Peroxide number

The peroxide number ( POZ ) is a key figure for the content of peroxidic functional groups in a fat or a fatty oil. It can be used as an indicator for assessing fat spoilage.

Formation of peroxide groups in fat

Peroxides and hydroperoxides in fats can result from the oxidation of unsaturated fatty acids with singlet oxygen ¹ O ₂ in an ene reaction . This reaction takes place by standing in the oxygen- containing air for a long time , which is why the peroxide number is a measure of how bad the fat is.

Formation of hydroperoxides on C = C double bonds of fatty acids with singlet oxygen (schematic representation).

specification

According to DIN regulations, the peroxide number is expressed in “milli- equivalents of oxygen per kilogram of fat”. The number corresponds to the amount of peroxidically bound oxygen atoms in millimoles per kilogram of fat. For example, if 1 kg of fat contains 0.5 mmol of peroxide groups, then O atoms are bound peroxidically in it, since each peroxide group has 2 oxygen atoms. This means that the peroxide number of the fat is POZ = 1. ${\displaystyle 2\cdot 0,5\,\mathrm {mmol} =1\,\mathrm {mmol} }$

execution

An iodine-starch reaction is carried out to detect the peroxides and determine the peroxide number . The sample is initially mixed with an excess of acidic potassium iodide solution, with iodide being oxidized to iodine by hydroperoxides and peroxides . Acetic acid is often used as the acid , while chloroform is a suitable solvent , as it dissolves most fats well.

${\displaystyle \mathrm {R{-}OOH\ +\ 2\ I^{-}\ +\ 2\ H_{3}O^{+}\longrightarrow \ R{-}OH\ +\ I_{2}\ +\ 3\ H_{2}O} }$

Then starch solution is added and the iodine formed is back- titrated with a standard solution containing thiosulfate ions, whereby the amount of converted iodide can be calculated.

${\displaystyle \mathrm {I_{2}\ +\ 2\ S_{2}O_{3}^{2-}\longrightarrow \ 2\ I^{-}\ +\ S_{4}O_{6}^{2-}} }$

At the same time, a blank sample is titrated and the peroxide number is calculated from the difference between the results.

literature

• Standard DIN EN ISO 27107: 2010-08: Animal and vegetable fats and oils - Determination of the peroxide number - Potentiometric endpoint determination (ISO 27107-2010 - beuth.de ).

Individual evidence

1. ↑ AP Author (ed.): Pathology of Oxygen. Academic Press, New York 1982, ISBN 0-12-068620-1 , pp. 25-26.
2. ↑ Regulation (EEC) No. 2568/91 (PDF) of the Commission of July 11, 1991 on the characteristics of olive oils and the methods for their determination.