Peroxide detection is the first sign of rancidity in unsaturated fats and oils. Other methods exist, but peroxide value is the most commonly used. It determines the extent to which an oil sample has been subjected to primary oxidation; the extent of secondary oxidation can be determined using the p-anisidine test.
Fats and oils contain double bonds that contribute to autoxidation. Oils with a high unsaturation level are the most susceptible to autoxidation. The peroxide value is the best test for autoxidation (oxidative rancidity). In the autoxidation reaction, peroxides are intermediates.
Autoxidation is a free radical reaction involving oxygen that causes fats and oils to deteriorate, resulting in off-flavors and off-odors. Peroxide value, or the concentration of peroxide in an oil or fat, is useful for determining the extent of spoilage.
The most common cause of milk fat deterioration is rancidity, which is caused by oxidation and affects the flavour and quality of the fat. The acceptability of ghee is largely determined by the degree of oxidative deterioration. The first product formed by the oxidation of an oil or fat is generally thought to be a hydroperoxide. The peroxides decompose further to produce secondary oxidation products such as aldehydes and ketones, which give ghee an off flavour. The peroxide value (PV) of ghee is commonly used to assess rancidity. PV is reported in milliequivalents of peroxide oxygen per kg of fat or ml of 0.002 N sodium thiosulphate per gram of sample. The most common method for determining PV is iodometric titration, which measures the iodine produced by peroxides in a fat or oil from potassium iodide. PV is a product of primary oxidation and thus measures the rancidity or degree of oxidation of a fat but not its stability or shelf-life. The PV of fresh ghee would be zero.
Peroxide value is a measure of the oxidative rancidity of ghee. It is determined by the amount of peroxide present in the ghee, which is formed by the oxidation of fats. The peroxide value of ghee is usually expressed in terms of meq/kg.
The iodometric method and the oxygen absorption method are both recommended for determining the PV of ghee.
Iodometric method: Hydroperoxides are the first detectable autooxidation products and are sufficiently stable to continue accumulating for some time. Hydroperoxides are oxidizing agent and they liberate iodine from KI and the liberated iodine can be estimated by titrating against standard sodium thiosulphate (Na2S2O3) using starch as indicator. The amount of iodine liberated is proportional to the PV of the ghee sample.
However, this method is not satisfactory because significant flavour deterioration occurs at peroxide values below the limit that this method can accurately determine. No peroxides were detected after two months of storage at 37°C. The average peroxide value of ghee was reported to be 1.80 & 2.70 at the end of a 3- 4 month storage period. When compared to cow ghee, buffalo ghee samples showed a faster rate of peroxide formation.
Ghee with a high peroxide value will have an off-flavor and may be harmful to consume. Therefore, it is important to choose ghee with a low peroxide value to ensure that it is of the best quality.
Source : Team Safelabs