Determination of the Δ15 double bond in fatty acids of hydrogenated soybean oil

A method was developed to determine the extent of hydrogenation of the Δ15 double bond which occurs during partial catalytic hydrogenation of soybean oil. A linear relationship was found to exist between the linolenate content of commonly occurring C₁₈ unhydrogenated oils (containing no tetraene) and the propanal resulting from their ozonization reduction. The amount of propanal so produced is directly related to the amount of Δ15 double bond in these oils, as well as in hydrogenated soybean oils. Soybean oil was treated with ozone in carbon tetrachloride at —20 C and then reduced with triphenylphosphine. The ozonized-reduced sample was injected into a gas chromatograph, operated at 170C and equipped with a 12 ft × 1/4 in. column of 100/ 120 mesh porous polymer beads. The propanal peak was identified and its area used as a measure of the fatty acids containing Δ15 double bonds in unhydrogenated soybean and other oils of known linolenate content. A nearly stoichiometric amount of propanal results from ozonizing, reducing and chromatographing soybean oil as shown by comparison with a standard mixture of propanal and carbon tetrachloride. The relative standard deviation for the method is ±4.4%. We have also found this method applicable to other oils containing the omega-3 double bond. Presented at the AOCS-AACC Meeting, Washington, D.C., March, 1968. No. Utiliz, Res. Div., ARS, USDA.