Ép lạnh: tinh dầu vỏ citrus lấy bằng lực cơ học

The oldest hand method in Calabria and Sicily is the sponge method (spugna). The fruit is halved and emptied, and the peel is pressed against a sponge that soaks up the oil, which is then squeezed out. In southern France there was the écuelle à piquer: a bowl lined with spikes. The fruit is rolled in the bowl, the spikes pierce the glands, and the oil runs down to a tube at the centre of the base.

Machines replaced hands from the nineteenth century. In Italy the two main machine types are the sfumatrice and the pelatrice. The sfumatrice works on emptied half peels: the peel is folded and squeezed between rollers, and the glands burst under their own internal pressure. The pelatrice works on whole fruit: abrasive rollers rasp the outer peel and water sprays wash the oil away. Some bergamot processors in Reggio Calabria list lines of both types.

In the Americas, citrus oil is mostly a by-product of the juice industry. FMC extractors take juice and peel oil at the same time on one line; the Brown peel shaver rasps whole fruit. Whatever the machine, the output is an oil in water emulsion. FAO notes that one tonne of oranges gives about 200 to 300 litres of emulsion and finally about 3 to 6 litres of oil after centrifuging.

The volatile part of citrus peel oil is mostly monoterpene hydrocarbons, led by limonene. The character of each fruit comes from smaller fractions: aldehydes such as citral in lemon, esters such as linalyl acetate in bergamot.

Because it is not distilled, pressed oil keeps non-volatile matter: waxes, carotenoid pigments, coumarins and furocoumarins such as bergapten and bergamottin. These compounds would not reach the condenser if the oil were distilled. Limonene oxidises readily in air and light, so pressed oils need closed storage away from heat.

Lime is the clearest case of one fruit, two products. Expressed lime oil comes from the peel and keeps citral and the fresh peel character. Distilled lime oil, made mainly in Mexico, follows another route: whole fruit is crushed and the juice and oil mixture is steam distilled for several hours.

During that time the acid in the juice catalyses reactions in the oil. Citral falls sharply and terpene alcohols such as alpha terpineol rise. Furocoumarins do not pass into the distillate, so distilled lime oil is not phototoxic. Lemon comes in the same two forms: expressed lemon and distilled lemon.

Some furocoumarins in pressed oils, led by bergapten (5-methoxypsoralen), cause a phototoxic reaction when skin carrying the oil meets UV light. This is why expressed citrus oils have their own rules.

The IFRA Standard on citrus oils and other furocoumarin-containing essential oils sets a limit of 0.0015% (15 ppm) total bergapten in finished leave-on products applied to sun-exposed skin. The limit does not apply to rinse-off products. When several furocoumarin-bearing materials are combined, their effect is treated as additive. In the European Union, Annex II of Cosmetics Regulation 1223/2009 prohibits furocoumarins except for their normal content in natural essences; in sun protection and bronzing products total furocoumarins must stay below 1 mg/kg.

To use bergamot without this constraint, producers remove the furocoumarins, usually by vacuum distillation. The product is labelled FCF (furocoumarin free) or bergaptene free, as with bergamot FCF. See also FCF.

Expressed, cold pressed, pressed or CP indicate a pressed oil. Distilled indicates oil distilled from peel or whole fruit. FCF or bergaptene free indicates furocoumarins have been removed. Terpeneless, or figures such as 5x and 10x, indicate an oil concentrated by removing much of its limonene; see rectified and terpeneless. The producing region, such as Calabria or Mexico, is often given because the same fruit grown elsewhere has a different composition.