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Essential oils are contained in the glands, veins, sacs and glandular hairs of aromatic plants. Flowers, leaves and non-fibrous parts need little, if any, preparation prior to distillation. Tough stalks, woody parts, roots, seeds and fruits, however, need to be ‘comminuted’ (cut up, disintegrated or crushed – wood is grated) in order to rupture the cell walls, allowing the easy escape of the volatile oil. (Volatile is derived from the Latin volare – to fly.)

Distillation is still considered to be the most economical method of extracting essential oils from plant materials.⁴ Some plants have to be distilled immediately they are harvested, for example melissa; if left even a few hours, the essential oil is lost – the yield from melissa is, in any case, very low. Some plants are left a few days, e.g. lavender, so that surplus water in the plant can dry out – this, by the way, slightly affects the yield. Some, like black pepper seeds, clary (clary sage) and peppermint, can be totally dried before distilling without losing any essential oil. It can be seen from these few examples that there is an art to distillation and that, especially for low-yield plants, much skill is needed. The role of the distiller is to achieve an oil as close as possible to the oil as it exists in the plant.

We distil water every day each time we boil a pan or kettle – the heat lifts molecules of water from the surface and they evaporate into the air, or condense as distilled water on the pan lid.

During distillation, only very tiny molecules can evaporate, so they are the only ones which leave the plant. These extremely small molecules make up an essential oil. Oils containing more of the smallest and therefore most volatile of these tiny molecules are termed ‘top notes’ in the perfumery world; those containing more of the heaviest and least volatile of the tiny molecules are called ‘base notes’. Those in between are known as middle or sometimes ‘heart’ notes.

FIGURE 2.1: Steam distillation

When plants are heated by steam in a still (alambic in France), the essential oils present in the plant material are freed, evaporating into the steam. These tiny molecules are carried along a pipe together with the steam and as they get further away from the heat source they begin to cool. To hasten this process, the pipe passes through a large vat of cold water (the important addition to distillation contributed by Avicenna around AD 1000) and condenses back into liquid form. As the density of essential oil differs from that of water, it either floats on the top or sinks to the bottom (mostly the former), where it can be drawn off. The result is a pure, genuine, whole and natural essential oil – an aromatherapist’s dream! This is the oil used in any reference or research carried out before the 19th century, when there were no synthetics to adulterate or ‘ennoble’ nature’s gifts.

Distillation is more complex than I have made it sound (books have been written on this subject alone) but the underlying principle is simple to understand and remains unchanged.