Vegetable oil used for culinary purposes is mostly contained in plant seeds including corn, soy, rapeseed that yields canola oil and olives. How the oil is extracted from these seeds varies from simple methods of grinding and pressing used over millennia to current, high-volume methods using sophisticated pressing machines and solvents.
Oil was traditionally extracted simply by crushing oil-bearing seeds with a pestle or millstone then pressing the oil from them; a version of this is still used in India. The Greeks and Romans added levers to presses. This method of extracting oil lasted through the 19th century when an Englishman, Joseph Bramah, invented the hydraulic press. An American, V. D. Anderson, invented the screw press in 1876.
Modern systems use variations of grinding and pressing to remove oil and more efficient, sophisticated pressing machinery called expellers, improved versions of Anderson's invention. High-volume commercial extraction uses heat and solvents to extract oil.
When oil is extracted from plants by squeezing them with a hydraulic press, the plants heat up; if the temperature is kept under 120 degrees Fahrenheit it is called cold pressed. First cold pressed means that the oil is obtained the first time an oil is pressed from a plant. Cold pressed oils have the best color, flavor and odor. The term "virgin" oil has no legal definition. First cold pressed oil is filtered to remove bits of seeds or olives, but it does not need to be refined.
Expeller Pressed Oil
The organic food community makes a distinction between pressed oil and oil extracted by an expeller press. In this method, which is more efficient, oil-bearing seeds are heated to 200 to 250 Fahrenheit then pushed through a worm press by a rotating shaft. Oil extracted in this manner is called "expeller pressed" or "pressed."
Oil Extracted by Solvents
The seed or other oil-bearing parts of a plant are ground and steam cooked. A petroleum-based solvent is added to dissolve everything except the oil. The solvents ordinarily used are heptane, hexane, octane or pentane, which are four types of naptha, a petroleum distillate. Trichlorethylene, a synthetic solvent, is sometimes used. The solvent is then separated from the oil. Using solvents to extract oil is the least expensive and most often used by companies that extract large volumes of oil commercially.
Refining Extracted Oil
The process of extracting oil with solvents is not complete until it is refined; this is accomplished by adding sodium hydroxide to it and heating it to 450 Fahrenheit. This process thins the oil but makes it more susceptible to becoming rancid. The oil is then filtered through activated carbons or clays to remove vitamins A, E and F plus phosphorous compounds and other impurities. Chemical agents are then added to correct the problem of the oil becoming rancid.