Some vegetable oils, such as olive, peanut, and some 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.
Cooking oil manufacturing process
Cleaning and grinding
1 .Incoming oil seeds are passed over magnets to remove any trace metal before being dehuller, 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 dehuller. 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
3. 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 from 68,950 to 20,6850 kilopascals as the oil is squeezed out from the slots in the barrel, where it can be recovered.
Extracting additional oil with solvents
4 .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.
Removing solvent traces
5 .Ninety percent 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.
Refining the oil
6. The oil is next refined to remove color, odor, and bitterness. Refining consists of heating the oil to between 107 and 188 degrees Fahrenheit (40 and 85 degrees Celsius) 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.
7. Oils are also degummed at this time by treating them with water heated to between 188 and 206 degrees Fahrenheit (85 and 95 degrees Celsius), steam, or water with acid. The gums, most of which are phosphatides, precipitate out, and the dregs are removed by centrifuge.
8 .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.
9. Finally, the oil is deodorized. In this process, steam is passed over hot oil in a vacuum at between 440 and 485 degrees Fahrenheit (225 and 250 degrees Celsius), 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.
Packaging the oil
10 .The completely processed oil is then I V measured and poured into clean containers, usually plastic bottles for domestic oils to be sold in supermarkets, glass bottles for imports or domestic oils to be sold in specialty stores, or cans for imports (usually olive oil).
