Two clumps of grass, hip high, with long, sharp edged leaves, both growing on Vietnamese soil. Crush a leaf and both suggest lemon. Distilled, they give two oils whose compositions barely overlap: on one side citral, on the other citronellal and geraniol.
The clump and the leaf
A lemongrass clump grows in a dense tuft, dozens of stems splitting from the base, each carrying long, narrow leaves. The leaf edge is sharp enough to cut a hand. The tight white base is the part that goes to the kitchen; the green blades above, often discarded in cooking, are the part that holds more oil.
These grasses belong to the family Poaceae, the same family as rice, corn and sugarcane. It is an easy detail to forget. The other aromatic plants in writing about Vietnam are trees: cassia, star anise, cajuput, may chang. Lemongrass is a grass. It grows fast, can be cut many times a year, and needs no long years to produce its material.
Lemongrass grows everywhere in the country, in home gardens, beside rice fields, in dedicated growing areas. In Vietnamese cooking it is a base seasoning, present from grilled dishes to broth. Java citronella is rarer in the kitchen. It is grown for its oil, and the growing area is concentrated in Lâm Hà, Lam Dong.
Both oils are steam distilled from the leaves, fresh or dried. For lemongrass, the library records an oil yield of about 0.2 to 0.4 percent of leaf weight. That means one tonne of leaves gives roughly two to four kilograms of oil. Fresh and dried leaves give different scent profiles; distilling right after cutting keeps a fuller character.
From the outside, the two clumps are hard to tell apart for anyone who does not grow them. The difference lies inside the leaf, in small oil cells the eye cannot see.
Cymbopogon: one genus, many species
The genus Cymbopogon contains many aromatic grasses, and each species follows its own chemical path. Lemongrass, citronella, palmarosa and gingergrass are all close relatives. What separates them is not their shape but the molecule that dominates the leaf oil.
Lemongrass on the market comes from two species. West Indian lemongrass, Cymbopogon citratus, grows in the Caribbean, Brazil, Africa and parts of Southeast Asia. East Indian lemongrass, Cymbopogon flexuosus, grows in India, Sri Lanka and Southeast Asia. The lemongrass grown in Vietnam is C. citratus. The lemongrass page in the library records both species.
Citronella also comes in two commercial types. The Sri Lanka type, Cymbopogon nardus, has the longest trading history. The Java type, Cymbopogon winterianus, is produced in large volumes in Java, Taiwan and China, and now makes up most commercial citronella oil. The citronella grown in Lâm Hà is C. winterianus; the citronella page in the library records both types.
Palmarosa, Cymbopogon martinii, shows how far the genus can travel. Its oil holds 74.5 to 81 percent geraniol, and its scent leans clearly towards rose. Same genus, same grass shape, but no longer a lemon scent.
So common names mislead easily. A label reading “lemongrass” sometimes does not name the species. The Vietnamese word “sả” can refer to any of the species above. The Latin name is the only firm point of reference.
Citral in lemongrass
The lemon scent of lemongrass comes almost entirely from one pair of molecules. Citral is not a single substance but a mixture of two isomers: geranial, the (E) form, and neral, the (Z) form. In nature the two usually occur together at a ratio of about 60 to 40 up to 65 to 35. The shared formula is C₁₀H₁₆O.
Lemongrass oils from several regions of Vietnam gave 61 to 76 percent citral, higher in leaf oil than stem oil. For West Indian lemongrass in general, the library records geranial from 36.7 to 55.9 percent and neral from 25 to 35.2 percent. East Indian lemongrass usually carries more citral.
The rest of West Indian lemongrass oil holds one significant constituent: beta myrcene, from 5.6 to 19.2 percent. Myrcene adds a green, slightly resinous line that takes the edge off the lemon. East Indian lemongrass has less myrcene and so smells more direct. There is also geraniol up to 6.7 percent, geranyl acetate, limonene and linalool, each at a few percent.
The library describes the scent of citral as sharp lemon, bright and fresh. Geranial carries the sharpness; neral, as the library describes it, is softer and sweeter, with a lemony finish. When both forms stand together at the high share found in lemongrass, the scent opens quickly, harsh in the first seconds, then settles into a green base.
