Long-chain polyunsaturated fatty acids (LC-PUFA) of the n-3 series, particularly eicosapentaenoic (EPA) and docosahexaenoic (DHA) acid, have specific activities especially in the functionality of the central nervous system. Due to the occurrence of numerous methylene-interrupted ethylenic double bonds, these fatty acids are very sensitive to air (oxygen) and temperature. Non-volatile degradation products, which include polymers, cyclic fatty acid monomers (CFAM) and geometrical isomers of EPA and DHA, were evaluated in fish oil samples obtained by deodorization under vacuum of semi-refined fish oil at 180, 220 and 250 7C. Polymers are the major degradation products generated at high deodorization temperatures, with 19.5% oligomers being formed in oil deodorized at 250 7C. A significant amount of CFAM was produced during deodorization at temperatures above or equal to 220 7C. In fact, 23.9 and 66.3 mg/g of C20 and C22 CFAM were found in samples deodorized at 220 and 250 7C, respectively. Only minor changes were observed in the EPA and DHA trans isomer content and composition after deodorization at 180 7C. At this temperature, the formation of polar compounds and CFAM was also low. However, the oil deodorized at 220 and 250 7C contained 4.2% and 7.6% geometrical isomers, respectively. Even after a deodorization at 250 7C, the majority of geometrical isomers were mono-and di-trans. These results indicate that deodorization of fish oils should be conducted at a maximal temperature of 180 7C. This temperature seems to be lower than the activation energy required for polymerization (intra and inter) and geometrical isomerization.

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Omega‐3 long‐chain polyunsaturated fatty acids (LC‐PUFA) are sensitive to heat and may be destroyed by thermal processes such as deodorization. For example, deodorization of fish oil may induce polymerization, geometrical isomerization and cyclization of eicosapentaenoic (EPA) and docosahexaenoic (DHA) acids. In this paper, we review our main findings on the effects of deodorization at three different temperatures on semi‐refined fish oil LC‐PUFA. Cyclic structures have been elucidated and mechanisms responsible for ring formation have been discussed. Polymers were found to be the most abundant degradation products formed during fish oil deodorization. A method for quantitative measurement of geometrical isomers of EPA and DHA by gas‐liquid chromatography (GLC) has been developed and validated. Overall assessment of the results obtained with this method suggests that deodorization at temperatures above 180°C affects the quality and the content of LC‐PUFA in fish oil.