Oils are extracted in many different ways. Sometimes the method used depends on the oil. Cooking oils are often extracted using a solvent like hexane, while aromatic oils are extracted with various methods including steam distillation and the traditional cold pressing method. In all of these options, the goal is to produce a pure oil free from contaminants and traces of the original plant material.
Hexane and Utility Oils
For utility oils such as cooking oils or oils used for skin care, the most common method of extraction is to use a solvent. The solvent is combined with the substance to draw out the oil, then is itself extracted to leave only the oil. The most common solvent is the petroleum-based hexane, but this solvent comes with several problems, as it is harmful to the environment and dangerous for workers to breathe in, and it finds its way into processed oils in trace amounts.
Essential Oils
For essential oils, or oils derived from plants and their seeds or flowers for aromatic purposes, there are several different methods of extraction available. While solvent extraction works for aromatic oils, too, healthier options are available and are often preferred, especially by small manufacturers and amateurs. These methods are discussed in the sections below.
Steam Distillation
With this method the plant substance is placed in a still, where pressurized steam is used to separate the oil and carry it away as vapor. This vapor is then cooled back down to a liquid, and the oil that was recovered floats to the top, where it is skimmed off. This method produces a pure oil, but takes time and a large quantity of the plant being used.
Cold Pressing
Cold pressing requires a rind, so it can only be used with citrus fruits and certain seeds. The rind is ground into small pieces and then pressed. Under the pressure, the oil will be squeezed out along with trace amounts of the rind. The oil is then allowed to separate and harvested.
Carbon Dioxide
A more modern method, this involves pressurizing carbon dioxide until it turns into a liquid. The pressurized liquid is combined with the plant material and allowed to extract the oil like a solvent. When depressurized, the carbon dioxide naturally turns back into a gas, and the oil extract is left behind.
Further Steps
These methods, when used on a wide scale, do not produce uniform results. Even with methods using oil skimming or solvents, the oil is often mixed with other residues and some of the original matter from which it was derived. To solve this problem, when oil is mass-produced it goes through several steps call clarification, in which the oil is strained and purified.
Sometimes the oil is simply left in bins for a short periods so the pure substance can rise to the top, while other processes use a mechanical clarify. The extract left after the pure oil has been removed also has uses, such lending food products a particular flavor.
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