Peroxides are primary reaction products formed during the initial stages of oxidation and give an indication of the progress of lipid oxidation.
During oxidation the peroxide value will reach a maximum followed by a decrease as the peroxides further react and decompose.
Determination of peroxide relies on the fact that peroxides can liberate iodine from potassium iodide. The lipid to be tested is dissolved in a suitable solvent and excess potassium iodide is added. The amount of iodine liberated is then measured by a relatively simple titration with sodium thiosulphate and a starch indicator. The peroxide value (PV) is expressed as milliequivalents of iodine per kg of lipid (meq/kg)
The PV may be used as an indicator in its own right to measure the extent of oxidation in oil or it can be followed over time to determine the Induction Period (IP) which is generally regarded as the point of onset of oxidation as antioxidants are exhausted.
PVs may be measured on samples under real time conditions or after storage at elevated temperatures to perform accelerated shelf life tests.
PV testing is most suited to liquid oils; however solid matrices may be evaluated by extracting the oils present in a suitable solvent first.
This method although simple does have drawbacks in that it tells you nothing about the history of the oil – peroxides are transient species and erroneously low results may be reported because of subsequent peroxide decomposition – especially if the lipid has seen high processing temperatures. Indeed the PV of oil may be significantly reduced by heating in the absence of oxygen!