Through distillation, the most common way of extracting essential oils from a plant, the plant material is placed upon a grid inside a still. Once the still is sealed, a combination of steam or water, along with gentle pressure, slowly breaks through the plant material releasing the essential oils within from their microscopic protective sacs. Remember, essential oils are volatile aromatic compounds, meaning they move from a liquid to a gaseous state quickly and easily. The vapors formed through the distillation process rise upward in a gaseous state through a connecting pipe that leads into a condenser. The condenser then cools the vapors back into liquid form. Since water and oil do not mix, the essential oil settles on the surface of the water where it can be siphoned off.
A more modern method of extraction, used specifically for citrus fruits (which would lose their characteristic aromas and beneficial properties once heated), is expression. Unlike steam distillation, this process uses no heat, thus is sometimes referred to as "cold-pressing." During expression, fruits are sent through a machine where their peels are scraped/cut, allowing the essential oils within the peels to be released from their sacs. The fruit is then sprayed down with water to aid the collection of essential oil. The entire mixture (some solids from the peels, water and essential oils) is then sent through a centrifuge to separate the essential oil from any solids, water, or juice from the fruit.
The CO2 (carbon dioxide) method of extraction is costly, but is also quick and efficient compared to steam distillation. CO2 gas has the unique ability to reach a supercritical state (neither completely gas or liquid, but rather having properties of both), at a relatively low temperature and pressure. This allows even the most sensitive oils to be collected via this method. First, the CO2 gas is heated and pressurized to a supercritical state, turning it into a solvent, which is then added to the plant material, removing the essential oils from the plant. The resulting mixture is filtered to remove any leftover plant material and the pressure and heat are reduced. This causes the CO2 to return to its gaseous state, evaporate out of the oil, leaving a pure essential oil behind.