Comparative Oxidative Stability of Fatty Acid Alkyl Esters by Accelerated Methods

Several fatty acid alkyl esters were subjected to accelerated methods of oxidation, including EN 14112 (Rancimat method) and pressurized differential scanning calorimetry (PDSC). Structural trends elucidated from both methods that improved oxidative stability included decreasing the number of double bonds, introduction of trans as opposed to cis unsaturation, location of unsaturation closer to the ester head group, and elimination of hydroxyl groups. Also noticed with EN 14112 was an improvement in oxidative stability when a larger ester head group was utilized. Methyl esters that contained ten or less carbons in the fatty acid backbone were unacceptable for analysis at 110 C (EN 14112) due to excessive sample evaporation. With respect to PDSC, a correlation was noticed in which the oxidation onset temperature (OT) of saturated fatty esters increased with decreasing molecular weight (R 0.7328). In the case of the monounsaturates, a very strong inverse correlation was detected between molecular weight and OT (R 0.9988), which was in agreement with EN 14112. Lastly, a strong direct correlation (R 0.8759) was elucidated between OT and oil stability index (OSI, EN 14112, 80 C). The correlation was not as strong (R 0.5852) between OSI obtained at 110 C and OT.