chapter  6
Methods for trans Fatty Acid Analysis
Pages 32

I. Introduction ........................................................................................................................ 157 II. Analysis Using Spectroscopy............................................................................................. 158

A. Infrared Spectroscopy................................................................................................. 158 1. Conventional IR Methods.................................................................................... 159 2. Ratioing of Single-Beam FTIR Spectra............................................................... 161

B. FTIR-Partial Least Squares Regression ..................................................................... 164 C. Near-Infrared Spectroscopy........................................................................................ 164

III. Analysis Using Chromatography ....................................................................................... 165 A. Gas Chromatography.................................................................................................. 165

1. Separations Using Packed Columns .................................................................... 165 2. Separations Using Capillary Columns................................................................. 165 3. Equivalent Chain Length ..................................................................................... 167 4. Relative Response Factors ................................................................................... 168 5. Official Capillary GC Methods............................................................................ 168

B. Thin-Layer Chromatography...................................................................................... 169 C. High-Performance Liquid Chromatography............................................................... 170

1. Reversed-Phase HPLC......................................................................................... 170 2. Silver Ion HPLC .................................................................................................. 170

IV. Analysis Using Off-Line Combined Techniques ............................................................... 171 A. Ozonolysis=Gas Chromatography .............................................................................. 171 B. Silver Ion Chromatography=NMR ............................................................................. 171 C. Gas Chromatography=IR Spectroscopy...................................................................... 172

V. Analysis Using Online Hyphenated Techniques................................................................ 172 A. GC-FTIR.................................................................................................................... 172 B. GC-EIMS ................................................................................................................... 174 C. SFC-FTIR .................................................................................................................. 178

VI. Trans Isomers in Commercial Products ............................................................................. 179 VII. Conclusions ........................................................................................................................ 180 References ..................................................................................................................................... 181

Trans fatty acids are present in a variety of food products and dietary supplements; some are derived from natural sources, such as dairy products, but most come from products that contain commercially hydrogenated fats. Margarines used to be the major source of trans fats; however, in recent years processed foods, such as snacks and fast foods, are more likely to be the major sources of dietary trans fats [1,2]. The nutritional properties of trans fatty acids have been debated for many

amounts of and high-density lipoprotein (LDL, HDL) contained in serum. Some studies have shown that trans fatty acids elevate levels of serum LDL cholesterol and lower HDL cholesterol [3-6]. Such results drew a great deal of attention, which eventually led to the mandatory labeling of trans fatty acids on food products in the United States, Canada, and many other countries [7-10]. For nutrition labeling purposes, trans fats are defined as the sum of all unsaturated fatty acids that contain one or more isolated, nonconjugated, double bonds with a trans double-bond geometric configuration. However, conjugated fatty acids with a trans double bond, such as conjugated linoleic acid (CLA) isomers, are excluded from this definition of trans fats. In the United States, label declaration of trans fats are not required for products that contain less than 0.5 g trans fat per reference amount and per labeled serving, as long as no claims are made for fat, fatty acids, or cholesterol. In Canada, to claim that a product is trans fat free, it must contain less than 0.2 g of trans fat per serving and per reference amount. Unlike other countries, Denmark imposed an upper limit on the amount of trans fats in foods (Danish Food Act, Executive order No. 160 of 11 March 2003), namely oils and fats must contain less than 2 g of trans fatty acids per 100 g of total fat, and fat-free labels would be allowed if a food product contains less than 1.0 g of trans fat per 100 g of the fat or oil.