Peroxide value

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There are a variety of approaches available allowing measurement of the extent to which rancidity reactions have occurred during the storage of a fat or oil. The most widely used is the analysis of the peroxide value of the oil.

The double bonds found in fats and oils play a role in autoxidation. Oils with a high degree of unsaturation are most susceptible to autoxidation. The best test for autoxidation (oxidative rancidity) is determination of the peroxide value. Peroxides are intermediates in the autoxidation reaction.

Autoxidation is a free radical reaction involving oxygen that leads to deterioration of fats and oils which form off-flavours and off-odours. Peroxide value, concentration of peroxide in an oil or fat, is useful for assessing the extent to which spoilage has advanced.

The peroxide value is defined as the amount of peroxide oxygen per 1 kg of fat or oil. Traditionally this was expressed in units of milliequivalents, although if we are using SI units then the appropriate option would be in millimoles per kg (NB. 1 millimole = 2 milliequivalents). Note also that the unit of milliequivalent has been commonly abbreviated as mequiv or even as meq.

The peroxide value is determined by measuring the amount of iodine which is formed by the reaction of peroxides (formed in fat or oil) with iodide ion.

2 I^- + H₂O + ROOH -> ROH + 20H^- + I₂

Note that the base produced in this reaction is taken up by the excess of acetic acid present. The iodine liberated is titrated with sodium thiosulphate.

2S₂O₃^2- + I₂ -> S₄O₆^2- + 2 I^-

The acidic conditions (excess acetic acid) prevents formation of hypoiodite, which would interfere with the reaction.

The indicator used in this reaction is a starch solution where amylose forms a blue to black solution with iodine and is colourless where iodine is titrated.

A precaution that should be observed is to add the starch indicator solution only near the end point (the end point is near when fading of the yellowish iodine colour occurs) because at high iodine concentration starch is decomposed to products whose indicator properties are not entirely reversible.

Correlation of rancid taste and peroxide value depends on the type of oil and is best tested with taste panels. The odours and flavours associated with typical oxidative rancidity are mostly due to carbonyl-type compounds. The shorter-chain aldehydes and ketones isolated from rancid fats are due to oxidative fission and are associated with advanced stages of oxidation. The carbonyl-type compounds develop in low concentrations early in the oxidative process.

Peroxide values of fresh oils are less than 10 milliequivalents /kg, when the peroxide value is between 20 and 40 milliequivalents/kg, a rancid taste is noticeable.

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