Peroxide value

Detection of peroxide gives the initial evidence of rancidity in unsaturated fats and oils. Other methods are available, but peroxide 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 from p-anisidine test.^[1]

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.

Definition

The peroxide value is defined as the amount of peroxide oxygen per 1 kilogram 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 kilogram (N.B. 1 milliequivalents = 0.5 millimole; because 1 mEq of O2 =1 mmol/2=0.5 mmol of O2, where 2 is valence). Note also that the unit of milliequivalent has been commonly abbreviated as mequiv or even as meq.

Method

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 + HOOH -> HOH + 2OH⁻ + 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.

2S₂O₃²⁻ + I₂ -> S₄O₆²⁻ + 2 I⁻

The acidic conditions (excess acetic acid) prevents formation of hypoiodite (analogous to hypochlorite), 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.

AOCS Standard Method

Reagents and solution

1. Acetic Acid - chloroform solution (7.2ml Acetic Acid and 4.8ml Chloroform).

2. Saturated Potassium Iodide solution. Store in the dark.

3. Sodium thiosulfate solution, 0.1N. Commercially available.

4. 1% Starch solution. Commercially available.

5. Distilled or deionized water.

Procedure

1. Weigh 2.00 (±0.02)g of sample into a 100 ml glass stoppered Erlenmeyer flask. Record weight to the nearest 0.01g.

2. By graduated cylinder, add 12 ml of the acetic acid - chloroform solution.

3. Swirl the flask until the sample is completely dissolved (careful warming on a hot plate may be necessary).

4. Using 1 ml Mohr pipette, add 0.2 ml of saturated potassium iodide solution.

5. Stopper the flask and swirl the contents of the flask for exactly one minute.

6. Immediately add by graduated cylinder, 12 ml of either distilled or deionized water, stopper and shake vigorously to liberate the iodine from the chloroform layer.

7. Fill the burette with 0.1N sodium thiosulfate.

8. If the starting color of the solution is deep red orange, titrate slowly with mixing until the color lightens. If the solution is initially a light amber color, go to step 9.

9. Using a dispensing device, add 1 ml of starch solution as indicator.

10. Titrate until the blue gray color disappears in the aqueous (upper layer).

11. Accurately record the mls of titrant used to two decimal places.
^[2]

CALCULATIONS: S = titration of sample B= titration of blank Peroxide value = (S - B) X N thiosulfate X 1000/ weight of sample or (S - B) X N thiosulfate X 500

Taste

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.

References