The Changes Of Trans-Fatty Acids During Heating Process

The effect of microwave and conventional heating on the fatty acids and trans-fatty acids (TFAs) compositions of edible oil was investigated. The change of content of trans-fatty acids and various fatty acids in oil after heating treatment were detected by gas chromatography, carbonyl value was detected by2,4-dinitrophenylhydrazine. The formation rate of trans-fatty acids and carbonyl value was detected by Infrared spectra.Edible oil was heated by Microwave oven for2min,4min,6min and8min, and the finally temperatures were120℃,156℃,186℃and212℃, then the oil which was heated to the same temperature under conventional heating process was compared to the microwave heating process. During microwave heating, contents of unsaturated fatty acids decreased much more than conventional heating. The total TFAs content increased from0.049g/L to0.085g/L by microwave oven for8min. Likewise, the microwave heating treatment caused a significant increase in carbonyl value in edible oil compared with conventional heating treatment at high temperature and shows a higher degree of deterioration. However, microwave heating for4min to156℃, the carbonyl value is not significantly different between microwave heating oil and conventional heating oil. The ratio of trans-double bond to carbonyl double bond decreases for microwave heating treatment., but the change of carbonyl value is not significant, it can be infer that the carbonyl value grew faster than trans-double bonds. These results suggest that edible oils heated under156℃will not induce the intensive change and be safe to the human heath.Because for the156℃under heating treatment, the change of various aspects is not significant, we did a group of comparison of different heating time under120℃,150℃180℃and212℃respectively, it shows that the content of saturated fatty acids grew up slowly with the increasing time and temperature, while the content of unsaturated fatty acids decreased slowly. Carbonyl value is also proportional to the heating temperature and time, and3hours of heating was significantly higher than1hour heating. Trans fatty acids increased with temperature and time increase up and the content of C18:2is increased more quickly than C18:1.The experimental results showed that the during microwave heating process, microwave make the activation energy of cis-trans isomerization lower and makes it easier to transform into trans fatty acids and the extra energy makes the fat cracking occurred, resulting in more carbonyl compounds. and this effect will be exacerbated as the temperature increases.