University of Minnesota researchers A. Saari Csallany, a professor of food chemistry and nutritional biochemistry, and graduate student Christine Seppanen have shown that when highly unsaturated vegetable oils are heated at frying temperature (365 F) for extended periods--or even for half an hour--a highly toxic compound, HNE (4-hydroxy-trans-2-nonenal) forms in the oil.
Previously, vegetable oils such as soybean, sunflower and corn were regarded as heart-healthy because of their high levels of linoleic acid, a polyunsaturated fatty acid. HNE is incorporated into fried food in the same concentration as it forms in the heated oil. Also, Csallany and her colleagues have found three toxic HNE-related compounds (known as HHE, HOE and HDE) in heated soybean oil. They will present their work at a poster session from 9 a.m. to 2 p.m. Wednesday, May 4, at the 96th annual meeting of the American Oil Chemists Society in the Salt Lake City Convention Center.
"HNE is a well known, highly toxic compound that is easily absorbed from the diet," said Csallany. "The toxicity arises because the compound is highly reactive with proteins, nucleic acids--DNA and RNA--and other biomolecules. HNE is formed from the oxidation of linoleic acid, and reports have related it to several diseases, including atherosclerosis, stroke, Parkinson's, Alzheimer's, Huntington's and liver diseases."
Csallany's work underscores the risk of repeated heating, or reusing, highly unsaturated oils for frying because HNE accumulates with each heating cycle. In future studies, Csallany and her colleagues plan to determine how long polyunsaturated oil must be heated at lower temperatures in order to form HNE and its related compounds. The study was funded by the University of Minnesota.