Soy oil makes up an estimated 70% to 80% of all cooking oil used commercially in the United States. And while generally regarded as a healthier choice than edible oils that are higher in saturated fat, soy oil’s good-for-you polyunsaturated fatty acids are prone to oxidation at the high temperatures typically used for frying.
Oxidation can make the oil heavy and gummy, which can lead to off-flavors and odors and cause the oil to form a messy foam during deep-frying. The combined effects of oxidation may shorten the usable life of the oil, which, in some commercial settings, is reused until health regulations or quality problems dictate discarding it.
Researchers at USDA’s Agricultural Research Service’s National Center for Agricultural Utilization Research are investigating natural antioxidant compounds that might effectively protect soybean oil’s polyunsaturated fats from oxidation during frying.
In preliminary experiments the scientists found sesamol, extracted from sesame seed oil, provided better antioxidant protection for soy oil than nine other natural antioxidants that the team tested. Follow-up experiments tested sesamol against a synthetic antioxidant, TBHQ (tert-butylhydroquinone), which is commonly added to soy cooking oil sold in bulk for commercial use. Tests with french fries showed that sesamol, when added to soy oil at the rate of 6,600 parts per million, provided better protection than TBHQ added at the allowable maximum of 200 parts per million.
Two indicators of oxidation—polymerization of certain soy oil molecules and loss of specific soy-oil protons—were used to evaluate samples of the oil throughout the 8-hour frying test.
The researchers warn that even though sesamol is a natural, edible compound, more research is needed to ensure that using it at levels that provide antioxidant protection for the soybean oil would, at the same time, meet federal GRAS (Generally Recognized As Safe) standards.