Recent epidemiological studies have suggested the adverse effects of an excess intake of trans fatty acids on human health. This study investigates the relation between the formation of trans fatty acids and heating. The mechanism of heat-induced cis/trans isomerization was first resolved by using the model lipids (triacylglycerols). Then the formation of trans fatty acids in edible oil during heating and frying were examined to accurately evaluate the content of trans fatty acids found in fried or heated food.
When one kind of unsaturated triacylglycerol—triolein (9-cis, 18:1)—was heated at around 180°C, small amounts of isomerization products dependent on heating temperature and heating period were obtained. Heat-induced isomerization is strongly correlated with the thermal oxidation of double bonds. And several edible antioxidants such as δ-tocopherol, sesamol, and rosemary extract effectively suppressed heat-induced trans isomerization.
In comparison with triacylglycerols, many small formations in trans fatty acids were observed in commercially available several edible oils when heated at around 180°C. The antioxidants coexisting in each edible oil also help suppress the formation of heat-induced trans fatty acids.
A frying model system was then used to estimate increases in trans fatty acids during cooking. Sliced raw potatoes (100 g) were fried in commercially available corn oil at 180°C, and 30 frying cycles were performed. A small change in the content of trans fatty acids in the frying corn oil suggests that an ordinary frying process using unhydrogenated edible oils has little impact on the dietary intake of trans fatty acids.