6.0 KiB
| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Cooking oil | 2/3 | https://en.wikipedia.org/wiki/Cooking_oil | reference | science, encyclopedia | 2026-05-05T14:15:11.297968+00:00 | kb-cron |
=== Storing and keeping oil === All oils degrade in response to heat, light, and oxygen. To delay the onset of rancidity, a blanket of an inert gas, usually nitrogen, is applied to the vapor space in the storage container immediately after production – a process called tank blanketing. In a cool, dry place, oils have greater stability, but may thicken, although they will soon return to liquid form if they are left at room temperature. To minimize the degrading effects of heat and light, oils should be removed from cold storage just long enough for use. Refined oils high in monounsaturated fats, such as macadamia oil, keep up to a year, while those high in polyunsaturated fats, such as soybean oil, keep about six months. Rancidity tests have shown that the shelf life of walnut oil is about 3 months, a period considerably shorter than the best before date shown on labels. By contrast, oils high in saturated fats, such as avocado oil, have relatively long shelf lives and can be safely stored at room temperature, as the low polyunsaturated fat content facilitates stability.
== Types and characteristics == Cooking oils are composed of various fractions of fatty acids. For the purpose of frying food, oils high in monounsaturated or saturated fats are generally popular, while oils high in polyunsaturated fats are less desirable. High oleic acid oils include almond, macadamia, olive, pecan, pistachio, and high-oleic cultivars of safflower and sunflower. Cold-Pressed vs. Refined Cooking Oils
Cold-pressed oils are extracted mechanically without the use of heat or chemical solvents, preserving nutrients and natural flavors, whereas refined oils undergo additional processes like bleaching and deodorization, which can strip beneficial compounds.
=== Smoke point === The smoke point is marked by "a continuous wisp of smoke". It is the temperature at which an oil starts to burn, leading to a burnt flavor in the foods being prepared and degradation of nutrients and phytochemicals characteristic of the oil. Above the smoke point are flash and fire points. The flash point is the temperature at which oil vapors will ignite but are not produced in sufficient quantities to stay lit. The flash point generally occurs at about 275–330 °C (527–626 °F). The fire point is the temperature at which hot oil produces sufficient vapors they will catch on fire and burn. As frying hours increase, all these temperature points decrease. They depend more on an oil's acidity than fatty-acid profile. The smoke point of cooking oils varies generally in association with how oil is refined: a higher smoke point results from removal of impurities and free fatty acids. Residual solvent remaining from the refining process may decrease the smoke point. It has been reported to increase with the inclusion of antioxidants (BHA, BHT, and TBHQ). For these reasons, the published smoke points of oils may vary.
Oils are extracted from nuts, seeds, olives, grains or legumes by extraction using industrial chemicals or by mechanical processes. Expeller pressing is a chemical-free process that collects oils from a source using a mechanical press with minimal heat. Cold-pressed oils are extracted under a controlled temperature setting usually below 105 °C (221 °F) intended to preserve naturally occurring phytochemicals, such as polyphenols, tocotrienols, plant sterols and vitamin E which collectively affect color, flavor, aroma and nutrient value.
== Extraction and refinement ==
Cooking oil extraction and refinement are separate processes. Extraction first removes the oil, typically from a seed, nut or fruit. Refinement then alters the appearance, texture, taste, smell, or stability of the oil to meet buyer expectations.
=== Extraction === There are three broad types of oil extraction:
Chemical solvent extraction, most commonly using hexane. Pressing, using an expeller press or cold press (pressing at low temperatures to prevent oil heating). Decanter centrifuge. In large-scale industrial oil extraction, combinations of pressing, chemical extraction and/or centrifuging are used in order to extract the maximum amount of oil possible.
=== Refinement === Cooking oil can either be unrefined, or refined using one or more of the following refinement processes (in any combination):
Distilling, which heats the oil to evaporate off chemical solvents from the extraction process. Degumming, by passing hot water through the oil to precipitate out gums and proteins that are soluble in water but not in oil, then discarding the water along with the impurities. Neutralization, or deacidification, which treats the oil with sodium hydroxide or sodium carbonate to pull out free fatty acids, phospholipids, pigments, and waxes. Bleaching, which removes "off-colored" components by treatment with fuller's earth, activated carbon, or activated clays, followed by heating, filtering, then drying to recoup the oil. Dewaxing, or winterizing, improves clarity of oils intended for refrigeration by dropping them to low temperatures and removing any solids that form. Deodorizing, by treating with high-heat pressurized steam to evaporate less stable compounds that might cause "unusual" odors or tastes. Preservative addition, including antioxidants such as BHA, BHT, and tocopherol to help preserve oils that have been made less stable due to high-temperature processing. Filtering, a non-chemical process which screens out larger particles, could be considered a step in refinement, although it does not alter the state of the oil. Most large-scale commercial cooking oil refinement will involve all of these steps in order to achieve a product that's uniform in taste, smell and appearance, and has a longer shelf life. Cooking oil intended for the health food market will often be unrefined, which can result in a less stable product but minimizes exposure to high temperatures and chemical processing.
== Waste cooking oil ==