Many plants have compounds within them that produce familiar odours or flavors. These aromas are the result of volatile essential oils. These essential oils are often isolated from the respective plant to produce concentrated natural flavorings. These essential oils may be a combination of organic compounds.
Spices are well known for their flavor and odour. By using a process of steam distillation, one can separate the essential oil from the organic bulk. One requirement of steam distillation is that the compound to be removed is insoluble in water. This results in both compounds distilling at a lower boiling point than the single pure compound.
Each immiscible liquid will exert its own vapour pressure over the liquid, independent of the other liquid. The total pressure is the sum of the individual vapour pressures (Dalton's Law of Partial Pressures). The total pressure must be equal to the atmospheric pressure. Thus, the two compounds will distill at a lower temperature than their normal boiling points.
The oil is then separated from the condensed water using solvent extraction.