ISO 27107:2010 — Animal and Vegetable Fats — Potentiometric Peroxide Value

1. Scope and Principle of Potentiometric Peroxide Value Determination

ISO 27107:2008 specifies a potentiometric end-point determination method for the peroxide value (PV) of animal and vegetable fats and oils. Peroxide value is a critical quality indicator measuring primary oxidation products in lipids, expressed in milliequivalents of active oxygen per kilogram of sample. The method is applicable to all animal and vegetable fats and oils, fatty acids and their mixtures with peroxide values from 0 to 30 meq of active oxygen per kilogram, and is also applicable to margarines and fat spreads with varying water content. It does not apply to milk fats or lecithins which require different analytical approaches.

The potentiometric method offers significant advantages over traditional visual titration. Objective end-point detection eliminates analyst subjectivity and variability in colour perception, which is particularly valuable for dark-coloured oils where the starch-iodine colour change is difficult to observe. The method also provides permanent records of titration curves allowing retrospective review of results. The principle involves dissolving the sample in an isooctane/acetic acid mixture, adding potassium iodide which is oxidized by peroxides present in the sample, and titrating the liberated iodine with standardized sodium thiosulfate solution. The end-point is detected electrochemically using a combined platinum electrode system.

2. Reagents, Apparatus, and Analytical Procedure

The standard provides detailed specifications for all reagents with emphasis on removing dissolved oxygen — a major source of positive bias. All solvents must be degassed by vacuum sonication or inert gas purging before use. The potassium iodide solution must be prepared fresh daily and tested for iodine/iodate contamination immediately before use. Two concentrations of sodium thiosulfate standard solution are specified: 0.1 N for general use and 0.01 N for low PV samples. The 0.01 N solution must be prepared fresh and its titre determined daily due to limited stability. The standard specifies potassium iodate as the primary standard for titre determination.

The automatic titrator must be capable of dynamic titration — fast addition at the beginning, slow near the end-point — to minimize analysis time while maintaining accuracy. A combined platinum electrode is specified with regular maintenance procedures to ensure stable potential readings. Sample preparation is critical: solid fats must be melted carefully to 10 degrees C above their melting point using a microwave oven without introducing air or exposing to direct sunlight. The peroxide value should be determined as the first test on any oil sample since it is sensitive to storage conditions and handling. All steps must be carried out in diffuse daylight or artificial light, avoiding direct sunlight exposure.

The titration procedure specifies precise timing for each step — the reaction between the sample and potassium iodide proceeds for exactly 60 seconds before titration begins, and the titration should be completed within a defined time window to ensure consistent results. The standard also specifies blank determinations to correct for any background iodine liberation from reagents. Results can be expressed in both milliequivalents of active oxygen per kilogram and millimoles per kilogram, where 1 meq/kg equals 0.5 mmol/kg. The active oxygen mass fraction in mg/kg can be calculated by multiplying the PV in meq/kg by 8 (the equivalent mass of oxygen).

3. Engineering Insights for Oil Quality Control Laboratories

Peroxide value is one of the most widely used indicators of oil quality and shelf life in the food industry, but it has important limitations. PV measures only primary oxidation products — hydroperoxides — which are formed in the early stages of lipid oxidation and can decompose during advanced oxidation into secondary products such as aldehydes and ketones that contribute to rancid off-flavours. A low PV does not necessarily indicate good quality if the oil has undergone advanced oxidation where hydroperoxides have already decomposed. For comprehensive quality assessment, complementary tests for secondary oxidation products — anisidine value or thiobarbituric acid value — are recommended alongside PV measurement.

The potentiometric method eliminates the subjectivity of visual end-point detection, improving inter-laboratory reproducibility. However, the method is empirically dependent on sample mass, reaction time, and temperature — these must be strictly controlled to obtain consistent results. The standard’s specification of fixed sample masses (5 g for PV above 1, 10 g for PV up to 1) addresses a known source of variability in earlier visual methods where different sample masses produced different results. Laboratories should validate the method for their specific instrument configuration and establish control limits using reference materials with known peroxide values.

Sample handling is critical for accurate PV determination. Oils are susceptible to photo-oxidation and auto-oxidation during storage and handling. Samples should be stored in full, airtight containers protected from light, preferably under nitrogen headspace. The peroxide value should be determined immediately after opening the sample container. If storage is necessary, samples should be kept at low temperature and analyzed within a few days. The standard references ISO 661 for sample preparation procedures. For inter-laboratory comparison or regulatory compliance, participation in proficiency testing schemes using the potentiometric method is recommended to validate laboratory performance against peers.

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