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University researchers have found that a toxic compound known as HNE, which has been linked to heart disease and stroke, builds up steadily in intermittently heated oil for up to five hours at frying temperature.
Change your oil every day
Reusing vegetable oil for frying could have health consequences
By Deane Morrison
May 15, 2006
If you follow the nutritional news, you've probably heard that vegetable oils are heart-friendly. In general, that's true-but even good friends can turn on you when things get hot. Work by two University food chemists shows that some nasty compounds form in soybean oil heated to frying temperature (365 F) and that they keep building up after cooling and reheating, similar to long-term heating. Leading the ongoing research is A.Saari Csallany, professor of food science and nutrition, who is evaluating edible oils and fats for the formation of harmful compounds under various circumstances. It's a difficult task. Not only are human diets complex, but even the oil from a single plant source like olives or peanuts is a mix of several different fatty acids with different properties. But oil chemists like Csallany and her former graduate student and colleague Christine Seppanen are identifying potential hazards so that consumers can make more informed choices about fats and oils. Both commercial and home kitchens fry foods in a vegetable oil such as soybean oil, a readily available product. What Csallany and Seppanen found is that a toxic compound known as HNE builds up steadily in intermittently heated oil for up to five hours at frying temperature. It doesn't matter if the five hours of heating is continuous or if the oil is alternately heated for an hour and stored at room temperature overnight over the course of several days; HNE accumulates similarly. "This toxic compound--HNE--has been shown to incorporate into food during frying and is readily absorbed from the diet," says Csallany. "It is capable of reacting with amino acids [the building blocks of proteins], DNA, and other biomolecules. Reports in the literature have linked it to several diseases, including heart disease, stroke, Parkinson's disease, Alzheimer's disease, Huntington's disease, and liver and other diseases." Several chemically reactive and toxic sister compounds of HNE (HHE, HOE, HDE, and HNE) also build up, although to lesser levels. The problem is that in the presence of oxygen, a chemical reaction called peroxidation occurs. During this process, oxygen is incorporated into polyunsaturated fatty acids--such as linoleic acid-in the oil, and therefore the oil begins to degrade. At frying temperature, several more chemical reactions take place and the toxic molecule HNE forms. The more the frying oil is exposed to air, the more chances for HNE to form. "Whether at home or in a restaurant, one should not reuse oils that are highly unsaturated and contain linoleic acid because they form HNE and related compounds," says Csallany. "Perhaps the best advice in general is to eat only limited quantities of fried foods." Csallany and Seppanen have recently published their work on reheated soybean oil at frying temperature in the Journal of the American Oil Chemists Society. For more on the fats and oils, visit the American Dietetic Association's Web site.