Steam distillation is the most commonly used method for collecting essential oils. It is a delicate and time-intensive process requiring careful attention to detail. Use of this method is prevalent not only because it yields exceptionally pure and clean products, but because it allows for collection of temperature-sensitive aromatic compounds. Unlike simple distillation, steam distillation involves a pressurized system. When the system is pressurized, essential oils can be distilled at temperatures well below their normal boiling point; thus protecting the integrity of their delicate and complex chemical profiles.
When using steam distillation, it is vital to pay careful attention to the heat source. The temperature of the system must remain within a strict range; too low and the essential oils will not be distilled, too high and there is risk of damaging the essential oils or collecting unwanted, non-aromatic compounds. Temperatures required for optimal steam distillation typically fall between 140° F and 212° F (60° C and 100° C). One benefit of steam distillation is that the temperature can be continuously adjusted and precisely controlled to ensure that the system always remains within the optimal temperature range.
Similarly, pressure must also be rigidly controlled. To do so, steam distillation requires the use of a “closed system.” This means that the system is pressurized to a level above that of atmospheric pressure. The required pressure varies from plant to plant, but typically falls between 15 pounds/in2 (PSI) and 20 PSI. Increasing the pressure of the distillation system can significantly reduce the distillation time. The time needed for distilling an oil can be reduced by up to one fourth if a pressurized system is used.
In addition to pressure and temperature, distillation time is also an important consideration. Each plant is unique, requiring anywhere from just a few hours to several days to fully distill. The wood oils typically have the longest distillation times requiring up to 36 hours or more to fully distill. On the other hand, lavender can be fully distilled in just 2 hours.
Steam distillation is conducted in a distillation still that uses water to remove the essential oil from the plant material. Heat is applied to the water, which produces steam. The steam rises and moves through a chamber holding the plant material. As the steam forces its way through the plant material, it ruptures small glands that hold the essential oil. Since essential oils contain only aromatic compounds that are readily volatile, the essential oil is easily carried by the steam into the condensing tube. Once there, the liquid is condensed and accumulates in the collecting still. After removal from the still, a mixture of essential oil and water will be present. Because essential oils are not water soluble, the mixture will naturally separate into two layers; a layer of pure essential oil on top and a layer of water underneath. The watery layer is called hydrosol and is often sold as floral water.