Bốn cách lấy hương từ một bông hoa

About a tonne of bitter orange blossom gives one kilogram of neroli oil. The same flowers, through a solvent, give a darker liquid carrying indole and methyl anthranilate that the distilled oil lacks. One flower, many molecules, and each way of taking its scent keeps a different part.

One Flower, Many Molecules

A bitter orange blossom, Citrus x aurantium var. amara, holds dozens of aromatic molecules. Some are light and evaporate quickly; some are heavy and barely move. Some dissolve in oil, some in water. Some are stable under heat, some change when boiled. No method brings them all back at once.

So the question of how to take scent from a flower is really a question of which part to keep. Steam distillation keeps what travels with steam. An organic solvent keeps what dissolves in it, waxes included. Compressed CO2 keeps what dissolves in a gas pressed close to a liquid state. Fat keeps what the flower gives off while it lies there.

The ISO 9235:2021 standard draws these lines more strictly than everyday speech. Under it, an essential oil comes from only three routes: steam distillation, mechanical pressing of citrus peel, and dry distillation. Concretes, absolutes and CO2 extracts use solvents, so they are not essential oils in the ISO sense.

This piece takes the bitter orange blossom as its axis and walks through four methods. It also opens a series on process: what happens between the field and the bottle.

Steam and the Still

Steam passes through the mass of flowers, penetrates the oil bearing structures and carries away the molecules that can volatilise. Two liquids that do not mix, such as water and oil, boil together below the boiling point of either. This is how molecules boiling above 200 °C still reach the condenser while the still sits at about 100 °C. That is the principle of steam distillation.

The vapour passes a condenser, turns back to liquid and runs into a separator, where oil and water form two layers. The usual separator is the Florentine flask. What cannot travel with steam, such as waxes, pigments, sugars and most very heavy molecules, stays in the still.

From bitter orange blossom the product is neroli. The yield is about 0.1 percent of fresh flower weight. The oil is rich in linalool, 31 to 54 percent, with limonene, linalyl acetate and beta pinene. Its smell is recorded as clear white flower, fresh, lightly sweet, with a green note and a slight bitterness.

The water leaving the separator is orange flower water, the hydrosol. It carries the water soluble molecules. With rose, 2 phenylethanol dissolves readily in water, so most of it ends up in rose water rather than in the oil. Heat and water can also create new molecules: the blue chamazulene in German chamomile oil forms during distillation and is not present as such in the flower. An essential oil is not a copy of the plant.

Solvent, Concrete and Absolute

Flowers are spread in layers on trays inside an extractor and washed with solvent several times. The solvent, usually hexane today, dissolves aroma molecules along with waxes, pigments and fats on the petal surface. Once the solvent is distilled off under reduced pressure, what remains is a waxy mass, often orange, brown or yellow, called a concrete. Yields are usually only 0.1 to 0.5 percent of fresh flower weight.

A concrete keeps its waxes, so it does not fully dissolve in alcohol. The second step is to wash the concrete with ethanol, chill it deeply below 0 °C so the waxes set and separate, filter them off, and distil the ethanol under vacuum so the aromatic part is not heated for long. The product is an absolute: a thick liquid, darker than an essential oil, soluble in alcohol. For rose, a 2003 study recovered 58 to 64 percent absolute from the concrete.

Orange flower absolute differs clearly from neroli. 2 phenylethanol makes up 4.5 to 35 percent, the molecule that distillation largely leaves in the water. Methyl anthranilate 3 to 15 percent. Indole 0.1 to 1 percent, giving a characteristic animal note absent from neroli. The scent is fuller, closer to the fresh flower. Two materials from the same petal can sit side by side in a formula as two different substances.

Solvents raise the question of residues. Producers measure them by gas chromatography and report them on the technical data sheet. Benzene, once used to extract flowers, has been prohibited by IFRA as a fragrance ingredient since 1988, with a residue ceiling of 1 ppm in the fragrance mixture. Orange flower absolute can also contain up to 1 percent benzyl cyanide, which IFRA limits to 0.01 percent in the finished product.

