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

1993; Wiley; Volume: 70; Issue: 4 Linguagem: Inglês

10.1007/bf02552718

1558-9331

Robert L. Wolff,

Food Chemistry and Fat Analysis

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 temperature, 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.