Vegetable oil

Oils extracted from plants have been used since ancient times and in many cultures. Archaeological evidence shows that olives were turned into olive oil by 6000 BCE[4] and 4500 BCE in present-day Israel and Palestine.[5]

In addition to use as food, fats and oils (both vegetable and mineral) have long been used as fuel, typically in lamps which were a principal source of illumination in ancient times. Oils may have been used for lubrication, but there is no evidence for this. Vegetable oils were probably more valuable as food and lamp-oil; Babylonian mineral oil was known to be used as fuel, but there are no references to lubrication. Pliny the Elder reported that animal-derived fats such as lard were used to lubricate the axles of carts.[6]

Many vegetable oils are consumed directly, or indirectly as ingredients in food – a role that they share with some animal fats, including butter, ghee, lard, and schmaltz. The oils serve a number of purposes in this role:

• Shortening – as in giving pastries a crumbly texture.
• Enriching – adding calories and satisfaction in consumption
• Texture – altering how ingredients combine, especially fats and starches
• Flavoring – examples include olive, sesame, or almond oil
• Flavor base – oils can also "carry" flavors of other ingredients, such as peppers,[7] since many flavors are due to chemicals that are soluble in oil.

Oils can be heated to temperatures significantly higher than the boiling point of water, 100 °C (212 °F), and used to fry foods. Oils for this purpose must have a high flash point. Such oils include both the major cooking oils – soybean, rapeseed, canola, sunflower, safflower, peanut, cottonseed, etc. – and tropical oils, such as coconut, palm, and rice bran. The latter are particularly valued in Asian cultures for high-temperature cooking, because of their unusually high flash points.

Unsaturated vegetable oils can be transformed through partial or complete "hydrogenation" into oils of higher melting point, some of which, such as vegetable shortening, will remain solid at room temperature.

The hydrogenation process involves "sparging" the oil at high temperature and pressure with hydrogen in the presence of a catalyst, typically a powdered nickel compound, such as Raney nickel. Chemically, hydrogenation is the reduction of a carbon-carbon double bond to a single bond, by addition of hydrogen atoms. Since the surface of the metal catalyst is covered with hydrogen atoms, when the double bonds of the unsaturated oil come into contact with the catalyst, it reacts with the hydrogen atoms, forming new bonds with the two carbon atoms; each carbon atom becomes single-bonded to an individual hydrogen atom, and the double bond between carbons can no longer exist. In organic chemistry, unsaturation is considered as a pair of hydrogen atoms missing from the (hypothetical) fully-saturated carbon chain. The level to which an organic molecule is deficient in hydrogen, is called the degree of unsaturation (DoU); as the degree of unsaturation decreases, the oil progresses toward being fully hydrogenated (when DoU = 0). A fully hydrogenated oil, also called a saturated fat, has had all of its double bonds converted into single bonds. If a polyunsaturated oil is left incompletely-hydrogenated (not all of the double bonds are reduced to single bonds), then it is a "partially hydrogenated oil" (PHO). PHOs are the primary dietary source of artificial trans fat in processed foods.[8] An oil may be hydrogenated to increase resistance to rancidity (oxidation) or to change its physical characteristics. As the degree of saturation is raised by full or partial hydrogenation, the oil's viscosity and melting point increase.

The use of hydrogenated oils in foods has never been completely satisfactory. Because the center arm of the triglyceride is shielded somewhat by the end fatty acids, most of the hydrogenation occurs on the end fatty acids, thus making the resulting fat more brittle. A margarine made from naturally more saturated oils will be more plastic (more "spreadable") than a margarine made from hydrogenated soy oil. While full hydrogenation produces largely saturated fatty acids, partial hydrogenation results in the transformation of unsaturated cis fatty acids to unsaturated trans fatty acids in the oil mixture due to the heat used in hydrogenation. Partially hydrogenated oils and their trans fats have been linked to an increased risk of mortality from coronary heart disease,[9] among other increased health risks.

In the US, the Standard of Identity for a product labeled as "vegetable oil margarine" specifies only canola, safflower, sunflower, corn, soybean, or peanut oil may be used.[10] Products not labeled "vegetable oil margarine" do not have that restriction.

Vegetable oils are used as an ingredient or component in many manufactured products.

Many vegetable oils are used to make soaps, skin products, candles, perfumes and other personal care and cosmetic products. Some oils are particularly suitable as drying oils, and are used in making paints and other wood treatment products. They are used in alkyd resin production. Dammar oil (a mixture of linseed oil and dammar resin), for example, is used almost exclusively in treating the hulls of wooden boats. Vegetable oils are increasingly being used in the electrical industry as insulators as vegetable oils are not toxic to the environment, biodegradable if spilled and have high flash and fire points. However, vegetable oils are less stable chemically, so they are generally used in systems where they are not exposed to oxygen, and they are more expensive than crude oil distillate. Synthetic tetraesters, which are similar to vegetable oils but with four fatty acid chains compared to the normal three found in a natural ester, are manufactured by Fischer esterification. Tetraesters generally have high stability to oxidation and have found use as engine lubricants. Vegetable oil is being used to produce biodegradable hydraulic fluid[11] and lubricant.[12]

One limiting factor in industrial uses of vegetable oils is that all such oils are susceptible to becoming rancid. Oils that are more stable, such as ben oil or mineral oil, are thus preferred for industrial uses. Castor oil has numerous industrial uses, owing to the presence of hydroxyl group on the fatty acid. Castor oil is a precursor to Nylon 11. Castor oil may also be reacted with epichlorohydrin to make a glycidyl ether which is used as a diluent and flexibilizer with epoxy resins.

