National Food Research Institute, Food Material Science Division, Kannondai 2-1-12, Tsukuba, Ibaraki 305-8642, Japan.
Chem Phys Lipids. 2010 Sep;163(7):741-5. doi: 10.1016/j.chemphyslip.2010.06.006. Epub 2010 Jul 1.
We investigated the heat-induced cis/trans isomerization of double bonds in monounsaturated lipids. When triolein (9-cis, 18:1) was heated around 180 degrees C, small amounts of isomerization products were obtained depending on the heating period. The heat-induced isomerization of triolein was considerably suppressed by the addition of different antioxidants or under nitrogen stream, and these additives simultaneously inhibited the thermal oxidation of double bonds in triolein. Therefore, an intermediate of the thermal oxidation reaction might be responsible for the heat-induced isomerization of the double bonds in triolein. The thermodynamics of the heat-induced isomerization of triolein (9-cis, 18:1) and trielaidin (9-trans, 18:1) were investigated using Arrhenius plot. The Arrhenius activation energies of cis double bonds in triolein and trans double bonds in trielaidin were 106 kJ/mol and 137 kJ/mol, respectively. The calculated internal rotational barrier heights of these double bonds were similar to those of the double bond of 2-butene radical and significantly lower than those of non-radicalized double bonds in 2-butene. These results suggest that heat-induced cis/trans isomerization of triolein and trielaidin occurs mainly through the formation of radical species, which are the intermediates produced during thermal oxidation. The activation energy difference between the two forms suggests that trans trielaidin radicals are more stable than cis triolein radicals. The high thermodynamic stability of the trans double bonds in lipid radicals would influence the population of cis and trans isomers in edible oils and contribute to slight accumulation of trans-18:1 isomers during heating or industrial processing.
我们研究了单不饱和脂肪中双键的热诱导顺/反异构化。当三油酸甘油酯（9-顺式，18:1）在 180 摄氏度左右加热时，根据加热时间的不同，会得到少量的异构化产物。添加不同的抗氧化剂或在氮气流下，三油酸甘油酯的热诱导异构化会受到很大抑制，这些添加剂同时抑制了三油酸甘油酯中双键的热氧化。因此，热氧化反应的中间产物可能负责三油酸甘油酯中双键的热诱导异构化。使用阿仑尼乌斯图研究了三油酸甘油酯（9-顺式，18:1）和三异油酸甘油酯（9-反式，18:1）的热诱导异构化的热力学。三油酸甘油酯中顺式双键和三异油酸甘油酯中反式双键的阿仑尼乌斯活化能分别为 106kJ/mol 和 137kJ/mol。这些双键的计算内旋转势垒高度与 2-丁烯自由基的双键相似，明显低于非自由基化的 2-丁烯双键。这些结果表明，三油酸甘油酯和三异油酸甘油酯的热诱导顺/反异构化主要通过自由基的形成发生，自由基是热氧化过程中产生的中间体。两种形式的活化能差异表明，反式三异油酸甘油酯自由基比顺式三油酸甘油酯自由基更稳定。脂自由基中反式双键的高热力学稳定性会影响食用油中顺式和反式异构体的比例，并导致在加热或工业加工过程中反式 18:1 异构体略有积累。
