Heat‐induced geometrical isomerization of α‐linolenic acid: Effect of temperature and heating time on the appearance of individual isomers

Abstract

The formation of linolenic acid geometrical isomers (LAGIs) was studied in linseed oil that was heated under vacuum in sealed ampoules at different temperatures (190-260°C) for several durations (2-16 h). A temperature of about 190°C seems to be necessary to induce the formation of LAGIs. At higher temperatures, disappearance of linolenic acid follows a first-order kinetic. The formation of LAGIs increases with both heating time and temperature, degrees of isomerization of linolenic acid higher than 50-60% could easily be obtained by simply heating the oil under vacuum. Side reactions remain at a low level. The mean probabilities of isomerization of individual ethylenic bonds are similar to those determined in linolenic acid-containing oils marketed in European countries, 41.9, 4.7 and 53.3% for double bonds in positions 9, 12 and 15, respectively. The di-trans t,c,t (trans, cis, trans) isomer is formed via the mono-trans c,c,t and t,c,c isomers by a two-step reaction. The proportions of the c,c,t and t,c,c isomers (relative to total LAGIs) decrease linearly with the heating time. The proportion of the c,t,c isomer is only slightly affected by this parameter; however, it increases with temperature. The proportion of the t,c,t isomer increases linearly with heating time at each tested temper- ature, at the expense of the c,c,t and t,c,c isomers. However, there is no simple relationship linking the disappearance of each of the mono-trans isomers and the formation of the di-trans isomer. KEY WORDS: Deodorization, geometrical isomers, heated oil, linolenic acid, linseed oil, trans fatty acids.Linolenic acid geometrical isomers (LAGIs) have been detected in edible soybean and rapeseed oils marketed in North America (1), France (2), some other European countries (Belgium, Germany, Great Britain) (3), Poland (2) and Israel (4). They may also occur in foods containing such oils (2,5) and in refined walnut oils (6). Although LAGIs can be generated in soybean and rapeseed oils that are overheated for prolonged periods in the laboratory (7) or in restaurants (8), it is now evident that the main dietary source resides in the "fresh" commercial oils themselves.The appearance of LAGIs in oils is linked to the deodorization step (1,9}. This operation is generally conducted at high temperatures (230 to 250°C) (10,11), under vacuum and in the presence of steam, for periods ranging from a few minutes to several hours. The resulting LAGIs have the structure cis- 9#is-12,trans-15, trans-9#is-12#is-15 (85-90% of total LAGIs), cis-9,trans-12,cis-15, trans-9, cis-12,trans-15 (10-15%) and trans-9,trans-9#is-15 and cis-9,trans-12,trans-15 (trace amounts, generally not detected without preliminary concentration) (1,5).

Fatty acid composition of commercial vegetable oils from the French market analysed using a long highly polar column

“…Both the individually analysed walnut oils showed TFA contents, with walnut II sample as high as 1.7 g total TFA/ 100 g oil. These trans-PUFA are mainly produced during the deodorisation step of oil refining process (Wolff, 1993a(Wolff, , 1993b. Avocado oil was the only oil to contain trans-18:1 isomers as low as 0.1% of total FAs.…”

Trans Polyunsaturated Fatty Acid Contents in Brazilian Refined Soybean Oil

“…The presence of di-trans 18:3 9t, 12c, 15t was observed in all lots of brands A, B, C, and D and it was significantly higher (P < 0.05) for brand C. This fatty acid originates mainly from acid 18:3 9c, 12c, 15t and its formation is rather favored by the use of deodorization temperatures higher than 245˚C. 15 In TPFA group 18:2, the total concentration ranged from 1.9 to 14.0 mg oil with a significant difference (P < 0.05) between brand C and brands A, B, and E. None of the brands evaluated presented acid 18:2 9t, 12t. Fatty acid 18:2 9c, 12t was found in concentrations larger than that of acid 18:2 9t, 12c in all brands, which represents a mean of 55% for this group.…”

Comparison of geometrical isomerization of unsaturated fatty acids in selected commercially refined oils

“…However, the high temperature used in this stage may induce the geometrical isomerization of unsaturated FA, such as C18:2 and C18:3 (Tasan and Demirci, 2003). During this stage, other factors which also affect the formation of trans FA include heating time, pressure and stripping steam dosage (Wolff, 1993;Bruggen et al, 1998;Martin et al, 2008). The rate of conversion also depends on the level of unsaturation of the FA molecule (Bruggen et al, 1998).…”