Trans fats

Unlike other dietary fats, trans fats are not essential, and they do not promote good health.^[12] The consumption of trans fats increases one's risk of coronary heart disease^[13] by raising levels of LDL cholesterol and lowering levels of HDL cholesterol.^[14] Trans fats from partially 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 States Food and Drug Administration (FDA), the National Heart, Lung and Blood Institute and the American 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]

Cooking with oil

Heating, as well as heating vessels rapidly change characteristics of cooking oil.^[21] Oils that are healthy 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's heat 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 withstand deep frying at higher temperatures and is resistant to oxidation 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 high smoke point:

• Avocado oil
• Mustard oil
• Palm oil
• Peanut oil (marketed as "groundnut oil" in the UK and India)
• Rice bran oil
• Safflower oil
• Olive oil
• Semi-refined sesame oil
• Semi-refined sunflower oil^[29]

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]

Storing and keeping oil

All oils degrade in response to heat, light, and oxygen.^[32] 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.^{[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 as macadamia oil,^[32] 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.^[32]

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.^[32]

Smoke point

The smoke point is marked by "a continuous wisp of smoke".^[53] 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.^[54]

Above the smoke point are flash and fire points.^[53] 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).^[55] The fire point is the temperature at which hot oil produces sufficient vapors they will catch on fire and burn.^[55] As frying hours increase, all these temperature points decrease.^[55] They depend more on an oil's acidity than fatty-acid profile.^[56]

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.^[54] Residual solvent remaining from the refining process may decrease the smoke point.^[56] 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.^[56]

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.^[54][78]

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 you will often see some combination of pressing, chemical extraction and/or centrifuging in order to extract the maximum amount of oil possible.^[104]

Refinement

Cooking oil can either be unrefined, or refined using one or more of the following refinement processes (in any combination):^[105]

• 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,^[106] 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.^[107]
• 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 doesn't 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.^[104] 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.

Recycling

Cooking oil can be recycled. It can be used in animal feed, soap, make-up, clothes, rubber, detergents, directly as fuel, and to produce biodiesel.^{[110][111][112]}

In the recycling industry, used cooking oil recovered from restaurants and food-processing industries (typically from deep fryers or griddles) is called yellow grease, recycled vegetable oil (RVO), used vegetable oil (UVO), or waste vegetable oil (WVO).^[113]

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.

Adulteration

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 from garbage.^[114]

In Kenya, thieves sell stolen electric transformers to operators of roadside food stalls for reuse of the oil in deep frying, suitable for prolonged use longer than regular cooking oil, but a threat to consumer health due to the presence of PCBs and polycyclic aromatic hydrocarbons.^[115]