General process information
Some vegetable oils, such as olive, peanut, and some coconut and sunflower oils, are cold-pressed. This method, which entails minimal processing, produces 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 bland, clear, and consistent oil.
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 corn, the kernel must undergo milling to separate the germ.
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.
The heated meal is then fed continuously into a screw press, which increases the pressure progressively as the meal passes through a slotted barrel. Pressure generally increases as the oil is squeezed out from the slots in the barrel, where it can be recovered.
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.
90% of the solvent remaining in the extracted oil simply evaporates, and, as it does, it is collected for reuse. The rest is retrieved with the use of a stripping column. The oil is boiled by steam, and the lighter hexane floats upward. As it condenses, it, too, is collected.
The oil is next refined to remove color, odor, and bitterness. Refining consists of heating the oil to between 40 and 85 degrees Celsius (107 and 188 degrees Fahrenheit) and mixing an alkaline substance such as sodium hydroxide or sodium carbonate with it.
Soap forms from the undesired fatty acids and the alkaline additive, and it is usually removed by centrifuge. The oil is further washed to remove traces of soap and then dried.
Oils are also degummed at this time by treating them with water heated to between 85 and 95 degrees Celsius (188 and 206 degrees Fahrenheit), steam, or water with acid. The gums, most of which are phosphatides, precipitate out, and the dregs are removed by centrifuge.
Oil that will be heated (for use in cooking) is then bleached by filtering it through fuller’s earth, activated carbon, or activated clays that absorb certain pigmented material from the oil. By contrast, oil that will undergo refrigeration (because it is intended for salad dressing, for example) is winterized—rapidly chilled and filtered to remove waxes. This procedure ensures that the oil will not partially solidify in the refrigerator.
Finally, the oil is deodorized. In this process, steam is passed over hot oil in a vacuum at between 225 and 250 degrees Celsius (440 and 485 degrees Fahrenheit), thus allowing the volatile taste and odor components to distill from the oil. Typically, citric acid at. 01 percent is also added to oil after deodorization to inactivate trace metals that might promote oxidation within the oil and hence shorten its shelf-life.
The most obvious byproduct of the oil making process is oil seed cake. Most kinds of seed cake are used to make animal feed and low-grade fertilizer.
Lecithin is a byproduct of the degumming process used in making soybean oil. This industrially valuable product is used to make animal feed, chocolate, cosmetics, soap, paint, and plastics—to name just a few of its diverse uses. Additional by product is the residual oil seed cake. The cake is high in protein and other nutrients, and researchers are working to develop methods of processing it into a palatable food that can be distributed in areas where people lack sufficient protein in their diets.