Qualitative and quantitative analysis of peanut oil triacylglycerols by reversed-phase liquid chromatography

Triacylglycerols (TGs) from an african peanut oil were analysed and fractionated by reversed-phase liquid chromatography (RPLC) using a differential refractometer as a detector. The fatty acids of the 33 collected fractions were analysed by gas chromatography after pentadecanoic acid (15:0) was added as an internal standard to quantitate the TG fractions. The three major fatty acids in the oil were octadecenoic acid (oleic acid, 44.5%), octadecadienoic acid (linoleic acid, 32.3%) and hexadecanoic acid (palmitic acid, 13.9%) together amounting to ca. 90% of the total fatty acids. Very long-chain saturated fatty acids (20:0-26:0) were also found. Thirty TGs could be easily identified from the fatty acid composition of the fractions alone. Dioleoyllinoleoylglycerol (18:1, 18:1, 18:2) was the main TG, amounting to nearly 17% of the oil, followed by palmitoyloleoyllinoleoylglycerol (16:0, 18:1, 18:2) (13%), oleoyldilinoleoylglycerol (18:2, 18:2, 18:1) (12%) trioleoylglycerol (18:1, 18:1, 18:1) (10%) and palmitoyldioleoylglycerol (16:0, 18:1, 18:1) (8%). These five TGs, which were virtually the only TGs in their respective fractions, together represented ca. 60% of the peanut oil TGs. Straight parallel lines were found for different series of TGs on plotting the logarithm of the relative retention time of the identified TGs (and those further identified) versus the number of double bonds. Other straight parallel lines were also observed on plotting the carbon numbers versus the equivalent carbon numbers of the oil TGs. These linear relationships were used to predict the different TGs present in the complex fractions. Their proportions in most instances were easily determined from the fatty acid composition of the fractions. In a very few instances a mathematical method had to be applied to solve the problem. Using the above-mentioned methods, 84 TGs could be identified and their percentages determined in the studied peanut oil. The very long-chain saturated fatty acids were always found associated with unsaturated fatty acids, preferentially with two molecules of linoleic acid. On the other hand, correction factors, determined from commercial simple TGs, were applied to peak areas before calculating the percentages of the 33 eluted TG fractions. For the major fractions of the oil, the series of values thus obtained was comparable to the series determined by the internal standardization procedure. Data reported here for peanut oil TGs are likely to be useful in identifying and quantifying the component TGs of other oils analysed by RPLC under the reported conditions.