Steam Distillation: How Essential Oils Are Extracted

Steam distillation is the most common method for extracting essential oils from plant material. It works by passing steam through the plant matter, which causes the aromatic compounds to volatilise. The steam and aromatic vapour are then cooled, causing them to condense back into liquid form. Because water and essential oil are immiscible, they separate naturally, and the oil is collected from the surface.

Temperature and pressure

The temperature and pressure of the steam affect which compounds are extracted and in what proportions. Lower temperature distillation preserves more of the delicate top notes and lighter compounds. Higher temperature distillation extracts more of the heavier, sesquiterpene compounds but can degrade some of the lighter aromatics.

This is why the distillation conditions are relevant to the GC/MS profile. Two batches of the same plant material distilled at different temperatures will produce oils with different compound percentages.

Cold pressing versus steam distillation

Citrus oils (lemon, bergamot, orange) are typically cold-pressed from the rind rather than steam distilled. Cold pressing preserves the full aromatic profile, including the furanocoumarins that make some citrus oils phototoxic. Bergamot FCF (furanocoumarin-free) is produced by an additional steam distillation step that removes these compounds, making it safe for use in leave-on products.

CO2 extraction

Supercritical CO2 extraction uses carbon dioxide under high pressure to extract aromatic compounds. It produces a more complete extraction than steam distillation and preserves heat-sensitive compounds that would be degraded by steam. CO2 extracts are generally more expensive and have a richer, more complex profile. Frankincense CO2 extract, for example, contains a broader range of boswellic acids than steam-distilled frankincense essential oil.