This thesis re-evaluated current analyses for hydroperoxides, the first stable product of lipid oxidation. The objective was to compare linearity of response, accuracy, limits of detection, active concentration range, reproducibility, and required conditions and handling for six commonly-used hydroperoxide assays, using cumene (lipid-soluble) and tert-butyl (water-soluble) hydroperoxides as test standards; optimized procedures were then applied to oxidized methyl linoleate. Traditional iodometric titration method with thiosulfate is the most accurate assay chemically. It is stoichiometric, linear, and useful for high peroxide concentrations, but unclear endpoints limits sensitivity and many handling issues must be controlled to provide reproducible results. It is the only method providing absolute quantitation of hydroperoxides. PeroxySafeTM and PeroxoQuantTM commercial kits based on the xylenol orange assay detected nanomoles of hydroperoxides, but samples with more than trace levels of hydroperoxides (the usual case with foods) must be diluted extensively before analysis. Variation of reaction response varied with hydroperoxide structure is a major disadvantage for this assay, and the Fe3+-xylenol orange complex was readily bleached by excess hydroperoxide, thus reducing apparent hydroperoxide levels. Reaction stoichiometry cannot be determined due to proprietary reagents of unspecified concentration. The ferric thiocyanate method (chemical reaction or Cayman LPOTM kit) is extremely sensitive, detecting as low as 5 nanomoles, but the reaction stoichiometry varies with solvent and hydroperoxide structure and concentration. Fe3+-SCN complexes bleached at high hydroperoxide concentrations, causing underestimation of peroxide values. Extensive dilution of samples is thus required for analyses of lipid extracts from most foods. Due to these complications, xylenol orange and Fe3+-thiocyanate assays may be useful for monitoring changes of single materials over time or comparing extracts with comparable fatty acid composition, but they cannot determine absolute hydroperoxide concentrations. No optical assay tested matched peroxide values determined by iodometric assay. Finally, hydroperoxides oxidize triphenylphosphine selectively and stoichiometrically to triphenylphosphine oxide that can be detected and quantitated by HPLC, detecting as low as 5 picomoles of hydroperoxide. The reaction has promise, but needs further investigation before adoption. Results for all methods highlight the importance of excluding oxygen during the assays and understanding the correct concentration range for each assay.
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Food Science
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Rutgers University Electronic Theses and Dissertations
Rutgers University. Graduate School - New Brunswick
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