The PV is still the most common chemical method of measuring oxidative deterioration of oils. Although hydroperoxides decompose to a mixture of volatile and non-volatile products and they also react further to endoper-oxides and other products, the PV measurement is a useful method of monitoring oxidative deterioration of oils, although it should normally be combined with a method of monitoring secondary oxidation products to provide a full picture of the progress of oxidation. The PV reaction mechanism are:
In several literatures, increased addition of α-tocopherol to an oil may increase the PV while reducing hexanal formation. This suggests that a high PV value may reflect either increased formation of hydroperoxides or reduced decomposition. Consequently, antioxidants may improve the flavour stability of an oil without it being evident from PV measurements.
The method of determining PV involves a titration of the oil containing potassium iodide in a chloroform–acetic acid mixture. The hydroperoxides oxidise the iodide to iodine, which is determined by titration with sodium thiosulphate. In order to avoid the use of chloroform, the AOCS has developed an alternative method which uses isooctane as solvent, although the method is limited to PV<70meq kg−1, as described in the AOCS guidelines.
The peroxide value (PV) determines the concentration of hydroperoxide, the primary oxidation products. The principle involves peroxides liberating iodine from potassium iodide:
ROOH + KI → ROH + KOH + I2
The amount of ROOH is then determined by measuring the amount of iodine formed, which is done by titration with sodium thiosulfate and using a starch indicator:
I2 + starch + 2Na2S2O3 (blue)→2NaI + starch + Na2S4O6(colorless).
The amount of peroxides is calculated back by the amount of sodium thiosulfate (Na2S4O6) consumed. It is expressed as peroxide value (PV) in units of milli-equivalents (meq) peroxide per 1 kg of fat extracted from the food. A general rule is that PV should not be above 10–20 meq/kg fat to avoid rancidity flavor.
Whereas the PV measures the components produced by the early stages of oxidation, the anisidine value (AnV) measures the aldehyde and ketonic breakdown products of peroxides. This, then, is the measure of the materials that give the rancidity to oils and fats. One gram of fat is reacted with 100 mL of a solution of p-anisidine (p-methoxyaniline) in iso-octane and the amount of reaction products determined spectrophotometrically at 350 nm in a 10-mm cell. The AnV is defined as 100 times the absorbance of the solution resulting from this reaction. For an oil to still be acceptable the AnV should be less than 10. Research found a significant correlation between AnV and both flavor scores and PV. Considering the autoxidation reaction as a whole and how the resulting components relate to PV and AnV it will be obvious that in the early stages of autoxidation the PV will increase but, as the peroxides break down into aldehydes and ketones, the AnV will increase and, eventually, the PV will begin to decrease again. This is why it is so important to know something about the history of the oil before drawing conclusions based on PV alone.
