smell of dried ginger oil is therefore not the sharp bite of a fresh slice but warm, round, a little earthy and woody. The heat on the tongue when chewing ginger comes from gingerols and shogaols, two groups of compounds that are practically non volatile. They stay in the residue during distillation, so ginger oil has the smell of ginger without its heat.

The lemon note in ginger comes from citral, made of geranial and neral. According to library data, citral is higher in fresh ginger. Australian and fresh Chinese ginger tend to be richer in citral and lemon, while Indian and African ginger lean earthier and warmer. Oil distilled from dried ginger and from fresh ginger can therefore differ clearly.

On safety, ginger has GRAS status, and at a test level of 4 percent no irritation or sensitisation was recorded. If the oil carries citral from fresh ginger, citral is restricted by IFRA and must be labelled as an allergen in the EU above threshold.

Ginger oil is produced mainly in China and India, in Jamaica, and also in England, where distillers work with dried ginger imported from Nigeria. Ginger is not among the materials commonly adulterated, which is unusual in the essential oil trade. So when two bottles of ginger oil smell different, the reason usually lies in the source of the rhizome and in whether it was distilled fresh or dried, rather than in something foreign added to it.

Turmeric: earth, wood and turmerone

Turmeric oil does not carry the yellow of turmeric powder in the way many expect. Curcumin, the yellow pigment that gives turmeric its colour, is not volatile, so it is absent from distilled turmeric oil. Curcumin is found only in the powder, in alcohol extracts and in CO2 extracts.

What crosses to the condenser is the turmerone group. The ar-turmerone constituent page records this molecule at 15.5 to 27.5 percent of turmeric oil, alongside its two isomers α-turmerone and β-turmerone. The prefix ar stands for aromatic, marking the aromatic ring in its structure. Formula C₁₅H₂₀O, a sesquiterpene ketone.

The library describes the smell of ar-turmerone as the signature note of turmeric: warm, woody, lightly peppery, earthy. The three turmerones together make up what a cook recognises the moment a jar of turmeric is opened. Beside them, α-zingiberene is present at 5 to 12 percent, a thread tying turmeric to its relative ginger.

Turmeric oil has one practical property worth remembering: it can leave a yellow orange stain on skin and fabric, and at high concentration the stain is stronger. The library lists a low risk of skin sensitisation, and advises storage in a dark, sealed, refrigerated bottle.

On the blotter, turmeric changes more slowly than ginger. The turmerones are sesquiterpene ketones, heavier than the light terpenes, so the warm, earthy smell holds a long time on the paper. In the first minute there is a brighter note somewhat like ginger, perhaps from the zingiberene, which then gives way to a dry woody ground. After a few hours, what remains on the strip is close to a jar of dried turmeric powder, only without the colour.

Galangal: camphor and pungency

Galangal oil sits in a region quite apart from the other two. In greater galangal, the leading molecule is not a sesquiterpene but 1,8-cineole, a light monoterpene oxide, at 30.2 to 33.6 percent. After it come camphor at 5.0 to 14.0 percent, β-pinene up to 12.9 percent, α-fenchyl acetate up to 12.7 percent and α-terpineol.

The result on the blotter is a cool, penetrating, slightly medicinal smell from the first seconds. It is what the perfumer's vocabulary calls camphoraceous, the smell of camphor and 1,8-cineole. Under that cool layer there is a little spicy sweetness from methyl cinnamate, recorded at 2.6 to 5.3 percent, and a light resinous ground from the pinenes.

Galangal has chemotypes too. The library records at least two lines: one rich in 1,8-cineole and one rich in (E)-β-farnesene. In some samples methyl cinnamate becomes a major component. Under the same name Alpinia galanga, two bottles of oil can open in two different directions.

If ginger leans warm and turmeric leans earthy, galangal leans cool. The three roots in one basket therefore cover three corners of the same spicy region: warm and round, earthy and woody, cool and medicinal. The library lists greater galangal with a low sensitisation risk and no known carcinogens.

Fresh ginger, dried ginger, CO2

The same ginger root can become three different materials depending on how it is handled. Fresh ginger distilled gives an oil rich in citral, bright with lemon. Dried ginger distilled gives a warmer, rounder oil, heavy in sesquiterpenes. And extraction with CO2 gives a third product, the closest of all to a freshly cut root.

CO2 extraction uses compressed carbon dioxide as the solvent. Under subcritical conditions CO2 takes mainly the volatile fraction, giving a select extract. Under supercritical conditions it also takes lipids and non volatiles, giving a total extract. On depressurisation the CO2 leaves entirely, with no solvent left in the product.

The library describes ginger CO2 as freshly cut ginger, lemon, hot spice, a little earth and wood. The total extract keeps the gingerols, so it carries a clear pungency on tasting and a warmer, rounder smell than the oil; the select is brighter and closer to fresh ginger. The gingerols in a total extract can feel hot on skin at high concentration.

Turmeric has a CO2 version too, and it is the form that keeps the curcumin distillation leaves behind. The piece CO2, a gas as a solvent goes further into the method. For the three roots here, CO2 shows one thing plainly: the scent and the taste of a spice can be separated, then set side by side again as the perfumer chooses.

Three rhizomes, one family, three leading molecules.

The scent crosses to the condenser. Taste and colour stay behind.

The kitchen sorted them before the laboratory did.