Automated measurement of lipid hydroperoxides in oil and fat samples by flow injection photometry

The status of oxidation of oil and fat samples is normally judged by their peroxide value (PV), an index that seeks to determine the peroxide content by measuring how much iodine is liberated by the sample from iodide in an a cidic medium in a specified time period. At peroxide levels of interest, no t only does this approach require considerable analyst skill, the method is inherently flawed because of the considerable differences in the rates at which different peroxides liberate iodine from iodide and potentially also the consumption of nascent iodine by unsaturated sites. We propose here a s ubstantially less biased method based on the oxidation of Fe(II) to Fe(III) by peroxides, followed by the colorimetric detection of the latter as the thiocyanate complex. The system is automated through now injection photomet ry. A methanol-butanol mixed solvent is used as the carrier stream. Samples , generally prediluted in the same solvent, are injected into this carrier; streams bearing Fe2+ and SCN- are separately introduced, mixed in-line, an d merged with the carrier stream. After a reaction time of 30 s, the optica l absorbance is detected by a light emitting diode (LED) based photometric detector. The method exhibits a wide dynamic range and good linearity (line ar r(2), 0.9943 for 0.1-120 mequiv/kg cottonseed oil hydroperoxides) with a good throughput rate (up to 60 samples/h). The method was successfully app lied to both vegetable (olive and cottonseed) oils and animal fats (lard an d poultry fat).