Depending on the type of oil you use to cook -- and whether or not you reheat that oil for multiple uses -- you could be exposing yourself to high levels of toxic aldehydes, chemicals known to cause neurodegenerative disease and cancer. Researchers from the University of the Basque Country (UBC) in Spain have found that some popular cooking oils, including soybean and sunflower oils, generate high levels of toxic aldehydes when heated and reheated.
Maria Dolores Guillen, a lecturer in the Pharmacy and Food Technology Department at UBC, and her colleagues observed that upon being heated, certain cooking oils undergo significant degradation of their fatty acid content. This degradation results in the production of toxic aldehydes like 4-hydroxy-[E]-2nonenal, 4-oxo-[E]-2-decenal, and 4-oxo-[E]-2-undecenal, the latter two of which were discovered in food for the first time as part of the study.
And while some of these aldehydes dissipate after being produced in cooked oil, many others linger and accumulate. Upon consumption, these toxins then reacts with human proteins, enzymes, and hormones, which can lead to serious health problems.
"It was known that at frying temperature, oil releases aldehydes that pollute the atmosphere and can be inhaled, so we decided to research into whether these remain in the oil after they are heated, and they do" noted Guillen. "It is not intended to alarm the population, but this data is what it is, and it should be taken into account."
This data, what was published in the journal Food Chemistry, involved the testing of olive, sunflower, and flaxseed oils heated at temperatures of 190 degrees Celsius (374 degrees Fahrenheit). For the olive and sunflower oils, the team heated them for a total of 40 hours spread over the course of five days, while they heated the flaxseed oil for 20 hours.
Using gas chromatography / mass spectrometry analysis protocols, they discovered that sunflower oil was the worst offender for toxic aldehydes, followed by flaxseed oil, also known as linseed oil, and olive oil. And the more times these oils were reheated, the worse their concentrations of toxic aldehydes became.
Similarly, a study out of the University of Minnesota's Department of Food Science and Nutrition found that soybean oil produces similar concentrations of other toxic aldehydes when fried at a temperature of 185 degrees Celsius (365 degrees Fahrenheit).
