Peroxide Value Test – Principle, Procedure, Result

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Peroxide Value (PV) Test is a chemical titration used to find the peroxides and hydroperoxides present in fats and oils. It is taken as an early index of oxidation and shows the freshness, quality, and keeping life of the sample. A high value means the oil is turning rancid.

Objectives of Peroxide Value Test

  • To measure the primary peroxides and hydro-peroxides formed in fats and oils.
  • To judge the present freshness and general quality of the lipid sample.
  • To help in predicting the probable shelf life before rancidity sets in.
  • To detect early oxidative spoilage even before off-flavour or odour appears.
  • To assess the oxidative stability during storage, heating, and handling.
  • To ensure consumer safety by checking that peroxide level is below harmful limit.
  • To evaluate loss of nutritional components destroyed by advanced oxidation.
  • To verify the sample meets food safety rules and trade grade standards.

Principle of Peroxide Value Test

Peroxide Value Test – Principle is based on the redox action of lipid peroxides on iodide present in an acidified solvent mix. The hydroperoxides inside the fat oxidize the iodide (I⁻) into free iodine (I₂). In this step glacial acetic acid together with chloroform (or isooctane) keeps the medium acidic and lets the oil dissolve properly.

The liberated iodine stays in the solution with a brown tint. It is titrated at once with standard sodium thiosulfate. Starch indicator is put when the colour turns pale; a deep blue forms first and then fades to colourless at the end point. The millilitres of thiosulfate used are taken to calculate the peroxide value, expressed as milliequivalents of active oxygen per kilogram of sample.

Requirements for Peroxide Value Test

Apparatus and equipment
Analytical balance (high-precision).
250 mL stoppered Erlenmeyer flask.
Class-A burette, 0.1 mL division.
Volumetric pipettes and measuring cylinder.
Stopwatch or timer.
Magnetic stirrer or glass rod for mixing.
Amber bottle for light-sensitive KI.
Hot plate or water bath to melt solid fat.

Chemical reagents
Glacial acetic acid–chloroform (3 : 2) solvent mix.
Fresh saturated potassium iodide solution.
Standard sodium thiosulfate 0.1 N (or 0.01 N).
1 % starch indicator.
Boiled and cooled distilled water.

Safety and working condition
Diffuse daylight or shaded lamp bench.
Fume hood while using chloroform.
Laboratory gloves, goggles, coat for personal protection.

Procedure of Peroxide Value Test

  1. Take about 5 g oil or melted fat into a clean 250 mL glass-stoppered flask.
  2. Add 30 mL mixed solvent (glacial acetic acid : chloroform = 3 : 2). Swirl till it dissolves.
  3. Pipette 0.5 mL fresh saturated KI. Stopper at once and shake without air entry.
  4. Keep the flask in dim light for exactly 60 s.
  5. Add 30 – 100 mL boiled-cooled distilled water to arrest the reaction and bring iodine into aqueous layer.
  6. Titrate the yellow solution with standard 0.1 N sodium thiosulfate while swirling till colour turns very pale.
  7. Put 1 mL (about) 1 % starch indicator; mixture becomes deep blue.
  8. Continue titration dropwise until the blue just disappears and stays clear for 30 s.
  9. Run a blank the same way using all reagents but no sample.
  10. Calculate peroxide value from (sample titre – blank titre) × normality × 1000 / weight of sample.

Calculation of Peroxide Value Test

Calculation of Peroxide Value

Peroxide value (PV) is calculated by the expression–

PV = [(S – B) × N × 1000] / m

where,
S = volume of sodium thiosulfate used for the sample (mL)
B = volume of sodium thiosulfate used for the blank (mL)
N = exact normality of the sodium thiosulfate titrant
m = weight of fat or oil taken (g)

Unit comes out in milliequivalents of active oxygen per kilogram of sample (meq O₂/kg).

When a titrant factor (F) is supplied for strength correction the relation is written as–

PV = [(S – B) × N × F × 1000] / m

Smaller PV means fresher oil; rising PV signals ongoing rancidity.

Result of Peroxide Value Test

Result (Interpretation)

PV < 2 meq O₂/kg – very fresh refined oil.
PV < 5 meq/kg – excellent virgin grade, cold-pressed type.
PV < 10 meq/kg – fresh and acceptable for normal table use.
PV 10 – 20 meq/kg – moderate oxidation, still usable in many virgin oils.
PV 20 – 30 meq/kg – clear rancidity risk, off-flavour starts.
PV > 26 meq/kg – labelled unfit for human intake by WHO/FAO limit.
PV > 30 meq/kg – advanced rancid, strong objectionable odour and taste.

Note – very low PV can also mean peroxide already decomposed; secondary tests are needed when quality in doubt.

Uses of Peroxide Value Test

  • Used to measure the first formed peroxides and hydro-peroxides in fats and oils.
  • Used to check present freshness and general quality of edible lipid products.
  • Used to predict and follow shelf life during storage and transport.
  • Used to catch early rancidity before off-odour or flavour appears.
  • Used to run routine quality control and meet trade or safety standards.
  • Used to confirm oils stay below the toxic limit set for consumer safety.
  • Used to judge oxidative stability under frying or other heat stress.
  • Used to test the effect of added antioxidants and of protective packaging.
  • Used along with p-Anisidine value to compute the TOTOX index for total oxidation.
  • Used to monitor stability of cosmetic, pharmaceutical, and dietary oil products.
  • Used to check oxidation level in animal feed fats and rendered by-products.

Advantages of Peroxide Value Test

  • Detects earliest hydro-peroxides before off-flavour appears.
  • Gives quick numeric picture of freshness and quality.
  • Helps forecast shelf life and set right expiry date.
  • Alerts to toxic breakdown products and keeps consumer safe.
  • Tracks oxidative stability for better processing and storage.
  • Serves many sectors—food, cosmetic, nutraceutical lines.

Limitations of Peroxide Value Test

  • Peroxides decompose quickly; very low PV may mean fresh or fully rancid oil.
  • High-temperature refining removes peroxides, so near-zero PV can mask earlier oxidation.
  • Added antioxidants can mop up iodine and push PV falsely low.
  • Trace metals such as copper or iron speed reagent oxidation, giving PV falsely high.
  • Endpoint judged by eye; operator variation shifts the number.
  • Strict 60-second wait, exact mass, right solvent—any slip changes the result.
  • Poor dissolution or moisture in sample stops full reaction, lowers titre.
  • Method needs chloroform and iodine waste; demands fume hood and special disposal.

Precautions of Peroxide Value Test

  • Keep sample in dark, cool, airtight bottle; uncap only at test time.
  • Melt solid fat just 10 °C above its melt point; avoid higher heat.
  • Use fresh saturated KI every day; keep in amber glass with crystals present.
  • Employ boiled-cooled distilled water to stay oxygen-free.
  • Do all chloroform work under fume hood; use isooctane when allowed.
  • Make sure glassware is clean, dry, ungrazed, and amber for light-sensitive reagents.
  • Swirl gently; do not aerate or create vortex.
  • Time KI reaction exactly 60 s with accurate stopwatch.
  • Add starch indicator only after iodine fades to pale yellow.
  • Run a daily blank; blank titre should stay below 0.1 mL.
  • Avoid metal contamination; note antioxidants may suppress iodine and give low PV.

References

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