Citral is easy to synthesise, so lemongrass oil is often adulterated with synthetic citral. The library notes the method for telling them apart is GC MS analysis with a chiral column. Genuine oil carries minor constituents that plain citral lacks.
Citronellal and geraniol in Java citronella
Java citronella oil shares its scent among three molecules rather than one. For the Java type, the library records citronellal from 34.8 to 42.8 percent, geraniol from 22.1 to 25.4 percent and citronellol from 9.7 to 11.5 percent. No molecule fills more than half the bottle.
Citronellal is also known as dihydrocitral. Structurally it is citral with two added hydrogen atoms, formula C₁₀H₁₈O. That small change alters the scent entirely. The library describes citronellal as fresh citronella, lightly floral, faintly woody. It still belongs to the lemon family, but rounder and less sharp than citral.
Geraniol is an alcohol, also C₁₀H₁₈O, smelling of sweet rose with lemony and waxy facets. It is the main constituent of palmarosa and appears in geranium and Damask rose. In Java citronella, geraniol makes up about a quarter of the oil, and that is why citronella always carries a floral line that lemongrass lacks.
Citronellol, the third molecule, is also an alcohol with a rose scent. Together, geraniol and citronellol account for about a third of Java citronella oil. The remainder includes limonene, geranyl acetate and a small amount of methyleugenol, up to 1.7 percent.
The Sri Lanka type varies more widely: citronellal from 5.2 to 46.8 percent, geraniol from 16.8 to 29.1 percent. The library notes that the Java type smells cleaner and more pleasant. The stability of its ratios is also one reason the Java type holds most of the market.
Two lemons on a blotter
Two blotters dipped side by side show the difference more clearly than any table. The lemongrass blotter opens sharp and bright, close to freshly zested lemon peel but drier, with a grassy edge. The citronella blotter opens softer, with a floral sweetness, and the lemon sits further back.
After a few minutes the difference grows. Citral evaporates relatively quickly; the lemongrass blotter moves towards the green base of myrcene and the minor constituents. The citronella blotter holds its floral part longer, because geraniol and citronellol are alcohols, slower to evaporate than aldehydes. A faint rose remains once the lemon has receded.
This is the most practical way to tell the two oils apart when the label is unclear. Sharp lemon, then green grass: lemongrass. Round lemon, then flowers: citronella. Anyone who has smelled both will rarely confuse them again.
The difference also decides where each belongs in perfumery. Lemongrass gives a short flash of light at the top. Citronella can bridge the top and the floral heart, thanks to geraniol. Both rarely stand alone in a refined formula, because their grassy edge easily covers other materials.
The same word, lemon, but two sets of molecules. Vietnamese calls both sả; the nose can tell them apart.
Citral and limits on skin
The two oils have skin limits more than twenty times apart, and the reason lies in the main molecule of each. Citral is classified as a moderate sensitiser. IFRA limits citral to 0.6 percent in leave on products; this may rise to 1.4 percent when citral is paired with d limonene at five to ten times its amount. Under EU rules the label must name citral above 10 ppm in leave on products or 100 ppm in rinse off products.
From this, the maximum dermal level for lemongrass oil is 0.7 percent. The library adds: of 1,606 contact dermatitis patients, 25, or 1.6 percent, reacted to lemongrass oil at 2 percent; of another 100 patients, none reacted at 1 percent. Lemongrass oil is not phototoxic.
Citronella oil has a maximum dermal level of 18.2 percent, based on a sample with 29.1 percent geraniol and a geraniol limit of 5.3 percent. The library places this oil at low risk of skin sensitisation, but notes it may contain methyleugenol. Geraniol is on the list of allergens that must be declared on EU labels, at the same 10 ppm and 100 ppm thresholds; the library rates citronellal as a weak to moderate sensitiser.
For lemongrass oil the library lists further notes: avoid dermal use on children under 2, take care with sensitive or damaged skin. The oil should be kept in a dark bottle, tightly closed, in a cool place, because citral oxidises easily. Oxidised oil may be more sensitising than fresh oil.
Two lemons, two limits. The figures 0.7 and 18.2 are chemistry's way of rewriting the difference the blotter already showed.
Two clumps of grass, from the same family as rice.
One holds citral, the other shares its scent among three molecules.
The word lemon in the name is only the starting point.