Detection of Sterol Epoxides in Foods by Colorimetric Reaction with Picric Acid

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Abstract
Picric acid (2, 4, 6-trinitrophenol) was reacted with sterol epoxides to form a chromophore with absorption maxima at 350 and 410 nm, which follow Beer's law. Linearity was obtained up to 0.2 mg/ml with a sensitivity of 2 μg of cholesterol α-epoxide/ml. Optimum conditions included a 24-h reaction at 23°C, a 150 + M/M excess of picric acid over sterol epoxide and a picrate removal step. Both cholesterol α-epoxide and cholesterol β-epoxide were synthesized and purified for analysis. The α-epoxide was seven times more reactive than the β-epoxide. Cholesterol and sitosterol α-epoxides had the same reactivity with picric acid. Epoxides were detected in commercially available french fries, but quantification required further purification.