Essential oil

Steam passes through the plant material, breaks or 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.

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

Hydrodistillation (water distillation): the material is immersed in water and boiled with it. This suits material that clumps under steam, such as rose petals. Rose oil in Bulgaria and Turkey is made this way.

Water and steam distillation: the material rests on a grid above the water; boiling water below sends steam up through the charge. The material does not touch boiling water.

Direct steam distillation: steam is raised in a separate boiler and piped into the base of the still. The operator controls steam flow and pressure. This is the most common industrial set-up, used for leaves, grasses, seeds and wood.

ISO 9235:2021 does not treat these as three products. Entry 3.12 covers essential oil obtained by steam distillation, with water added to the still (hydrodistillation) or without (direct steam). The example given is essential oil of orris, commonly called iris butter.

The usual separator is the Florentine flask. Condensate flows in, oil and water separate, and the water leaves through an overflow set at a fixed height. Most essential oils are lighter than water and float. Some are heavier, such as clove bud and cinnamon bark oils, and sink; flasks for these have the outlet arranged the other way round.

The water that leaves is the hydrosol, which ISO calls aromatic water or hydrolate. It holds 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. Some distilleries return this water to the still for a second pass, a practice called cohobation. ISO 9235:2021 adds an entry for native aromatic water: aromatic water distilled from the material's own water, with no water added.

Dry distillation uses no water or steam. Wood, bark, roots or gums are heated in a closed vessel; heat breaks down part of the material and the condensate is a tar. ISO 9235:2021 defines this form at 3.9, with birch tar oil as the example. The library's examples are birch tar and cade.

Crude dry-distilled oils contain polycyclic aromatic hydrocarbons (PAH). IFRA does not allow crude birch tar or crude cade oil as fragrance ingredients. Only rectified grades may be used, provided the total of two markers, benzo[a]pyrene and 1,2-benzanthracene, stays at or below 1 ppb in the finished product.

Yields differ by a factor of several hundred between materials. A study of Damask rose clonal selections recorded 0.030 to 0.045% oil, roughly 2,200 to 3,300 kg of flowers per kilogram of oil. Clove bud sits at the other end: another study recorded about 15 to 17% depending on tree age.

An essential oil is not a copy of the plant. Heat and water can form new compounds. A known case is chamazulene in German chamomile: this blue compound forms from matricin during distillation and is not present as such in the flower. Distillation time changes composition too, since light molecules come over first and heavy ones later; fractional distillation relies on exactly this.