FAQ

Some vegetable oils, such as olive, peanut, coconut and sunflower oils, are cold-pressed. This method, which entails minimal processing, produces a light, flavorful oil suitable for some cooking needs. Most oil sources, however, are not suitable for cold pressing, because it would leave many undesirable trace elements in the oil, causing it to be odiferous, bitter tasting, or dark. These oils undergo many steps beyond mere extraction to produce a bland, clear, and consistent oil.

Cleaning and grinding

1 Incoming oil seeds are passed over magnets to remove any trace metal before being dehulled, deskinned, or otherwise stripped of all extraneous material. In the case of cotton, the ginned seeds must be stripped of their lint as well as dehulled. In the case of corn, the kernel must undergo milling to separate the germ.
2 The stripped seeds or nuts are then ground into coarse meal to provide more surface area to be pressed. Mechanized grooved rollers or hammer mills crush the material to the proper consistency. The meal is then heated to facilitate the extraction of the oil. While the procedure allows more oil to be pressed out, more impurities are also pressed out with the oil, and these must be removed before the oil can be deemed edible.

Pressing

The heated meal is then fed continuously into a screw press, which increases the pressure progressively as the meal passes through a slotted barrel.

Extracting additional oil with solvents

Soybeans are usually not pressed at all before solvent extraction, because they have relatively little oil, but most oil seeds with more oil are pressed and solvent-treated. After the initial oil has been recovered from the screw press, the oil cake remaining in the press is processed by solvent extraction to attain the maximum yield. A volatile hydrocarbon (most commonly hexane) dissolves the oil out of the oil cake, which is then recovered by distilling the light solvent out. In using this machine, flakes of meal are sent through wedge-shaped cells of a cylindrical vessel. The solvent then passes through the matter to be collected at the bottom. The solvent percolates through the matter which is periodically dumped and replaced.