Biodiesel is obtained from vegetable oil or animal fats (biolipids) which are mainly fatty acid methyl esters (FAME), and transesterified with methanol. It can be produced from many types of oils, the most common being rapeseed oil (rapeseed methyl ester, RME) in Europe and soybean oil (soy methyl ester, SME) in the US. Methanol can also be replaced with ethanol for the transesterification process, which results in the production of ethyl esters. The transesterification processes use catalysts, such as sodium or potassium hydroxide, to convert vegetable oil and methanol into biodiesel and the undesirable byproducts glycerine and water, which will need to be removed from the fuel along with methanol traces. Biodiesel can be used pure (B100) in engines where the manufacturer approves such use, but it is more often used as a mix with diesel, BXX where XX is the biodiesel content in percent.