SOP: Peroxide Value in Oils and Fats

Peroxide value catches oxidation early, before rancid flavors give the game away. It measures the hydroperoxides formed as oils start to deteriorate, making it the front-line freshness test for frying oils, nuts, and any fat-containing product.

Chemical analysis converts invisible composition into numbers that specifications, labels, and safety decisions depend on. Consistent technique matters as much as good instruments: small variations in timing, temperature, or reagent preparation shift results enough to fail a batch that should pass, or pass one that should fail.

Purpose

To determine peroxide value by iodometric titration as an index of primary oxidation in edible oils and fats.

Scope

Applies to refined and crude vegetable oils, animal fats, frying oils, and extracted fat from foods. For very fresh oils below 1 meq/kg, use the micro method with larger sample weight.

Equipment and Materials

• Conical flasks with glass stoppers, 250 mL
• Burette, 10 mL, Class A
• Acetic acid and chloroform mixture, 3 to 2 by volume
• Saturated potassium iodide solution, freshly prepared
• Standardized sodium thiosulfate, 0.01 N (0.1 N for high values)
• Starch indicator, 1 percent, freshly prepared
• Analytical balance
• Dark cupboard or foil for reaction

Procedure

1. Weigh 5 g of oil (or an amount expected to give a titration of 0.5 to 5 mL) into a glass-stoppered flask. Record the exact weight.
2. Add 30 mL of the acetic acid and chloroform mixture and swirl to dissolve.
3. Add 0.5 mL of saturated potassium iodide solution, stopper immediately, and swirl for 1 minute.
4. Let stand in the dark for exactly 5 minutes.
5. Add 30 mL of deionized water and titrate with standardized sodium thiosulfate, shaking vigorously, until the yellow iodine color nearly disappears.
6. Add 1 mL of starch indicator and continue titrating to the disappearance of the blue color.
7. Run a blank with every batch and subtract its volume.
8. Work swiftly and shield from strong light, which accelerates iodine liberation.

Recording Results

Peroxide value in milliequivalents of active oxygen per kg equals (sample titration minus blank) times normality times 1000, divided by sample weight in grams. Report to one decimal place. Typical fresh refined oils read below 5 meq/kg.

Quality Control

Run a reagent blank with every batch and subtract it from sample results. Analyze every sample in duplicate; the relative percent difference between duplicates must not exceed 10 percent for concentrations well above the detection limit. Include a certified reference material or a validated in-house control sample with each batch and plot the result on a Shewhart control chart. Investigate any point outside two standard deviations and reject the batch if a point falls outside three standard deviations. Verify the calibration of volumetric glassware annually against Class A tolerances, and check the analytical balance daily with certified weights. Standardize titrants monthly and label with the expiry date. Document all quality control results in the batch record; sample results must not be reported when associated QC fails.

Troubleshooting

• High blanks: the potassium iodide solution is oxidized or the solvent contains peroxides. Prepare fresh KI daily and check solvent quality.
• Endpoint returns: air oxidation during titration. Titrate briskly and keep flasks stoppered between additions.
• Two-phase endpoint unclear: shake vigorously near the endpoint so iodine transfers fully to the aqueous phase.
• Erratic duplicates: sample not homogeneous or weighing errors with viscous oils. Warm gently to homogenize before weighing.

Safety Notes

• Chloroform is a suspected carcinogen and hepatotoxin. Use in the fume hood with gloves; consider the isooctane alternative method where permitted.
• Acetic acid is corrosive. Wear eye protection.
• Collect halogenated waste separately for licensed disposal.