Cooking oil (also known asedible oil) is a plant or animal liquidfat used in frying, baking, andother types of cooking. Oil allows higher cooking temperatures than water, making cooking faster and more flavorful, while likewise distributing heat, reducing burning and uneven cooking. It sometimes imparts its own flavor. Cooking oil is also used in food preparation and flavoring not involving heat, such assalad dressings and bread dips.
Cooking oil is typically a liquid at room temperature, although some oils that contain saturated fat, such ascoconut oil,palm oil andpalm kernel oil are solid.[1]
Mayo Clinic has highlighted certain oils that are high in saturated fats, includingcoconut,palm oil andpalm kernel oil. Those having lower amounts of saturated fats and higher levels of unsaturated (preferably omega-3) fats like olive oil, peanut oil, canola oil, soy andcottonseed oils are generally healthier.[7] The USNational Heart, Lung and Blood Institute[8] urged saturated fats be replaced with polyunsaturated and monounsaturated fats, listing olive and canola oils as sources of healthier monounsaturated oils while soybean and sunflower oils as good sources of polyunsaturated fats. One study showed that consumption of non-hydrogenated unsaturated oils like soybean and sunflower is preferable to the consumption of palm oil for lowering the risk ofheart disease.[9]
Cashew oil and other nut-based oils do not present a danger to persons with anut allergy, because oils are primarily lipids, and allergic reactions are due to surface proteins on the nut.[10]
The seeds of most cultivated plants contain higher levels ofomega-6 fatty acids thanomega-3, with some notable exceptions. Growth at colder temperatures tends to result in higher levels of omega-3 fatty acids in seed oils.[11]
Unlike other dietary fats, trans fats are notessential, and they do not promote good health.[12] The consumption of trans fats increases one's risk ofcoronary heart disease[13] by raising levels ofLDL cholesterol and lowering levels ofHDL cholesterol.[14] Trans fats frompartially hydrogenated oils are more harmful than naturally occurring oils.[15]
Several large studies[16][17][18][19] indicate a link between the consumption of high amounts of trans fat and coronary heart disease, and possibly some other diseases. The United StatesFood and Drug Administration (FDA), theNational Heart, Lung and Blood Institute and theAmerican Heart Association (AHA) all have recommended limiting the intake of trans fats. In the US, trans fats are no longer "generally recognized as safe", and cannot be added to foods, including cooking oils, without special permission.[20]
Lisbon oil merchant, c. 1900.Olive oilSunflower seed oil
Heating, as well as heating vessels rapidly change characteristics of cooking oil.[21] Oils that arehealthy at room temperature can become unhealthy when heated above certain temperatures, especially when heating repeatedly. The toxic risk is linked to oxidation of fatty acids and fatty acids with higher levels of unsaturation are oxidized more rapidly during heating in air.[22]So, when choosing a cooking oil, it is important to match the oil'sheat tolerance with the temperature which will be used.[23] and to change frying oil a few times per week.[22]Deep-fat frying temperatures are commonly in the range of 170–190 °C (338–374 °F), less commonly, lower temperatures ≥ 130 °C (266 °F) are used.[24]
Palm oil contains more saturated fats than canola oil, corn oil, linseed oil, soybean oil, safflower oil, and sunflower oil. Therefore, palm oil can withstanddeep frying at higher temperatures and is resistant tooxidation compared to high-polyunsaturated vegetable oils.[25] Since the 1900s, palm oil has been increasingly added into food by the global commercial food industry because it remains stable in deep frying, or in baking at very high temperatures,[26][27] and for its high levels of natural antioxidants, though the refined palm oil used in industrial food has lost most of its carotenoid content (and its orange-red color).[28]
The following oils are suitable for high-temperature frying due to their highsmoke point:
Less aggressive frying temperatures are frequently used.[30] A quality frying oil has a bland flavor, at least 200 °C (392 °F) smoke and 315 °C (599 °F) flash points, with maximums of 0.1% free fatty acids and 3% linolenic acid.[31] Those oils with higher linolenic fractions are avoided due to polymerization or gumming marked by increases in viscosity with age.[30] Olive oil resists thermal degradation and has been used as a frying oil for thousands of years.[30]
All oils degrade in response to heat, light, and oxygen.[32] To delay the onset ofrancidity, a blanket of aninert gas, usually nitrogen, is applied to the vapor space in the storage container immediately after production – a process calledtank blanketing.[citation needed][33]
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.[citation needed]
Refined oils high in monounsaturated fats, such asmacadamia oil,[32] keepup to a year, while those high in polyunsaturated fats, such assoybean oil, keep about six months. Rancidity tests have shown that the shelf life ofwalnut oil is about 3 months, a period considerably shorter than thebest before date shown on labels.[32]
By contrast, oils high in saturated fats, such asavocado oil, have relatively long shelf lives and can be safely stored atroom temperature, as the low polyunsaturated fat content facilitates stability.[32]
Cooking oils are composed of various fractions offatty acids.[34] 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.[24] Higholeic acid oils include almond, macadamia, olive, pecan, pistachio, and high-oleic cultivars of safflower and sunflower.[35]
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.[36]
^Warner and Gupta reported fishy and stale flavors in potato chips fried in this oil and attributed them to the unusual linoleic:linolenic acids ratio.[44]
The smoke point is marked by "a continuous wisp of smoke".[54] It is the temperature at which an oil starts to burn, leading to a burnt flavor in the foods being prepared and degradation ofnutrients andphytochemicals characteristic of the oil.[55]
Above the smoke point are flash and fire points.[54] 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).[56] The fire point is the temperature at which hot oil produces sufficient vapors they will catch on fire and burn.[56] As frying hours increase, all these temperature points decrease.[56] They depend more on an oil's acidity than fatty-acid profile.[57]
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.[55] Residual solvent remaining from the refining process may decrease the smoke point.[57] 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.[57]
^Specified smoke, fire, and flash points of any fat and oil can be misleading: they depend almost entirely upon the free fatty acid content, which increases during storage or use. The smoke point of fats and oils decreases when they are at least partially split into free fatty acids and glycerol; the glycerol portion decomposes to form acrolein, which is the major source of the smoke evolved from heated fats and oils. A partially hydrolyzed oil therefore smokes at a lower temperature than non-hydrolyzed oil. (Adapted fromGunstone, Frank D., ed. (17 March 2011).Vegetable Oils in Food Technology: Composition, Properties and Uses. Wiley, Inc.OCLC1083187382.)