CO2 in the Supercritical State

Compressed and warmed past its critical point, about 31 °C and 74 bar, CO2 is no longer a gas or a liquid in the usual sense. It moves into plant tissue like a gas and dissolves aromatic compounds like a liquid. When the pressure is released, CO2 turns back into gas and leaves. What remains in the separator is the CO2 extract. The ISO standard calls it a supercritical fluid extract.

The solvent power of CO2 rises with its density, so the composition of the extract is chosen by setting pressure and temperature. A select extract is made at lower pressure, about 90 to 200 bar, giving a mobile liquid close to the essential oil of the same species, often with a few heavier molecules that steam carries poorly. A total extract is made at about 250 to 350 bar and also takes waxes, fats and pigments, so it is often thick or sets on cooling.

The method needs no boiling water and leaves no organic solvent such as hexane behind. A CO2 extract carries fewer by products of heat and hydrolysis than an essential oil. With ginger, the CO2 extract keeps the gingerols, pungent compounds that do not travel with steam; with black pepper, the total grade carries piperine. With fresh flowers, studies such as a 2012 liquid CO2 extraction of jasmine show the method can be used, but most commercial CO2 extracts today come from dry material: seeds, roots, fruit, resins.

No solvent residue does not mean harmless. IFRA limits and cosmetic rules apply by composition, as for the essential oil of the same species, and a total extract may be richer in sensitising compounds than the oil. How CO2 works is discussed further in CO2, a gas used as a solvent.

Flowers on Fat: A Method Almost Lost

Before volatile solvents, fat was the main way to hold the scent of flowers that could not survive a still. Fat absorbs aroma molecules and keeps them. The product is a pomade: a mass of fat saturated with flower scent. The method is called enfleurage.

Cold enfleurage used the chassis: a glass pane set in a wooden frame. Both faces of the glass were spread with a thin layer of fat, purified so it had no smell of its own. Flowers were strewn on the fat and frames were stacked so each layer of flowers sat enclosed between two layers of fat. After about a day the spent flowers were removed and fresh ones laid down. The recharging went on for weeks until the fat was strongly scented. It suited jasmine and tuberose, which keep releasing scent after picking. According to a City of Grasse heritage record, cold enfleurage appeared there around 1750 for delicate flowers such as orange blossom, jasmine and tuberose.

Hot enfleurage dropped flowers into melted fat, stirred and kept them warm for a time, then strained them out and added a new batch. Heat moved the scent into the fat faster but could alter heat sensitive flowers. The pomade was then stirred repeatedly with alcohol; the alcohol drew out the scent while the fat largely stayed behind. That solution is extrait de pomade, and after chilling, filtering and removing the alcohol, the result is a pomade absolute.

Enfleurage took many hands, many weeks and much fat for a small amount of scent. Solvent extraction did the same job in hours. In Grasse the Chiris firm built a solvent extraction hall in 1898, opened in 1899; the concrete gradually replaced the pomade during the twentieth century. Today the method survives mainly in museums, classes and a few small workshops, although the Absolue Pays de Grasse geographical indication, approved by INPI in 2020, still accepts enfleurage alongside volatile solvents and supercritical CO2.

Four Scents of the Same Flower

Set four blotters side by side, each from one method, and the orange blossom appears four times. Neroli is bright, green and clear, mostly linalool and light terpenes. Orange flower water is soft and muted, carrying the water soluble molecules. The absolute is full and sweet, with the animal depth of indole and the roundness of methyl anthranilate. A CO2 extract or a pomade, where they exist, would sit at other points along the same axis.

No method is more faithful than another. Each is a filter, and each filter has its own blind spot. Steam leaves behind the heavy part and the water soluble part. Solvent brings wax and needs a second washing step. CO2 depends on the pressure chosen. Fat demands time that modern industry can no longer pay for.

This matters when reading a label. The words jasmine essential oil on a bottle almost certainly do not mean an essential oil in the ISO sense, since jasmine is hardly distilled on a commercial scale; its main form is the absolute. A complete label should give the Latin name, the plant part and the method. A label that says absolute should lead to the next question: absolute of what.

At Lê Mai the words tinh dầu on the shelf are used in the broad everyday sense. But behind every bottle there is still one of these four routes, and that route decides which flower, among the many a flower could be, went into the bottle.

Steam, solvent, compressed gas, fat.

Four filters for the same petal.

The scent in the bottle is the part the method chose to keep.