ABSTRACT
The word “lipids” generally describes a class of nonpolar organic molecules that are not soluble in water but are soluble in nonpolar organic solvents such as hexane, chloroform, and ether (Moreau, 2005). Lipids usually contain long chains of hydrocarbons, but may also contain oxygen, nitrogen, phosphorus, and sulfur. Plant seed oils, usually called vegetable oils, and animal fats, are composed of a mixture of many different lipids, but the main component is triacylglycerols (TAGs). TAGs (Figure 9.1a) make up anywhere from 85% to 98% of a crude (unre„ned) plant seed oil or animal fat. The term “oil” is used for triacylglycerols that remain liquid at room temperature, while the term “fat” refers to triacylglycerols that are solid at room temperature. In corn, sorghum, and other seeds, TAGs are stored in the germ. They are composed of three fatty acids esteri„ed to each hydroxyl position of a glycerol molecule. The „ve major fatty acids: palmitic, stearic, oleic, linoleic, and linolenic, which are 16-or 18-carbons in length, and have zero to three double bonds, make up the majority of plant TAGs. The percentage of each fatty acid in the TAGs varies depending on plant species and variety; these percentages are known as the “fatty acid composition.” Fatty acids can be hydrolyzed from TAGs by heat and moisture, acid or alkali, or by enzymes called lipases, to form free fatty acids (FFA) and diacylglycerols, monoacylglycerols, or free glycerol, depending on how completely the fatty acids were hydrolyzed from the TAG molecule. Other lipids that are commonly found in vegetable oils include, but are not limited to: diacylglycerols, monoacylglycerols, FFA, phytosterols, tocopherols and tocotrienols (vitamin E), waxes, phospholipids, carotenoids, acyl alcohols, acyl aldehydes, and hydrocarbons (Moreau, 2005). Examples of the structures of phospholipids, carotenoids, tocopherols, and tocotrienols are shown in Figure 9.1b through e. The other lipids mentioned above are found in much lower quantities in crude vegetable oils. Whereas TAGs mainly function as carbon storage, lipids such as phospholipids, tocopherols, phytosterols, and carotenoids have other important functions within plant cells, and they are found in tissues other than just the seeds.
