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Comparison of lipid peroxidation measures for corn oil subjected to different heating times and temperatures
Lipid peroxidation results in the production and degradation of numerous compounds, but assays commonly used to evaluate the extent of peroxidation measure a small proportion of these compounds. Therefore, a study was conducted to compare several indicators of peroxidation when heating refined corn oil at 185°C for 12 h (RO) or 95°C for 72 h (SO) with air flow of 12 L/min sampled hourly or at 8 h intervals, respectively. Samples were assayed for PUFA, FFA, peroxide value (PV), anisidine value (AnV), thiobarbituric acid reactive substances (TBARS), hexanal, 4-hydroxynonenol (HNE), and 2,4-decadienal (DDE). The correlation procedure of SAS was used to evaluate associations among assays and within assays over time for each heat treatment. Unperoxidized refined corn oil (0 h) contained 53.9% PUFA, 1.1% FFA, 2.1 mEq O2/kg PV, 0.24 AnV, 17.8 mg malondialdehyde eq/kg TBARS, 1.7 ug hexanal/g, 1.1 ug HNE/mL, and 12.6 uM DDE. Regardless of treatment, PUFA was negatively correlated (r < - 0.9, P < 0.05), whereas TBARS, hexanal, and HNE were positively correlated (r > 0.6, P < 0.05) with time. However, FFA was positively associated (r = 0.86, P < 0.01) with time for RO, but not for SO, and AnV was positively correlated (r = 0.96, P < 0.01) with time for SO, but not for RO. Peroxide value was negatively correlated (r = - 0.81, P < 0.01) with time for RO, but positively correlated (r = 0.94, P < 0.01) with time for SO. Regardless of treatment, hexanal (r < - 0.9) and HNE (r < - 0.6) were negatively associated (P < 0.05) with PUFA. The AnV was negatively correlated with PUFA (r = - 0.95, P < 0.01) for SO, but not RO. In RO, FFA content was positively correlated (P < 0.02) with TBARS (r = 0.87), hexanal (r = 0.79), and HNE (r = 0.64), but not for SO. These results indicate that thermal processing and storage conditions should be considered when selecting indicators of peroxidation, but this information is seldom available. The PV, AnV, and FFA assays are variable indicators of peroxidation in oil exposed to different heat treatments. However, HNE and hexanal increased with heating duration and reflected PUFA degradation for RO and SO; thus indicating that hexanal and HNE are reliable indicators of peroxidative damage in corn oil regardless of time and temperature conditions.
Keywords: corn oil, lipid peroxidation, peroxidation assays