Quantitative analysis of hydroperoxy-, keto- and hydroxy-dienes in refined vegetable oils.
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 90 wt%. Transmethylation with NaOMe resulted to be appropriate as derivatization procedure, although inevitably also gives rise to losses of hydroperoxydienes, which were lower than 10 wt%, and formation of keto- and hydroxy-dienes as a result. An amount of 0.6-2.1 wt% 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 0.3 μg/mL for hydroperoxy- and keto-dienes and 0.6 μg/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 1.1 mg/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.
Instituto de Grasa, Consejo Superior de Investigaciones Científicas, Avda. Padre García Tejero, 4, E-41012 Sevilla, Spain.
Arturo
A
Morales
Susana
S
Marmesat
M Carmen
MC
Dobarganes
Gloria
G
Márquez-Ruiz
Joaquín
J
Velasco
D000475:Alkenes / Q000032:analysis*; D006861:Hydrogen Peroxide / Q000032:analysis*; D006878:Hydroxides; D057230:Limit of Detection; D016014:Linear Models; D019787:Linoleic Acid / Q000737:chemistry; D000432:Methanol; D019301:Oleic Acid / Q000737:chemistry; D010084:Oxidation-Reduction; D010938:Plant Oils / Q000737:chemistry*; D017680:Potassium Compounds; D015203:Reproducibility of Results; D013229:Stearic Acids / Q000737:chemistry
D000475:Alkenes; D006878:Hydroxides; D010938:Plant Oils; D017680:Potassium Compounds; D013229:Stearic Acids; D019301:Oleic Acid; C031183:stearic acid; D019787:Linoleic Acid; D006861:Hydrogen Peroxide; C029943:potassium hydroxide; D000432:Methanol