Analysis of thermally abused soybean oils for cyclic monomers

Abstract

Cyclic monomers derived from the intramolecular condensation of the C₁₈ polyunsaturated fatty acids have been reported to elicit toxic responses when fed to laboratory animals at low dietary levels. This study was undertaken to quantitate the cyclic monomers formed by thermal oxidation induced during deep fat frying to assess the potential toxicity of commonly used vegetable oils. Two separate experiments were designed to study the effects of unsaturation and both intermittent and continuous heating on cyclic monomer formation. Both lightly hydrogenated soybean oil (iodine value [IV]=107) and refined, bleached and deodorized soybean oil were studied. The heated oil sustained substantial chemical and physical alterations, as indicated by IV decreases from 10–15 units, increases in free fatty acids of 5–10‐fold and in noneluted material of 18–21%. Selected samples were completely hydrogenated and analyzed for cyclic monomers by gas chromatography. Under chromatographic conditions sufficiently sensitive to detect a cyclic monomer standard at less than 0.5% by weight, no cyclic monomers were detected in any of the heated oil samples. However, after concentration by low temperature crystallization of the hydrogenated samples to remove a major portion of the saturated components interfering in cyclic monomer resolution, about 0.3–0.6% cyclic acids, as well as 0.4–0.9% polar materials, were detected in the heated soybean oils. Components appearing in the gas chromatogram with the same retention time as those in a cyclic monomer standard were further identified by gas chromatography‐mass spectrometry as disubstituted cyclic C‐18 acids.