Fat hydrogenation. Fat hydrogenation is the process of combining unsaturated fat with hydrogen in order to partially or completely convert it into saturated fat. Typically this hydrogenation is done with liquid vegetable oils resulting in solid or semi-solid fats.

Changing the degree of saturation of the fat changes some important physical properties, such as the melting range, which is why liquid oils become semi-solid. Solid or semi-solid fats are preferred for some baked goods such as biscuits and pie dough because how the fat mixes with flour produces a more desirable, crumbly texture in the baked product. Because partially hydrogenated vegetable oils are cheaper than animal fats, are available in a wide range of consistencies, and have other desirable characteristics such as increased oxidative stability and longer shelf life, they are the predominant fats used as shortening in most commercial baked goods.

The process is typically carried out at very high pressure, with the help of a nickel catalyst that is removed from the final product.

Process
Hydrogenating vegetable oil is done by raising a blend of vegetable oil and a metal catalyst, typically nickel, 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 atom becomes single-bonded to an individual hydrogen atom, and the double bond between carbons can no longer exist.

Full hydrogenation results in the conversion of all of the unsaturated fats into saturated fats by transforming all of the double bonds in the fat into single bonds. Partial hydrogenation reduces some, but not all, of the double bonds by the partial replacement with single bonds. The degree of hydrogenation is controlled by restricting the amount of hydrogen, reaction temperature and time, and the catalyst.

Issues
Cis–trans isomerization of some of the remaining unsaturated carbon bonds to their trans isomers during the partial hydrogenation process produces trans fat, which has been demonstrated to have cardiovascular health risk. The conversion from cis to trans bonds is chemically favored because the trans configuration has lower energy than the natural cis one. At equilibrium, the trans/cis isomer ratio is about 2:1.

Numerous studies have concluded that these trans fatty acids have negative health effects.  As a result, many countries have enacted trans fat regulation that aims to eliminate or severely restrict their amount in industrialized food products, such as mandatory labeling of trans fats on food products. The United States Food and Drug Administration has concluded that partially hydrogenated oils are not generally recognized as safe, and since 2018 categorizes them as food additives, not food.

A number of old and new ingredients are available to replace partially-hydrogenated oil with significant levels of trans fat.
 Improved hydrogenation processes can produce partially-hydrogenated oil with much lower amounts of trans fat.
 Oils rich in monounsaturated fatty acids (especially oleic acid) obtained from plant breeding have higher stability compared to their polyunsaturated-rich ancestors, offering a similar improvement to shelf life.
 Tropical oils (palm oil, palm kernel oil, coconut oil) are naturally rich in saturated fat. They can be further fractionated to increase the concentration of desired fatty acids.
 Interesterification can be used to mix multiple types of fats, obtaining an oil with intermediate properties. For example, soybean oil and fully-hydrogenated soybean oil can be interesterified to obtain a semi-hard oil.

Dietary recommendations
Many health organizations recommend limiting or replacing dietary intake of trans fats and saturated fats, in favor of unsaturated fats.

History

Nobel Prize laureate Paul Sabatier worked in the late 1890s to develop the chemistry of hydrogenation. Whereas Sabatier considered hydrogenation of only vapors, the German chemist Wilhelm Normann showed in 1901 that liquid oils could be hydrogenated, and patented the process in 1902. Normann's hydrogenation process made it possible to stabilize affordable whale oil or fish oil for human consumption, a practice kept secret to avoid consumer distaste. During the years 1905–1910, Normann built a fat-hardening facility in the Herford company. At the same time, the invention was extended to a large-scale plant in Warrington, England, at Joseph Crosfield & Sons, Limited. It took only two years until the hardened fat could be successfully produced in the plant in Warrington, commencing production in late 1909. The initial year's production totalled nearly 3,000 tonnes. In 1909, Procter & Gamble acquired the United States rights to the Normann patent; in 1911, they began marketing the first hydrogenated shortening, Crisco (composed largely of partially hydrogenated cottonseed oil). Further success came from the marketing technique of giving away free cookbooks in which every recipe called for Crisco.

Before 1910, dietary fats in industrialized nations consisted mostly of butterfat, beef tallow, and lard. During Napoleon's reign in France in the early 19th century, a type of margarine was invented to feed troops using tallow and buttermilk. Soybeans began to be imported into the U.S. as a source of protein in the early 20th century, resulting in an abundance of soybean oil as a by-product that could be turned into a solid fat, thereby addressing a shortage of butterfat. With the advent of refrigeration, margarines based on hydrogenated fats presented the advantage that, unlike butter, they could be taken out of a refrigerator and immediately spread on bread. Some minor changes to the chemical composition of hydrogenated fats yielded superior baking properties compared to lard. As a result of these factors, margarine made from partially hydrogenated soybean oil began to replace butterfat. Partially hydrogenated fat such as Crisco and Spry, sold in England, began to replace butter and lard in baking bread, pies, cookies, and cakes in 1920.

Production of partially hydrogenated fats increased steadily in the 20th century as processed vegetable fats replaced animal fats in the U.S. and other Western countries. At first, the argument was a financial one due to the lower costs of margarines and shortenings compared to lard and butter, particularly for restaurants and manufacturers. However, during the 1980s regulators, physicians, nutritionists, popular health media, educational curricula and cookbooks began to promote diets low in saturated fats for health reasons. Advocacy groups in the U.S. responded by demanding the replacement of saturated animal and tropical fats with vegetable alternatives. The Center for Science in the Public Interest (CSPI) campaigned vigorously against the use of saturated fats by corporations, including fast-food restaurants, endorsing trans fats as a healthier alternative. The National Heart Savers Association took out full page ads in major newspapers, attacking the use of beef tallow in McDonald's French fries. They urged multinational fast-food restaurants and food manufacturers to switch to vegetable oils, and almost all targeted firms responded by replacing saturated fats with trans fats.

In the early 20th century, soybeans began to be imported into the United States as a source of protein; large quantities of soybean oil were a by-product. At the same time, there was not enough butterfat available for consumers. Margarine manufacturers found that hydrogenated fats worked better than the previously used combination of animal and liquid vegetable fats. Margarine made from hydrogenated soybean oil and vegetable shortenings such as Crisco and Spry, sold in England, began to replace butter and lard in baking bread, pies, cookies, and cakes by 1920.

Production of hydrogenated fats increased steadily until the 1960s, as processed vegetable fats replaced animal fats in the United States and other Western countries. At first, the argument was a financial one due to lower costs; advocates also said that the hydrogenated fats of margarine were healthier than the saturated fats of butter.

Since then the food industry has moved away from partially hydrogenated fats in response to the health concerns about trans fats, labeling requirements, and removal of trans fats from permitted food additives. They have been replaced with fully hydrogenated fats, vegetable oils that are naturally higher in saturated fat and therefore more solid at room temperature, such as palm oil and coconut oil, and interesterified fats, which cannot result in the formation of trans fats.

Alternatives to hydrogenation
The negative public image and strict regulations has not led to substantial interest in replacing partial hydrogenation. In principle, the level of trans fat may be altered by modification of the catalysts and conditions of hydrogenation.

See also
 Hydrotreated vegetable oil

References

External links
 

 
Edible oil chemistry