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Detection ofperoxide gives the initial evidence ofrancidity in unsaturated fats and oils. Other methods are available, butperoxide value is the most widely used. It gives a measure of the extent to which an oil sample has undergone primary oxidation; extent of secondary oxidation may be determined fromp-anisidine test.[1]
Thedouble bonds found in fats and oils play a role inautoxidation. Oils with a high degree ofunsaturation are most susceptible to autoxidation. The best test forautoxidation (oxidative rancidity) is determination of the peroxide value.Peroxides are intermediates in the autoxidation reaction.
Autoxidation is afree radical reaction involvingoxygen 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 1kilogram of fat or oil. Traditionally this was expressed in units ofmilliequivalents, although inSI units the appropriate option would be inmillimoles perkilogram (N.B. 1 milliequivalents = 0.5 millimole; because 1 mEq of O2 =1 mmol/2 of O2 =0.5 mmol of O2, where 2 is valence). The unit of milliequivalent has been commonly abbreviated as mequiv or even as meq.
The peroxide value is determined by measuring the amount ofiodine which is formed by the reaction of peroxides (formed in fat or oil) withiodide ion.
Thebase produced in this reaction is taken up by the excess ofacetic acid present. To measure the iodine, aredox titration is performed usingsodium thiosulfate.
The acidic conditions (excess acetic acid) prevents formation ofhypoiodite (analogous tohypochlorite), which would interfere with the reaction.
Astarch indicator solution is used, for whichamylose forms a blue to black solution with iodine and is colorless when the iodine is converted to iodidetitrated.
Peroxide values of fresh oils are less than 10 milliequivalents/kg; when the peroxide value is between 30* and 40 milliequivalents/kg, a rancid taste is noticeable.