Plant oils are extracted from vegetables, fruits, nuts and seeds of plants in various ways.
1. Cold Pressing (Mechanical)
Cold pressed oils have the highest nutritional value, and if you are eating them also the best flavour and aroma.
Cold pressing involves grinding the vegetable, fruit, seed or nut using heavy granite millstone or stainless steel presses in such a way that the friction doesn't produce a lot of heat. By keeping temperatures below 49°C (120°F) the goodness of the oil is preserved. The lower you keep the temperature the better the oil you get.
Some oils such as grapeseed oil are never produced by cold pressing because you simply don't get much oil out of them with the method.
2. Expeller Method (Mechanical)
Under the expeller method oils are extracted using high pressure. Heat may also be used. If it is, temperatures as high as 85°C (185°F) are applied.
3. Solvent Extraction (Chemical)
The amount of solvent left could range from 6-20% with some solvents. With hexane, however, the residue left is said to be 10ppm which is a very low concentration; it's equivalent to 0.001%.
Personally, if I could prove that the solvent used was hexane and the price was much lower than with other oils then I would be happy to take my chances on this one because I spend a small fortune on ingredients for my DIY Hair Care Recipes.
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You might also like:
whfoods.org, cold-pressed oils (about.com), Expeller Pressing (wikipedia),
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By Heather Katsonga-Woodward