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In organic chemistry, transesterification is the process of exchanging the alkoxy group of an ester compound by another alcohol. These reactions are often catalyzed by the addition of an acid or base.

Transesterification: alcohol + ester → different alcohol + different ester

Acids can catalyse the reaction by donating a proton to the carbonyl group, thus making it more reactive, while bases can catalyse the reaction by removing a proton from the alcohol, thus making it more reactive.

Transesterification is used in the synthesis of polyester, in which diesters undergo transesterification with diols to form macromolecules. For example, dimethyl terephthalate and ethylene glycol react to form polyethylene terephthalate and methanol, which is evaporated to drive the reaction forward. The reverse reaction (methanolysis) is also an example of transesterification, and has been used to recycle polyesters into individual monomers (see plastic recycling).

One of the first uses of transesterified vegetable oil (biodiesel) was to power heavy-duty vehicles in South Africa before World War II. The name "biodiesel" has been given to transesterified vegetable oil to describe its use as a diesel fuel.

It was patented in the U.S. in the 1940s by Colgate, though biolipid transesterification may have been discovered much earlier. In the 1940s, researchers were looking for a method to more readily produce glycerine, which was used to produce explosives for World War II. Many of the methods used today by producers and homebrewers have their origin in the original 1940s research.

Interesterified fats, when used as replacement fats for trans fats, not only lower desirable High density lipoprotein levels but also cause undesirable increases in blood sugar concentrations. Interesterified fats, a limited study shows, adversely effect the body's insulin concentratons, reducing the regulation of blood sugar and increasing the risk to diabetics. For comparable amounts of palm oil, as compared with interesterified soybean oil, the interesterified fat increased volunteers' blood sugar by 20 percent in studies documented the journal Nutrition & Metabolism. Although the research is preliminary, interesterified fats, which are used to replace trans fats in commercial foods, arguably have adverse health effects and warrant more research prior to extensive use.^[1]