Quantitative analysis of hydroperoxy-, keto- and hydroxy-dienes in refined vegetable oils

Abstract

Quantitative analysis of the main oxidation products of linoleic acid – hydroperoxy-, keto- and hydroxy-dienes – in refined oils is proposed in this study. The analytical approach consists of derivatization of TAGs into FAMEs and direct analysis by HPLC-UV. Two transmethylation methods run at room temperature were evaluated. The reactants were KOH in methanol in method 1 and sodium methoxide (NaOMe) in method 2. Method 1 was ruled out because resulted in losses of hydroperoxydienes as high as 90wt%. Transmethylation with NaOMe resulted to be appropriate as derivatization procedure, although inevitably also gives rise to losses of hydroperoxydienes, which were lower than 10wt%, and formation of keto- and hydroxy-dienes as a result. An amount of 0.6–2.1wt% of hydroperoxydienes was transformed into keto- and hydroxy-dienes, being the formation of the former as much as three times higher. The method showed satisfactory sensitivity (quantification limits of 0.3μg/mL for hydroperoxy- and keto-dienes and 0.6μg/mL for hydroxydienes), precision (coefficients of variation ≤6% for hydroperoxydienes and ≤15% for keto- and hydroxy-dienes) and accuracy (recovery values of 85(±4), 99(±2) and 97.0(±0.6) % for hydroperoxy-, keto- and hydroxy-dienes, respectively). The method was applied to samples of high-linoleic (HLSO), high-oleic (HOSO) and high-stearic high-oleic (HSHOSO) sunflower oils oxidized at 40°C. Results showed that the higher the linoleic-to-oleic ratio, the higher were the levels of hydroperoxy-, keto- and hydroxy-dienes when tocopherols were completely depleted, i.e. at the end of the induction period (IP). Levels of 23.7, 2.7 and 1.1mg/g oil were found for hydroperoxy-, keto- and hydroxy-dienes, respectively, in the HLSO when tocopherol was practically exhausted. It was estimated that hydroperoxydienes constituted approximately 100, 95 and 60% of total hydroperoxides in the HLSO, HOSO and HSHOSO, respectively, along the IP.