What is the name of the process that turns used cooking oil into biodiesel?

Transesterification is the process that turns used cooking oil into biodiesel.

Vegetable oils and animal fats contain triglycerides. During transesterification, those molecules react with a short-chain alcohol, usually methanol, in the presence of a catalyst such as sodium hydroxide or potassium hydroxide. The reaction produces fatty-acid methyl esters, commonly called biodiesel, and glycerol as a coproduct.

Used cooking oil must usually be filtered and treated before processing because water and free fatty acids can interfere with the reaction. The resulting fuel can be used in suitable diesel engines, often in blends with petroleum diesel.

Transesterification is not the same as pyrolysis, which uses heat to break molecules apart, or gasification, which converts carbon-based materials into synthesis gas. Biodiesel made this way is chemically different from renewable diesel, which is produced through hydrogen-based refining.