Vegetable oil is used in the production of some pet foods. AAFCO[13] defines vegetable oil in this context as the product of vegetable origin obtained by extracting the oil from seeds or fruits which are processed for edible purposes.

Vegetable oils are also used to make biodiesel, which can be used like conventional diesel. Some vegetable oil blends are used in unmodified vehicles but straight vegetable oil, also known as pure plant oil, needs specially prepared vehicles which have a method of heating the oil to reduce its viscosity. The use of vegetable oils as alternative energy is growing and the availability of biodiesel around the world is increasing.

The NNFCC estimates that the total net greenhouse gas savings when using vegetable oils in place of fossil fuel-based alternatives for fuel production, range from 18 to 100%.[14]

Oils can be removed via mechanical extraction, termed "crushing" or "pressing." This method is typically used to produce the more traditional oils (e.g., olive, coconut etc.), and it is preferred by most health-food customers in the United States and in Europe. There are several different types of mechanical extraction.[15] Expeller pressing extraction is common, though the screw press, ram press, and ghani (powered mortar and pestle) are also used. Oilseed presses are commonly used in developing countries, among people for whom other extraction methods would be prohibitively expensive; the ghani is primarily used in India.[16] The amount of oil extracted using these methods varies widely, as shown in the following table for extracting mowrah butter in India:[17]

The processing of vegetable oil in commercial applications is commonly done by chemical extraction, using solvent extracts, which produces higher yields and is quicker and less expensive. The most common solvent is petroleum-derived hexane. This technique is used for most of the "newer" industrial oils such as soybean and corn oils. After extraction, the solvent is evaporated out by heating the mixture to about 300 °F (149 °C).[19]

Supercritical carbon dioxide can be used as a non-toxic alternative to other solvents.[20]

Oils may be partially hydrogenated to produce various ingredient oils. Lightly hydrogenated oils have very similar physical characteristics to regular soy oil, but are more resistant to becoming rancid. Margarine oils need to be mostly solid at 32 °C (90 °F) so that the margarine does not melt in warm rooms, yet it needs to be completely liquid at 37 °C (98 °F), so that it doesn't leave a "lardy" taste in the mouth.

Hardening vegetable oil is done by raising a blend of vegetable oil and a catalyst in near-vacuum to very high temperatures, and introducing hydrogen. This causes the carbon atoms of the oil to break double-bonds with other carbons, each carbon forming a new single-bond with a hydrogen atom. Adding these hydrogen atoms to the oil makes it more solid, raises the smoke point, and makes the oil more stable.

Hydrogenated vegetable oils differ in two major ways from other oils which are equally saturated. During hydrogenation, it is easier for hydrogen to come into contact with the fatty acids on the end of the triglyceride, and less easy for them to come into contact with the center fatty acid. This makes the resulting fat more brittle than a tropical oil; soy margarines are less "spreadable". The other difference is that trans fatty acids (often called trans fat) are formed in the hydrogenation reactor, and may amount to as much as 40 percent by weight of a partially hydrogenated oil. Hydrogenated oils, especially partially hydrogenated oils with their higher amounts of trans fatty acids, are increasingly thought to be unhealthy.

In the processing of edible oils, the oil is heated under vacuum to near the smoke point or to about 450 °F (232 °C),[21] and water is introduced at the bottom of the oil. The water immediately is converted to steam, which bubbles through the oil, carrying with it any chemicals which are water-soluble. The steam sparging removes impurities that can impart unwanted flavors and odors to the oil. Deodorization is key to the manufacture of vegetable oils. Nearly all soybean, corn, and canola oils found on supermarket shelves go through a deodorization stage that removes trace amounts of odors and flavors, and lightens the color of the oil. However, the process commonly results in higher levels of trans fatty acids and distillation of the oil's natural compounds.[22][23][24]

People can breathe in vegetable oil mist in the workplace. The U.S. Occupational Safety and Health Administration (OSHA) has set the legal limit (permissible exposure limit) for vegetable oil mist exposure in the workplace as 15 mg/m³ total exposure and 5 mg/m³ respiratory exposure over an 8-hour workday. The U.S. National Institute for Occupational Safety and Health (NIOSH) has set a recommended exposure limit (REL) of 10 mg/m³ total exposure and 5 mg/m³ respiratory exposure over an 8-hour workday.[25]

Typical productivity of some oil crops, measured in tons (t) of oil produced per hectare (ha) of land per year (yr). Oil palm is by far the highest yielding crop, capable of producing about 4 tons of palm oil per hectare per year.

Extracting the oils first by expeller or cold pressing methods, then solvent expelling the rest of the oils from the leftover matter. This is a method used by larger capacity oil mills. As the energy consumption of the mechanical press increases as more oil is released, it is more efficient to extract the rest of the oil (past around 60%) by solvent extraction.[61]