Commercial oleic acid has a tendency to develop a dark color during heating. It has been found that this discoloration is partially due to oxidation and partially due to the presence of minor constituents. Their characterization as oxidized and polymerized fatty acids will be reported in a separate paper. The effects of the two factors are synergistic to each other and thus accentuate the darkening of the commercial oleic acid when it is heated under air. Removal of the minor constituents from commercial oleic acid by silicic acid treatment drastically improved its color stability. The generally acknowledged concept that the higher the polyunsaturated fatty acid content in the commercial oleic acid, the lower its color stability, is not correct. The color stability of commercial oleic acid is more predominantly dependent upon the content of minor constituents than that of polyunsaturated fatty acid.
Minor constituents in high quality tall oil fatty acids have been isolated successfully by liquid column chromatography, using silicic acid as the adsorbent. The minor constituents contained two types of compounds: those which were noneffective and those which were effective in causing the darkening of tall oil fatty acids during heating. The former consisted oftrans‐3,5‐dimethoxystilbene and rosin acids. The latter was separated into numerous fractions by the combination of chemical methods, silicic acid column chromatography, and low temperature fractional crystallization. The fractions were characterized by functional group analyses, chemical reactions, and UV and IR spectrometric methods. Most of the fractions contained two‐three times as much oxygen in the molecule as the original sample and were highly oxidized fatty acids. They had mol wt ranging 300–551 and contained double bonds, carbonyl, ester, peroxide, and hydroxyl groups. The effect of these minor constituents upon the color stability of tall oil fatty acids during heating was postulated as being due to the hydroxyl groups located in the α‐position to the double bond in the molecule.
Minor constituents were isolated from a mixture of commercial oleic acid manufactured from beef tallow by 10 different companies. Silicic acid was used as an adsorbent to isolate the minor constituents. They were first separated into acidic and nonacidic fractions. Each fraction was then separated into numerous subfractions by stepwise gradient elution liquid chromatography, using silicic acid as the adsorbent. The subfractions which had an adverse effect on the color stability of oleic acid during heating were characterized with functional group analysis, elemental analysis and IR spectrophotometry. The minor constituents amounted to ca. 1.18% of the commercial oleic acid. They were complex mixtures with multiple functional groups. Some of the subfractions contained 2\s‐3 times as much oxygen in the molecule as oleic acid. They had molecular weights ranging from 308 to 830 which are from monomers to trimers of oxidized oleic acid. These minor components contained carbonyl, ester and hydroxyl groups and double bonds. Some of the nonacidic minor constituents may contain amide groups. Most of the acidic subfractions were dark red, viscous liquids and the nonacidic subfractions were dark green or greenish‐brown, semisolid substances. A relationship was established that the greater the polarity of the minor constituents, the greater is its adverse effect on the color stability of oleic acid during heating.
The minor constituents from a high quality commercial oleic acid (SCOA) were isolated and fractionated by liquid column chromatography. They were chemically characterized and their effects on the color stability of oleic acid during heating were determined. The results were compared qualitatively and quantitatively with those from a mixture of commercial oleic acids (MCOA) manufactured by 10 companies. It was found that, qualitatively, SCOA and MCOA contained the same type of minor constituents. However, quantitatively, the MCOA contained 1.18% of minor constituents whereas the SCOA contained only 0.81% of minor constituents. The amount of effective minor constituents which had an adverse effect on the color stability of oleic acid during heating was 0.09% for SCOA vs 0.45% for MCOA.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.