Corn oil was used on a large scale in research on the relationship between dietary fat and blood cholesterol levels as the only highly polyunsaturated and nonhydrogenated oil readily available to researchers and patients in the mid-1950s.
Almost all corn oil is expeller-pressed, then solvent-extracted using hexane or 2-methylpentane (isohexane). The solvent is evaporated from the corn oil, recovered, and re-used. After extraction, the corn oil is then refined by degumming and/or alkali treatment, both of which remove phosphatides. Alkali treatment also neutralizes free fatty acids and removes color (bleaching). Final steps in refining include winterization (the removal of waxes), and deodorization by steam distillation of the oil at 232–260 °C (450–500 °F) under a high vacuum.
Some specialty oil producers manufacture unrefined, 100%-expeller-pressed corn oil. This is a more expensive product since it has a much lower yield than the combination expeller and solvent process, as well as a smaller market share.