Commercial fish oil is defined as the oils expressed primarily from menhaden, and secondarily from herring and tuna. They are abundant in triglycerides with long-chain polyunsaturated fatty acids containing more than 18 carbon atoms in the chain and with four or more double bonds. Essential fatty acids constitute about two percent of fish oil. Other components of fish oil include trace amounts of volatile, short-chain fatty acids, phospholipids, hydrocarbons, vitamins A and D, wax esters, and sterols, such as cholesterol. The composition of fish oil varies greatly according to its source. In general, fish oil for use in human food is hydrogenated to the degree necessary to produce a stable, palatable product of suitable plasticity. The objective is to partially saturate the polyene chains but not the monoene chains. The proportion of double bonds left unsaturated usually is measured by one of the several iodine value methods. However, hydrogenation invariably and unpredictably decreases the proportion of essential fatty acids. Hydrogenation also produces a wide range of chemically and sterically different products that vary according to source of the oil and subsequent treatment, e.g., conditions of storage, and refining. The monoeth- ylenic fatty acids of herring oil hydrogenated for margarine use (m.p. 32.5°C) have been reported to include two-thirds trans- and one-third cis-isomers. Since fish oil has been used as a source of food fat in many parts of the world, the literature on it is voluminous. In contrast, relatively little literature is available on hydrogenated fish oil. The important food uses of hydrogenated fish oil in many countries but not the United States are in margarines and shortenings. The most common use of fish oil and hydrogenated fish oil in the United States currently is in industrial products, including paints and lubricants rather than food.1
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