What's cooking? Cis-trans isomerization of unsaturated fatty acids

Trans fatty acids (TFA), which are widely used in the food industry, cause serious illnesses and their consumption should be minimized. This study focused on hidden sources of TFA such as edible oils after heating.  It was hypothesized that heat would induce cis-trans isomerization in unsaturated oils (cis fats) manifested by an increased TFA content. The experiment tested six refined, general purpose edible oils with different poly/monounsaturated fat concentrations under various conditions. The oils were heated at four temperatures (360, 390, 420, 450°F), with the highest one being around the oils’ smoke point, and three reaction times (30, 120 and 240 minutes). Each test was conducted under four environmental conditions: a glass container (open and covered), a glass container with copper rods, and a steel container. An infrared spectrometer was used to determine trans fat presence indicated by a peak at 966 Wavenumber (cm-1). The TFA change was determined by calculating the difference of TFA before and after heating. The spectrometer calibration curve was created by mixing research grade 100% cis (Triolein Glyceride) and 100% trans (Trielaidin Glyceride) fats to obtain 10 reference points (ranging from 0 to 13% TFA concentration). The “Area of Region” method of quantitative analysis was performed to determine TFA concentration.  For all tested oils the results showed a higher TFA content after heating, proving that geometrical cis-trans isomerization had occurred and confirming the hypothesis for the majority of the test conditions. The TFA increase had a linear dependency on time. The cis-trans isomerization rate depended on the temperature according to the Arrhenius equation. The activation energy was calculated for all the test settings and varied for each oil sample based on the environmental conditions (open/covered glass, catalysts). There was very little difference in the isomerization rate between high and low polyunsaturated oils during shorter heating times, but then after 120 min and over 420°F, the oil with the higher polyunsaturated content exhibited a higher isomerization rate. Applying the uncorrelated T-test confirmed the hypothesis for the majority of the test conditions, except for a subset of the conditions with a combination of the shortest time and lowest temperature. Since the experiment confirmed that TFA content increased with heat and temperature, then during normal use, oils used for cooking/frying contain more TFA than the original product.  Since they are absorbed by the food, people consume higher amounts of TFA then they would expect based on the current FDA nutritional label. Consumer awareness should be increased and the FDA regulated nutritional label should be enhanced to indicate increased TFA content after heating. Further studies should be conducted to assess the impact of adding antioxidants and the impact of the cookware type on TFA creation. Furthermore, the regulations for the Food Service industry should be reviewed and updated to consider heat induced TFA creation.