Formation of Hydroperoxy‐, Keto‐ and Hydroxy‐Dienes in FAME from Oils: Influence of Temperature and Addition of α‐Tocopherol

Abstract

AbstractThe influence of temperature (40, 60 and 80 °C) and addition of α‐tocopherol (0, 500 mg/kg) on the formation and distribution of the main oxidation products of linoleic acid, i.e. hydroperoxy‐, keto‐ and hydroxy‐dienes, were studied in samples of fatty acid methyl esters (FAME) derived from high‐linoleic (HL) and high‐oleic (HO) sunflower oils. In the range of temperatures studied, the formation of hydroperoxydienes showed monomolecular and bimolecular rate constants that ranged from 0.01 to 1 mmol1/2kg−1/2h−1 and from 0.02 to 0.9 h−1, respectively. The overall activation energies involved were similar for both samples and for the monomolecular and bimolecular periods (63–68 kJ/mol). The relative oxidation of methyl linoleate, which depended on the fatty acid composition of the FAME sample, was unaffected by temperature. At the three temperatures assayed, hydroperoxydienes constituted approximately 90 and 50% of total hydroperoxides in the HL and HO samples, respectively. Formation of keto‐ and hydroxy‐dienes was influenced by temperature in a similar way to hydroperoxydienes and, consequently, changes in the distribution of compounds were not observed. The addition of α‐tocopherol not only decreased the overall oxidation rate, but also affected the distribution of compounds. The content of hydroperoxydienes relative to that of total hydroperoxides was not affected by the presence of the antioxidant in the HL sample, whereas a significant increase (75%) was found in the HO sample compared with the control (50%). The addition of α‐tocopherol in both samples also resulted in a slight increase of keto‐ and hydroxy‐dienes in relation to hydroperoxydienes.