SOP: Active Oxygen Method (AOM) for Fat Stability
GIFSQ — Global Institute for Food Safety & Quality
Principle
The active oxygen method (AOM) measures the time in hours required for a sample of fat or oil to attain a predetermined peroxide value (100 m.e.) under specified conditions of test. Air is passed through the fat maintained at high temperature until the peroxide value has reached a level corresponding approximately to the point at which rancidity is organoleptically perceptible. For those fats to which the peroxide value does not apply, the end-point is determined organoleptically. The method is empirical and the conditions should be rigorously maintained.
Apparatus required
- Air purification train: compressed air passed through a stainless steel needle valve to three gas-washing jars in series — one bottle containing distilled water and two bottles containing 2% potassium dichromate in 1% H₂SO₄; to the last jar, a 5-bulb water-cooled vertical condenser; the condenser connected to a wide-mouthed bottle containing glass wool; the outlet end of the bottle connected (a) to 2 more gas-washing jars in series containing distilled water and (b) to the manifold which takes the sample tubes through a flowmeter
- Sample tubes: Pyrex or Corning tubes, 2.5 × 20 cm, calibrated or etched at the 20 ml level; fitted with a neoprene stopper having two holes, with an air inlet tube and an outlet tube
- Boiling water bath
- Heater maintaining 97.8°C
- Capillary manifold with flowmeter
Reagents
a. Distilled water, preferably glass-distilled (to prevent contamination with copper and/or other metals).
b. 2% potassium dichromate in 1% H₂SO₄ (for the air purification train).
c. Acetone or petroleum ether (for cleaning the tubes after determination); detergent solution if needed.
Procedure
Pour 20 ml of oil or melted fat into the sample tube (to melt the fat, do not exceed a temperature more than 10°C above the melting point)
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Insert the aeration tube and adjust the air inlet tube so that it is 5 cm below the surface of the oil; place the tube in a boiling water bath for 5 min and then transfer to the heater which maintains a temperature of 97.8°C
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Connect the aeration tube to the capillary manifold, adjust the air rate and note the time
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At a time slightly less than the time required for the development of the target value (judged by experience from the odour of the outlet air from the sample tube), determine the peroxide value using 1 g of the sample
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If the peroxide value is between 75 and 175 m.e., make another determination immediately; if the value is more than 175 m.e., discard the material and make a new determination
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If the first determination gives a value less than 75: determine the peroxide value using 5 g of sample at a time when it would be about 75; make a second determination exactly one hour thereafter, and find the peroxide value (the two determinations should give values between 75 and 225 m.e.); plot the peroxide value against time on coordinate graph paper, connect the two points and find the time required to give a value of 100 m.e.
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Repeat the procedure on the duplicate sample and report the AOM stability value as the average of the two values (the time required to attain a peroxide value of 100 m.e.)
Precautions
- Do not wash the apparatus using cleaning mixture; use glass-distilled water preferably to prevent contamination with copper and/or other metals.
- Clean the tubes and other parts coming in contact with the oil using acetone or petroleum ether immediately after the determination; use detergent solution if needed.
- With respect to organoleptic behaviour, fats can be grouped as stable and unstable. The stable fats generally contain oleic acid and a small amount of linoleic acid as unsaturated fatty acids; the unstable fats contain linolenic acid and higher unsaturated fatty acids in addition to oleic and linoleic acid. In stable fats the peroxide value increases gradually without being noticeable organoleptically. With animal fats, the peroxide value increases gradually up to 20 (induction period), after which there is a sharp increase; generally, rancidity occurs in animal fats when the peroxide value reaches 20. In vegetable fats no such sharp increase is observed: the peroxide value gradually increases up to a maximum and then decreases; rancid odour and taste become appreciable when the peroxide value has reached 100.
References
Ranganna, S., Handbook of Analysis and Quality Control for Fruit and Vegetable Products, 2nd edition.