Oils may be processed using mechanical pressing or heat and chemicals, a method that can affect its flavor and potentially its health benefits. Olive oil, for example, is prized for the complex flavors that are strongest when the oil is fresh from the fruit. That’s why higher grades (extra virgin and virgin) are given only to mechanically pressed oil that hasn’t been treated with heat or chemicals. Those premium, mechanically pressed oils also contain more antioxidants.
Processed or refined oils do have some pluses, though. They are less expensive, last longer, and can hold up to high-heat uses such as frying without smoking and breaking down into potentially toxic compounds. On the minus side, refined oils may have been extracted with hexane, an industrial solvent. A form of hexane is classified as an air pollutant by the Environmental Protection Agency and as a neurotoxin by the Centers for Disease Control and Prevention, and environmental groups have raised concerns about residues that might be left behind in the oil.
Testing by an organic advocacy group found trace amounts of hexane residue (less than 10 parts per million) in a sample of soybean oil. But almost all of the research on hexane toxicity has involved factory workers breathing in high concentrations of airborne hexanes. At very low exposure levels through food, there is no reason to think it should be a health problem, says the toxicologist John L. O’Donoghue, Ph.D., of the University of Rochester School of Medicine and Dentistry.
If you’re concerned about hexane in oil, look for labels that say it was ìexpeller pressed. Oils that carry the "USDA Organic" label are also produced without hexane. And see our reviews and Ratings of olive oil.