Overview
ISO 729:1988 specifies a laboratory method for the determination of the acidity of oils in oilseeds, expressed preferably as an acid value (mg KOH/g) or alternatively as a conventional acidity (% by mass). The method applies to oil extracted from seeds (extraction per ISO 659) and is based on dissolving the extracted oil in a diethyl ether/ethanol mixture and titrating free fatty acids with an ethanolic potassium hydroxide solution. The standard excludes cottonseed with adherent linters and is not applicable to oil palm or olive fruits; its use with lauric-rich oils (copra, palm kernel) is problematic.
Key topics and requirements
- Sample preparation: Oil must be the extract obtained according to ISO 659 (hexane/light petroleum extraction). Sampling should follow ISO 542.
- Reagents and solvents: Diethyl ether/ethanol (1:1 v/v) or, if necessary, ethanol/toluene as substitute; phenolphthalein (or alkali blue 6B for strongly colored oils); ethanolic KOH standard volumetric solution (normally 0.1 mol/L; 0.5 mol/L may be used when larger titrant volumes are needed).
- Titration procedure: Dissolve the weighed test portion in neutralized solvent mixture, titrate with ethanolic KOH until persistent endpoint color (phenolphthalein pink for ≥10 s).
- Safety note: Diethyl ether is highly flammable and forms peroxides; neutralize and handle with appropriate precautions.
- Calculation and reporting:
- Acid value (mg KOH/g) = V × c × 56.1 / m (where V = titrant volume mL, c = mol/L, m = test portion g).
- Acidity (%) is derived from the acid value using the molar mass of the acid adopted for expression (oleic acid commonly used by convention).
- Report mean of two determinations, to 0.01 units, and state whether result refers to oil from whole seed (as received) or pure seed oil. Include any deviations or incidents.
Practical applications and users
ISO 729 is used by:
- Food and agricultural testing laboratories for quality control of oilseed lots.
- Seed processors and crushers to monitor oil quality and free fatty acid levels during storage and processing.
- Regulatory bodies and purchasers specifying acidity limits for commercial transactions.
- Research groups studying oil deterioration, seed storage, or processing effects.
Practical benefits:
- Provides a standardized, comparable measure of free fatty acid content (indicator of hydrolytic rancidity).
- Integrates with ISO 659 oil-content procedures for streamlined workflows.
- Useful for acceptance testing, shelf-life evaluation, and process control.
Related standards
- ISO 659 – Oilseeds: Determination of hexane (light petroleum) extract (oil content)
- ISO 660 – Animal and vegetable fats and oils: Determination of acid value and of acidity
- ISO 542 – Oilseeds: Sampling
- ISO 5725 – Precision of test methods (repeatability and reproducibility)
Keywords: ISO 729, acidity determination, oilseeds, acid value, free fatty acids, ethanolic KOH titration, diethyl ether/ethanol, oil quality testing.
