Wong - Mechanism and Theory in Food Chemistry

Western Regional Research Center, Albany, California, USA

Commercial oils and fats of plant and animal origins consist exclusively of triacylglycerols (TAG), with the chemical structure of trihydric alcohol glycerol esterified with fatty acids. The common fatty acids in edible oils and their systematic names are presented in Table . Most fatty acids have a straight chain of an even number of carbons, commonly classified into short chain (26 carbons), medium chain (810), and long chain (1224). Depending on the absence or presence of double bonds, fatty acids are referred to as saturated and unsaturated, respectively. The latter may be monounsaturated (monoenoic, containing one double bond) or polyunsaturated (polyenoic, containing two or more double bonds). The double bonds are all in the cis configuration. Some fatty acids may contain functional groups, such as hydroxy- or keto- groups. The carbon atoms in the fatty acid chain are numbered from the carboxyl end. If the fatty acid is unsaturated, the double bond position is counted from the carboxyl end. For example, linoleic acid is 18:2 (indicating 18 carbons with two double bonds) or cis , cis -9,12-octadecadienoic acid (indicating the double bonds at the C9 and C12 positions counting from the carboxyl end). In another naming system, the double bonds are counted from the methyl end, denoted by x (omega representing the methyl terminal carbon and x = the position of the first double bond from the methyl end) or n-x (n = total number of carbon). For example, linoleic acid using this system is named 18:2(6) or (n-6), indicating 18 carbons, two double bonds, the first double bond at C6 counting from the terminal methyl group. Occasionally, the delta () system is used. Linoleic acid in this case is represented by 18:29,12, indicating 18 carbons, two double bonds at positions C9 and C12.

In the TAG structure, when the two primary hydroxyl groups of the glycerol moiety are esterified with different fatty acids as in most edible fats and oils, the molecule becomes asymmetric. In order to designate the configuration of a TAG molecule, the IUPAC-IUB Commission on Biochemical Nomenclature has recommended the stereospecific numbering ( sn ) system [), the secondary OH group on the carbon atom (in the mid position) is shown to the left, then the primary (end) carbons on the top and the bottom are designated as sn -1 and sn -3, respectively, and the mid position carbon becomes sn -2. For example, the triacylglycerol in the figure is described as sn -glycerol-1-stereate-2-oleate-3-palmitate or sn -SOP or simply SOP with the understanding that the sn system is used.

Oils and fats do not exist as a single molecular species of triacylglycerol, but as mixtures of TAG species (Table ). Soybean oil consists of 15 species of TAGs, with LLL, OLL, and PLL being more abundant. Sunflower oil contains 17 species, with the more abundant OLL, LLL, and OOL. Most of the data for lipid contents of oils and fats are reported as the total fatty acid composition in the TAG mixture as a whole. This type of information is useful in telling which major fatty acids present in a particular oil or fat. However, it does not tell the distribution of the fatty acids among the positions of the glycerol moiety in the TAG molecule. Stereospecific distribution has significant implication on the chemistry, biochemistry, and metabolism of the lipid, as described in later sections.