| Unsaturated fatty acids arc of extremely high content in vegetable oil. Although they are good in the human body, such as lowering cholesterol, they are highly susceptible to oxygen attack thanks to their quite a few of unsaturated double bonds, leading to the occurrence of primary oxidation and secondary oxidation. Due to the different oxidation mechanisms and the degree of oxidation involved, different compounds are formed at different stages, as well as undesirable sensory and biological effects. Therefore, it is a challenging task to assess the lipid oxidation status of edible vegetable oils.In this study, soybean oil and palm oil were used as samples. This study is to investigate ten kinds of carbonyl compounds, including seven monocarbonyl compounds(hexanal, nonanal, trans-2-hexenal, trans-2-nonenal, trans-2-decenal,trans,trans-2,4-decadienal and trans-2-undecenylal) and three dicarbonyl compounds(glyoxal, methylglyoxal and 2,3-butanedione).To simulate rapid thermal oxidation of lipids,the lipid samples were heated at 100, 120, 140, 160, 180 and 200? for 1 h, 3 h and 5 h,respectively. The carbonyl compounds in the oils were detected by high performance liquid chromatography, and the carbonyl values were simultaneously measured. It showed that under the same oxidation condition, the carbonyl values of soybean oil were higher than those of palm oil during the rapid thermal oxidation process. The carbonyl value increased gradually with the increase of temperature. In the range of experimental temperature gradients, the carbonyl values were from 2.96 to 45.89 meq/kg for soybean oil and from 2.47 to 43.18 meq/kg for palm oil. The monocarbonyl compounds of the two vegetable oils account for the absolute predominance (748.63 mg/kg, 160? 5 h, palm oil; 684.24 mg/kg,160? 5h, soybean oil), and the content of hexanal was the highest (551.67 mg/kg, 180 ?5 h,soybean oil; 488.41 mg/kg,160 ? 1 h,palm oil). The distribution of hexanal is consistent to that of total monocarbonyl compounds. The other six kinds of monocarbonyl compounds showed their own distribution characteristics due to the boiling point, the source of fatty acids, and the effect of viscosity of heating oil. Dicarbonyl compounds, the content of which in the two vegetable oils was below 1.50 mg/kg, were not significant under the set conditions, especially at high temperature (180, 200?) and long time (3 h, 5 h). Therefore, it was the oxidation process at 60? as simulation manipulation of ambient temperature, with detection of the peroxide values, to observe whether the correlation between them. It showed that there was no good correlation between dicarbonyl compounds and peroxide value in simulated auto-oxidation process.The initial fatty acid composition of the two vegetable oils was determined by gas chromatography, and the effect of the fatty acid difference on the carbonyl compounds produced by the oxidized lipids was analyzed. The two kinds of vegetable oils had different species and contents of fatty acids, so did those of carbonyls produced are particularly distributed under the experimental conditions. Saturated aldehydes were both dominant in the two vegetable oils,in which there was little difference in hexanal, however, content of nonylaldehyde in soybean oil was not as much as that in palm oil. The content of soybean oil was predominant in unsaturated aldehydes, in which trans-2-hexenal and trans-2-decenal were particularly prominent of the difference in the two vegetable oils. |
PDF Full Text Request