Stearic acid

Stearic acid (first syllable rhymes with either bear or gear) (IUPAC systematic name: octadecanoic acid) or 18:0 is a saturated fatty acid. It is a waxy solid, and its chemical formula is C₁₈H₃₆O₂. Its name comes from the Greek word stéar (genitive: stéatos), which means tallow. The salts and esters of stearic acid are called stearates.

Food sources

It occurs in many animal and vegetable fats and oils. One important source is cocoa.

Production

Stearic acid is prepared by treating animal fat with water at a high pressure and temperature, leading to the hydrolysis of triglycerides. It can also be obtained from the hydrogenation of some unsaturated vegetable oils. Common stearic acid is actually a mix of stearic acid and palmitic acid, although purified stearic acid is available separately.

Uses

Stearic acid is useful as an ingredient in making candles, plastics, dietary supplements, oil pastels and cosmetics, and for softening rubber.^[2] It is used to harden soaps, particularly those made with vegetable oil.

Stearic acid is also used as a parting compound when making plaster castings from a plaster piece mold or waste mold and when making the mold from a shellacked clay original. In this use, powdered stearic acid is dissolved in water and the solution is brushed onto the surface to be parted after casting.

Esters of stearic acid with ethylene glycol, glycol stearate and glycol distearate are used to produce a pearly effect in shampoos, soaps, and other cosmetic products. They are added to the product in molten form and allowed to crystalize under controlled conditions.

In fireworks, stearic acid is often used to coat metal powders such as aluminium and iron. This prevents oxidation, allowing compositions to be stored for longer.

It is used along with simple sugar or co syrup as a hardener in candies.

Stearic acid is one of most commonly used lubricants during injection molding and pressing of ceramic powders.^[3]

Reactions

Stearic acid undergoes the typical reactions of saturated carboxylic acids, notably reduction to stearyl alcohol, and esterification with a range of alcohols.

Metabolism

An isotope labeling study in humans^[4] concluded that the fraction of dietary stearic acid oxidatively desaturated to oleic acid was 2.4 times higher than the fraction of palmitic acid analogously converted to palmitoleic acid. Also, stearic acid was less likely to be incorporated into cholesterol esters. These findings may indicate that stearic acid is less unhealthy than other saturated fatty acids.