Biodiesel is obtained by transferring vegetable oils or animal fats into alkyl esters of their fatty acids. However, the amount of free glycerol as a by-product of the biodiesel production, together with remaining amounts of unreacted triacylglycerides and partially reacted mono- and diacylglycerides, play an important role for the fuel quality. These compounds should be absent in the ideal case.
Read more in: L.R. Miguel et al., On-line hydrogenation of biodiesel – toward a reference method for the determination of glycerides and total glycerol, J. Chromatogr. A, doi: 10.1016/j.chroma.2014.02.040 (2014)