Oils are extracted from nuts, seeds, olives, grains or legumes byextraction 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 aspolyphenols,tocotrienols,plant sterols andvitamin E which collectively affect color, flavor, aroma and nutrient value.[55][79]
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.
In large-scale industrial oil extraction you will often see some combination of pressing, chemical extraction and/or centrifuging in order to extract the maximum amount of oil possible.[105]
Cooking oil can either be unrefined, or refined using one or more of the following refinement processes (in any combination):[106]
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,[107] 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 withfuller'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.[108]
Preservative addition, including antioxidants such asBHA,BHT, andtocopherol 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.[105] 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.
A bin for spent cooking oil inAustin, Texas, managed by a recycling company.
Proper disposal of used cooking oil is an important waste-management concern. Oil can congeal in pipes, causingsanitary sewer overflow.[109] Because of this, cooking oil should never be dumped in the kitchen sink or in the toilet bowl. The proper way to dispose of oil is to put it in a sealed non-recyclable container and discard it with regular garbage.[110] Placing the container of oil in the refrigerator to harden also makes disposal easier and less messy.
Cooking oil can be recycled. It can be used in animal feed, soap, make-up, clothes, rubber, detergents, directly as fuel, and to producebiodiesel.[111][112][113]
In the recycling industry, used cooking oil recovered from restaurants and food-processing industries (typically fromdeep fryers orgriddles) is called yellow grease, recycled vegetable oil (RVO), used vegetable oil (UVO), or waste vegetable oil (WVO).[114]
Grease traps or interceptors collect fats and oils from kitchen sinks and floor drains. The result is called brown grease, and unlike yellow grease its severe contaminants make it much harder to recycle.
Gutter oil and trench oil are terms used in China to describe recycled oil processed to resemble virgin oil, but containing toxic contaminants and sold illegally for cooking; its origin is frequently brown grease fromgarbage.[115]
^Che Man, YB; Liu, J.L.; Jamilah, B.; Rahman, R. Abdul (1999). "Quality changes of RBD palm olein, soybean oil and their blends during deep-fat frying".Journal of Food Lipids.6 (3):181–193.doi:10.1111/j.1745-4522.1999.tb00142.x.
^Matthäus, Bertrand (2007). "Use of palm oil for frying in comparison with other high-stability oils".European Journal of Lipid Science and Technology.109 (4):400–409.doi:10.1002/ejlt.200600294.
^abDetwiler, S. B.; Markley, K. S. (1940). "Smoke, flash, and fire points of soybean and other vegetable oils".Oil & Soap.17 (2):39–40.doi:10.1007/BF02543003.
^Brown, Amy L. (2010).Understanding Food: Principles and Preparation. Belmont, CA: Wadsworth Publishing. p. 468.ISBN0-538-73498-1. Retrieved2011-01-16.The smoke point of an oil depends primarily on its free fatty acid content (FFA) and molecular weight. Through repeated use, as in a deep fryer, the oil accumulates food residues or by-products of the cooking process, that lower its smoke point further. The values shown in the table must therefore be taken as approximate, and are not suitable for accurate or scientific use
^A. G. Vereshagin and G. V. Novitskaya (1965) The triglyceride composition of linseed oil. Journal of the American Oil Chemists' Society 42, 970-974.[1]
^National Research Council, 1976,Fat Content and Composition of Animal Products, Printing and Publishing Office, National Academy of Science, Washington, D.C.,ISBN0-309-02440-4; p. 203,online edition
^"09038, Avocados, raw, California".National Nutrient Database for Standard Reference, Release 26. United States Department of Agriculture, Agricultural Research Service. Archived fromthe original on January 10, 2014. Retrieved14 August 2014.
Warner, K. (1999). "Impact of High-Temperature Food Processing on Fats and Oils".Impact of Processing on Food Safety. Advances in Experimental Medicine and Biology. Vol. 459. pp. 67–77.doi:10.1007/978-1-4615-4853-9_5.ISBN978-1-4613-7201-1.PMID10335369.
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