Oil Methods of Extraction revisited

The overwhelming majority of essential oils are volatile and so steam distilled even the Citrus. Cold pressing of Citrus Bergamot Oil produces the oil most commonly used in aromatherapy and perfume . The edible carrier oils used in massage are cold pressed.

The method of extraction depends greatly on the quantity of the oil in the plant, how it is stored and the extent to which it is volatile.

For example: 

In many edible oils (sunflower, olive, almond, canola etc) the oil is not volatile and is present in the seed or fruit at up to 40% by weight which makes it suited to pressing. 

Essential oils are present at much lower concentrations (eg Tea Tree Oil is only around 1% of the harvested biomass).  Most EO's are volatile making them ideal for steam extraction. 

It would be impractical (actually impossible) to extract TTO by pressing and uneconomical to use solvent extraction.  While CO2 extraction is possible in TTO the chemical profile in the resulting oil differs from standard profiles obtained by steam extraction and CO2 extraction is again uneconomical given the volume of biomass (leaf material) needed to produce a Kg of the oil. 

The extraction method is also driven by the ultimate usefulness of the oil .  Tea Tree oil is a useful antiseptic while Rose Oil gives us something very precious indeed!

The method of extraction used is therefore usually tailored to the characteristics of the source material and based on economical considerations for practical, efficient extraction and end use rather than quality per se. 

CO2 extracts present specific characteristics, quite different to essential oils (composition, taste or smell, etc.). CO2 extracts have found their own market in food uses, but quite a few in cosmetic and perfume uses.

The way to extract essential oils depends primarily on the physical characteristics of the raw materials to extract, but also on the chemical characteristics of the extract and finally the traditional methods. For the same plant, the composition of the extract will be different by disltillation time.  

Cold pressing is used for citrus as well as Carrier oils, but there is much Steam Distillation of Citrus with different qualities obtained. Steam distillation is generally used for roots flowers or leaves with an good concentration of oils and a physical facility to extract it by water steam, but a lot of other flowers give bad results with steam distillation and need other stronger solvents to obtain an extract (concrete). 

"Purity" in the sense of capturing the useful minor components depends also on the traditional vs modern handling and diligence of preparation of the biomass as well as the method of extraction. The whole Organic vs Pure debate turns on this. Organic methods are thought and do seem to yield a more "energetic" and useful oil for aromatherapy purposes apart from the issue of treatment residues.

Again while Lemon oil may be steam distilled without much loss of "purity" given its usefulness and composition the more precious Bergamot oil commonly used in aromatherapy and perfume tends to be cold pressed using traditional methods by the main producers so as to capture the useful components present in more minor proportions in the oil. A longer (and more expensive) distillation can be another factor in producing a "purer" oil for example in frankincense.

Yes CO2 or solvent extraction is the most accurate representation of the oil in the biomass and useful where the characteristic desired is obvious. There is obvious no need to improve on the smell of the rose or jasmine and the producer would not find a market for anything but. However some of the most useful components for aromatherapy and perfumery can be the product of the steam distillation process itself rather than found naturally in the plant for example the anti inflammatories in the chamomiles. A "purer" oil in this sense would not satisfy the purists.