🍎 Peroxide value of oils and fats — iodometry with a visual endpoint

Food chemistry PN-EN ISO 3960

Overview

The standard specifies the iodometric method of determining the peroxide value of animal and vegetable oils and fats with visual detection of the endpoint. The peroxide value is a measure of the amount of oxygen chemically bound to an oil or a fat in the form of peroxides, particularly hydroperoxides — the primary products of the oxidation of lipids — and is treated as an indicator of the degree of rancidity (oxidation) of the fat. The method concerns all fats and oils, fatty acids and their mixtures with a peroxide value from 0 to 30 meq of active oxygen per kg, and also margarines and spreadable fats of various water content; it is not suitable for milk fat or for lecithins.

The peroxide value is a dynamic parameter, depending on the history of the sample, and the determination is a highly empirical procedure — the result depends on the mass of the test portion. For that reason ISO/TC 34/SC 11 fixed the mass of the test portion at 5 g for a peroxide value greater than 1 and 10 g for a value less than or equal to 1; because of this the results may be somewhat lower than according to the earlier standards. The result is usually expressed in milliequivalents of active oxygen per kg; the value in millimoles per kg is half the value in milliequivalents.

The peroxide value is used in the national quality standards for edible fats: the laboratory instructions give maximum values among others for refined rapeseed oil 5.0 and for olive oil 20.0. The laboratory instructions of the technical universities (Wrocław, Gdańsk) and of the University of Gdańsk describe a variant of the performance with chloroform and acetic acid according to the earlier edition of the standard; the current edition uses isooctane.

Method principle

A test portion of the fat is dissolved in a mixture of glacial acetic acid and isooctane (60 ml + 40 ml) and a saturated solution of potassium iodide is added. The peroxides oxidise the iodide to iodine, which is titrated with a standardised solution of sodium thiosulfate 0.01 mol/l (prepared fresh from a solution of 0.1 mol/l or standardised daily) with visual detection of the endpoint against a starch solution. All the reagents must be free of dissolved oxygen — the acetic acid, the isooctane and their mixture are degassed in an ultrasonic bath under vacuum or by bubbling nitrogen or carbon dioxide through them. The peroxide value is calculated from the volume of thiosulfate used for the sample and for the blank; in the instructions of the technical universities: PV = 1000·c·(V − V0)/m [meq O2/kg], where PV is the peroxide value, c the concentration of the thiosulfate (mol/l), V and V0 the volumes used for the sample and for the blank (ml), m the mass of the sample (g).

Applications

• Assessment of the freshness and of the degree of oxidation of vegetable oils, animal fats, margarines and spreadable fats
• Control of the commercial quality of edible fats at producers, receivers and in official laboratories (the limits in the product standards and in the Codex Alimentarius)
• Study of the changes of fat during heating, frying and storage
• Calculation of the Totox index together with the anisidine value (Totox = 2·PV + AV, where PV is the peroxide value and AV the anisidine value)
• Exercises in food analysis — the characteristic values of fats

Key parameters

Standard

Standard number

PN-EN ISO 3960:2017-03

Title (PL)

Oleje i tłuszcze roślinne oraz zwierzęce — Oznaczanie liczby nadtlenkowej — Jodometryczne (wizualne) oznaczanie punktu końcowego

Title (EN)

Animal and vegetable fats and oils — Determination of peroxide value — Iodometric (visual) endpoint determination

Step-by-step procedure

1. Preparation of the reagents

   Degas the acetic acid, the isooctane and their mixture 60 : 40. Prepare a saturated solution of potassium iodide and check it: to 0.5 ml of the solution in 30 ml of the mixture add two drops of starch — if a blue colour appears requiring more than one drop of thiosulfate, discard the solution.

2. Titrant

   From the thiosulfate solution of 0.1 mol/l prepare fresh a solution of 0.01 mol/l in boiled water (if possible bubbled through with nitrogen) and establish its titre against potassium iodate; determine the titre daily.

3. Sample

   Prepare the sample in accordance with ISO 661. Weigh into a 250 ml conical flask with a ground joint 5 g of the fat (an expected peroxide value > 1) or 10 g (≤ 1) to the nearest 0.0001 g.

4. Dissolution and addition of the iodide

   Dissolve the sample in the acetic acid–isooctane mixture, add the saturated solution of potassium iodide, close the flask immediately and mix (in the laboratory instructions: mix for about 1 min, then leave it in a dark place for 5 min).

   ⏱ Time: 5 min

5. Dilution and titration

   Add water, a few drops of the starch solution (a dark navy blue colour) and titrate the iodine liberated with the thiosulfate solution 0.01 mol/l until the colour disappears and remains gone for at least 30 s, shaking vigorously.

6. Blank

   Carry out a blank with the same reagents without the fat; the consumption of thiosulfate for the blank is subtracted from the consumption for the sample.

7. Calculation

   Peroxide value = 1000·c·(V − V0)/m [meq O2/kg] (V, V0 — the volumes of thiosulfate for the sample and for the blank, ml; c — the concentration, mol/l; m — the mass of the sample, g). Give the result as the mean of two determinations; the value in mmol/kg is half the value in meq/kg.

8. Test report

   Give the identification of the sample, the method of sampling, the reference to the standard, the mass of the test portion and the result; the peroxide value depends on the history of the sample, so record the conditions of storage.

Required equipment and apparatus

Reagents, media and consumables

Health and safety (OHS)

• Glacial acetic acid is corrosive, isooctane flammable — work under a fume hood, goggles, gloves, away from sources of ignition
• Degassing with an inert gas (CO2, nitrogen) — in a ventilated room
• The iodine liberated in the flask irritates the respiratory tract — keep the flask closed until the titration

Frequently asked questions