Essential oils are amazing compounds - one of the true wonders of nature. They are the "essence" of the plant - what makes it special. The oils are stored in the cells of the plants - sometimes in the leaves or needles (like in mint or pine), in the flowers (like rose or chamomile), bark (like birch or cinnamon), roots (like ginger or spikenard), in the skin or rind (like in citrus fruits) or in the seeds (like cardamom or anise).
For thousands of years, aromatic plants and oils have been used in perfumes and in a wide array of medicinal, culinary, religious applications. But how do we get the essential oils out of the plant's cells and into a bottle? The majority of essential oils are obtained through two methods: distillation and expression.
Expression works for citrus oils like orange, lemon and lime, but the majority of the rest of the essential oils come from some sort of distillation either water or steam.
The simplest distillation method is a simple water distillation. The plant material is immersed in water and boiled. The steam and the essential oils rise out of the hot water and are then cooled where they condense and are collected. Two products are created here - the essential oil and the condensed water which contains water soluble essences of the plant material. These floral waters are called hydrosols. This method works well with flower blossoms and finely powdered plant material.
Water distillation can be done under vacuum (reduced) pressure which allows the temperature to be less than boiling, which helps to protect and preserved some of the more delicate plant materials.
Steam distillation, the most common method of extracting essential oils, uses a very similar setup to water distillation, except instead of being immersed in the hot water, steam is passed through the plant material. (See diagram.) The steam breaks open the cells containing the essential oil, and the steam and oil then pass into the cooling chamber where, like with water distillation, two products are created - essential oil and hydrosol.