Stearic acid is a very common amino acid composed of 18 carbon molecules linked together in a chain. Because of its structure, stearic acid is sometimes referred to as octadecanoic acid. The substance is used in the manufacturing of more than 3,200 skin and hair care products sold in the United States. On product labels, it is sometimes listed under other names, including Century 1240, cetylacetic acid, Emersol 120, Emersol 132, Emersol 150, Formula 300 and Glycon DP.
In nature, stearic acid is found in the fats and oils of plants and animals. Animal fat samples typically consist of 30 percent stearic acid. Most plant oils receive 5 percent of their volume from the amino acid, with the exception of cocoa butter and shea butter, which contain as much as nine times more stearic acid.
Because stearic acid is found in so many plants and animals, it is relatively inexpensive to produce. To isolate the substance, fat or oil that contains the amino acid is heated and pressurized. Then, the material is placed in boiling water inside of a distillation apparatus. This device catches the steam given off by boiling the fat or oil samples and then carries it through a series of chilled coils. The sudden drop in temperature causes the stearic acid to condense and become a liquid. It can then be further cooled to produce a waxy solid substance.
One of the largest uses of stearic acid is in the production of soaps. When added to these products, the amino acid helps to thicken and harden the other ingredients to form a solid bar. Stearic acid also has important cleansing properties that make it useful in soaps. The ingredient acts as a surfactant, a substance that lowers the surface tension of oils.
The surface tension is the force that allows oils and liquids to retain their shape in the presence of natural forces like gravity. Oils have a higher surface tension than ordinary water, which is why water droplets do not readily mix with oils. By lowering the surface tension of oil, stearic acid allows water to combine with the oil molecules and wash them away. As a result, stearic acid helps to remove dirt, sweat and excess sebum from the skin and hair. This makes it a useful ingredient in cleansers, body washes and shampoos as well as bar soaps.
Because stearic acid helps water and oil mix, the ingredient is also added to many liquid cosmetics and skin and hair care products in low concentrations to function as an additive rather than an active ingredient or cleansing agent. In these products, stearic acid helps prevent the formulas from separating into liquid and oily layers. As a result, products that contain stearic acid require less shaking prior to use and remain more potent when stored for extended periods of time.
Less commonly, stearic acid is added to products to make them less transparent. When utilized for this purpose, stearic acid can improve the appearance of a lotion, gel or cream to make it more appealing to users. In cosmetics like foundations and concealers, stearic acid's ability to increase the opacity of formulas helps them better hide skin imperfections.
Because stearic acid is derived from natural sources and not produced in industrial settings, it is sometimes used as an alternative to chemical ingredients in natural skin care products. It is important to note, though, that often, the ingredient is sourced from by-products obtained during the processing of meats, particularly pork. For this reason, it is not frequently used in vegan cosmetics and skin care products; however, stearic acid sourced from plants is suitable for inclusion in formulas that are animal-free.
The U.S. Food and Drug Administration has concluded that stearic acid is safe for topical use in skin care products in limited quantities. The ingredient is also approved for use as a food additive to give products a chewier or gummier texture. The leading skin care product safety evaluation group, the Cosmetic Ingredient Review (CIR) Expert Panel, has found that the amount of stearic acid commonly used in cosmetics and skin and hair care products does not pose any health risks. Although stearic acid is not toxic, in some clinical studies it has produced skin irritation in rabbits, indicating that some people with sensitive skin may be unable to tolerate the ingredient.