What chemical process converts vegetable oils into biodiesel?

The chemical process that converts vegetable oils into biodiesel is transesterification.

Vegetable oils are mainly triglycerides: molecules built from glycerol joined to three fatty-acid chains. During transesterification, an alcohol—usually methanol—reacts with those triglycerides. A catalyst such as sodium hydroxide or potassium hydroxide helps split the molecules and rearrange them into fatty-acid methyl esters, the compounds commonly sold as biodiesel, plus glycerol as a coproduct.

This step matters because raw vegetable oil is much thicker than conventional diesel. Its high viscosity can cause poor atomization, injector deposits, and lubrication problems in engines designed for diesel fuel. Converting the oil into smaller ester molecules makes the fuel flow and spray more like petroleum diesel.

A common mix-up is confusing transesterification with esterification. Esterification combines an alcohol with free fatty acids, often as a pretreatment when the oil contains substantial acidity; transesterification is the main conversion of triglycerides. “Hydrogenation” instead adds hydrogen to chemical bonds and is used in processes such as hardening some edible oils. Feedstocks include soybean, rapeseed, palm, and used cooking oils, and the exact fuel properties depend on the fatty-acid composition.