chapter  13
40 Pages

Omega-3 and Trans Fatty Acids

ByVITTORIO MARIA MORETTI AND CARLO CORINO

Contents 13.1 Nomenclature and Classifi cation of Fatty Acids ........................................................... 234

13.1.1 Omega-3 Fatty Acids ....................................................................................... 234 13.1.2 Trans Fatty Acids ............................................................................................ 236

13.2 Sources of Omega-3 Fatty Acids .................................................................................. 237 13.3 Sources of Trans Fatty Acids ........................................................................................ 237 13.4 Analysis of Omega-3 and Trans Fatty Acids ................................................................. 239

13.4.1 Lipid Extraction and Preparation of Fatty Acid Esters .................................... 239 13.4.2 Direct Gas Chromatography ........................................................................... 240 13.4.3 Silver Ion Th in-Layer Chromatography ............................................................247 13.4.4 Silver Ion Solid-Phase Extraction .................................................................... 249 13.4.5 Silver Ion High-Performance Liquid Chromatography ................................... 249 13.4.6 Mass Spectrometry of Fatty Acid Derivatives ...................................................253 13.4.7 Infrared Spectroscopy ..................................................................................... 254

13.5 Occurrence of Omega-3 and Trans Fatty Acids in Meat and Poultry Products ............ 254 Acknowledgments ................................................................................................................... 262 References ............................................................................................................................... 262

13.1 Nomenclature and Classifi cation of Fatty Acids Th e systematic nomenclature of fatty acids is based on the number of carbon atoms in the hydrocarbon chain and on the number and position of double bonds relative to carboxyl group(s). Substituted groups and their positions and geometric confi guration at double bonds are designated.1 Fatty acids are referred as derivatives of hydrocarbons of the same number of carbon atoms, in which the fi nal letter “e” of the hydrocarbons is substituted by “anoic” for the saturated and “enoic” for unsaturated fatty acids. For example, octadecane becomes octadecanoic acid (saturated) or octadecenoic acid (unsaturated). Th e Greek letter Δ (delta), followed by one or more numbers, is used to designate the presence and the position of one or more double bonds, counting from the carboxyl group. Th e double bond positions are designated with numbers before the fatty acid name (e.g., Δ9,12,15-octadecatrienoic acid, or simply 9,12,15-octadecatrienoic acid, or 9,12,15-18:3). Conversely, the Greek letter ω or the letter n are used to indicate the position of the fi rst double bond counting from the terminal methyl group of the molecule. Th us, Δ9-octadecenoic acid becomes 18:1ω-9 or 18:1n-9; Δ9,12-octadecadienoic acid becomes 18:2ω-6 or 18:2n-6; Δ9,12,15-octadecatrienoic acid becomes 18:3ω-3 or 18:3n-3. Th e “ω” or “n” nomenclature has been introduced as a convenient way to express the metabolic conversions between fatty acid families. Th is terminology involves two assumptions.2 First, all double bonds are in cis confi guration and, second, if more than one double bond is present in the molecule, all double bonds are separated by a methylene (–CH2-) group; in this case, double bonds are referred as methylene interrupted. An example of a methylene interrupted fatty acid is linoleic acid (18:2n-6; Δ9,12-octadecadienoic acid).