Trans fatty acids present in the diet arise from two origins. The first is from bacterial biohydrogenation in the forestomach of ruminants, which is the source of trans fatty acids present in mutton and beef fats. These are present at a concentration of 2-9% of bovine fat. Tra¬ęs-11-octadecenoic acid is the main isomer produced although trans-9- and trans-10-octadecenoic acid are also produced. Thus, trans fatty acids occur in nature and cannot be considered to be foreign substances.

The second origin is from the industrial catalytic hydrogenation of liquid oils (mainly of vegetable origin, but also of fish oils). This produces solid fats and partially hydrogenated oils and is undertaken to increase the thermal stability of liquid oils and to alter their physical properties. The margarines, spreads, shortenings, and frying oils produced are thus more useful in the food processing industry than liquid oils. Chemically, a range of trans isomers is produced that, for vegetable oils containing predominantly C18 unsaturated fatty acids, is qualitatively similar to those produced by biohydrogena-tion, although the relative proportions of the isomers may differ. Use of fish oils containing a high proportion of very long-chain (C20 and C22) fatty acids with up to six double bonds produces more complex mixtures of trans, cis, and positional isomers. However, the use of hydrogenated fish oils in food processing is declining, owing to a general fall in edible oil prices and to consumer preference for products based on vegetable oils.

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