Biodiesel is a type of fuel made from vegetable oil and/or animal fat which consists of long alkyl esters (see “Organic Chemistry” to review functional groups). Biodiesel fuel is commonly distributed for sale as a mixture with petrodiesel and is labeled with a “B factor” describing the fraction of biodiesel present. B100 is pure biodiesel, while B20 would represent a mixture of 20% biodiesel with 80% petrodiesel.