Oil Extraction from Nut

Many oil-bearing seeds and nuts are broken up by grinding, flaking, or rolling, then subjected to mechanical pressing to liberate the oil. The modern continuous screw press exerts pressures as high as 30,000 pounds per square inch. In modern press extraction, oilseeds or nuts are cleaned, and the shells or hulls removed; the kernels or meats are ground to a coarse meal that is pressed with or without preliminary heating. Cold-pressed oil, also called cold-drawn, or virgin, oil, is purer and has a better flavour than oil expressed with the aid of heat. After pressing the meals made from oily seeds or nuts, the remaining cake contains about 5 to 15 percent oil. Most of the oil present in these residues, and in meals made from seeds and nuts that naturally contain little oil, can be removed by extraction with volatile solvents, especially petroleum benzin (also known as petroleum ether, commercial hexane, or heptane). The solvent is percolated through the meal, dissolving the oil, which is finally recovered from the solution by evaporating the solvent. The solvent is also recovered and used over again.

part of article taken from http://www.britannica.com/EBchecked/topic/426152/oil-extraction