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Heat induced cis-trans isomerisation of sunflower oils depending on temperature, reaction time and original content of linoleic acid was investigated. The content of isomeric fatty acids was determined by gas chromatography and the content of polymers by gel permeation high-performance liquid chromatography. The content of trans fatty acids increased with time and with temperature and a rate of cis-trans isomerisation and polymerisation depends on the temperature according to Arrhenius equation. The content of polymers was significantly lower in sunflower oil with high content of oleic acid because of the low concentration of linoleic acid in oil. In both oils the content of conjugated linoleic acid initially increased depending on time and temperature, however after certain time the stationary state occurred. Polymerisation of polyenoic fatty acids takes place directly with heat induced cis-trans isomerisation.

The aim of the present study is to describe the mechanism controlling heat‐induced formation of sunflower oil triacylglycerol and fatty acid methyl ester oligomers. The unique combination of high‐performance size‐exclusion chromatography with hyphenated electrospray ionization mass spectrometry (MS), atmospheric pressure chemical ionization‐MS, and high‐temperature gas chromatography‐MS techniques allows differentiating between radical coupling species and Diels–Alder cycloadducts. Targeted analysis of thermally degraded sunflower oils confirms the exact structures of various acyclic oligomers accompanied by less‐abundant products of pericyclic transformations. A series of model experiments simulate the impact of dienophile nature on the course of Diels–Alder reactions. Thus, α‐tocopherylquinone, δ‐tocopherylquinone, and methyl‐(E)‐11‐oxoundec‐9‐enoate are synthesized as naturally occurring dienophiles bearing electron‐withdrawing groups. The geometry of poor dienophiles does not affect concerted cyclization, while the structure of electron deficient dienophiles can overcome low reactivity. Practical Application: In the absence of oxygen, heat‐induced degradation of polyunsaturated triacylglycerols proceed predominantly via a radical pathway, whereas concerted reactions represent minor mechanisms. Sunflower oil triacylglycerol molecules in the system without propagation stage can be effectively protected by natural and/or synthetic antioxidants. Application of chelates is also recommended. However, antioxidant‐derived quinones, such as α‐tocopherylquinone, can enter the Diels–Alder reaction even more easily than dienophiles without electron‐withdrawing groups. Unsaturated core aldehydes possess the same reactivity. Examination of the mechanism controlling high‐temperature degradation of triacylglycerols is especially important for processing engineers in edible oil refineries and food technologists. New perspective may help them to minimize undesirable changes in polyunsaturated species.

Filip V., Kyselka J., Hrádková I., Berčíková M., Cihelková M. (2015): A versatile apparatus for the bench scale bleaching and deodorsation of vegetable oils. Czech J. Food Sci., 33: 537-544.A versatile isothermal glass apparatus provided with sintered glass on the bottom was designed for the bench scale adsorptive bleaching and deodorisation of vegetable oils. The stream of argon bubbles agitated the sample in both cases and protected the vegetable oil effectively against undesirable autoxidation and photooxygenation changes. However, a variable concentration of hexanal (27-30 μmol/kg) was always present in half-refined oils due to the heterolytic fragmentation of alkyl hydroperoxides. The colour on the Lovibond scale and the content of phosphorus (< 5 mg/kg) were efficiently decreased during the laboratory bleaching of rapeseed oil. It was found out that the adapted glass apparatus connected with a vacuum pump (1 hPa, a stream of argon gas instead of stripping steam) was able to reduce the content of carbonyl compounds below detectable concentration at 220-240°C. Furthermore, the extent of minor geometrical and positional isomerisation reactions was negligible (0.035-0.130 wt%). The results of the bench scale bleaching and deodorisation experiments were completely comparable with the industrial equipment under reduced pressure of the air.