Glycerol (/ˈɡlɪsərɒl/)[6] is a simple triol compound. It is a colorless, odorless, viscous liquid that is sweet-tasting and non-toxic. The glycerol backbone is found in lipids known as glycerides. It is also widely used as a sweetener in the food industry and as a humectant in pharmaceutical formulations. Because of its three hydroxyl groups, glycerol is miscible with water and is hygroscopic in nature.[7] Modern use of the word glycerine (alternatively spelled glycerin) refers to commercial preparations of less than 100% purity, typically 95% glycerol. [8] Structure Although achiral, glycerol is prochiral with respect to reactions of one of the two primary alcohols. Thus, in substituted derivatives, the stereospecific numbering labels the molecule with a sn- prefix before the stem name of the molecule.[9][10][11] Production Glycerol is generally obtained from plant and animal sources where it occurs in triglycerides, esters of glycerol with long-chain carboxylic acids. The hydrolysis, saponification, or transesterification of these triglycerides produces glycerol as well as the fatty acid derivative: Triglyceride 3 NaOH / H2O Rightward reaction arrow Δ 3 × soap 3 × glycerol Triglycerides can be saponified with sodium hydroxide to give glycerol and fatty sodium salt or soap. Typical plant sources include soybeans or palm. Animal-derived tallow is another source. Approximately 950, 000 tons per year are produced in the United States and Europe; 350, 000 tons of glycerol were produced per year in the U.S. alone from 2000 to 2004.[12] The EU directive 2003/30/EC set a requirement that 5.75% of petroleum fuels were to be replaced with biofuel sources across all member states by 2010. It was projected in 2006 that by 2020, production would be six times more than demand, creating an excess of glycerol as a byproduct of biofuel production.[7] Glycerol from triglycerides is produced on a large scale, but the crude product is of variable quality, with a low selling price of as low as US$0.02–0.05 per kilogram in 2011.[13] It can be purified, but the process is expensive. Some glycerol is burned for energy, but its heat value is low.[14] Crude glycerol from the hydrolysis of triglycerides can be purified by treatment with activated carbon to remove organic impurities, alkali to remove unreacted glycerol esters, and ion exchange to remove salts. High purity glycerol (greater than 99.5%) is obtained by multi-step distillation; a vacuum chamber is necessary due to its high boiling point (290 °C).[7]