ISO 3656:2011

Overview

ISO 3656:2011 - "Animal and vegetable fats and oils - Determination of ultraviolet absorbance expressed as specific UV extinction" specifies a standardized spectrophotometric method to measure UV absorbance of fats and oils. The standard defines how to prepare test solutions, which solvents and wavelengths to use, and how to calculate specific extinction (K values) to assess oil quality, refinement and oxidation. The method is widely used for quality control, authenticity testing and regulatory compliance in the edible oil and fats sector.

Key topics and technical requirements

• Scope: Measurement of UV absorbance for animal and vegetable fats and oils to express results as specific UV extinction (K232, K268, K270).
• Wavelengths & solvents:
  • Use isooctane (2,2,4‑trimethylpentane) for K232 and K268.
  • Use cyclohexane for K232 and K270.
  • Solvents must have low inherent UV absorbance (e.g., < 0.12 at 232 nm and < 0.05 at 250 nm).
• Sample prep:
  • Prepare according to ISO 661; typical test portion 0.05–0.25 g into a 25 ml flask to obtain absorbance 0.2–0.8.
  • Ensure clarity (filter if turbid) and homogeneous solution.
• Instrumentation:
  • UV-visible spectrometer with quartz cells of 10 mm pathlength; measure over 220–360 nm (or as needed).
  • Verify instrument wavelength and photometric accuracy using certified holmium oxide filters and potassium dichromate reference cells.
• Calculation:
  • Specific extinction Kλ = Eλ / (c · s), where Eλ is measured extinction, c is concentration (g/100 ml), s is pathlength (cm). Report to two decimal places.
• ΔK (variation of specific extinction):
  • Included (particularly for olive oil analysis) to detect changes in maximum absorption (wavelength m depends on solvent).
• Quality & precision:
  • Precision and reproducibility established from interlaboratory trials (ISO 5725 procedures); repeatability and reproducibility limits are provided.
• Safety & reagents:
  • Follow hazardous substance handling rules; use analytical-grade reagents.

Applications and users

• Quality control in edible oil refining and bottling operations to detect refining residues, oxidation, or adulteration.
• Regulatory testing for compliance with standards (e.g., IOC/EC regulations for olive oil) and national food control laboratories.
• Laboratories performing routine oil analysis (food testing labs, contract testing, R&D).
• Research into lipid oxidation, shelf life and refining process effects. Practical outcomes include identification of conjugated dienes/trienes (indicators of refining or oxidation) and monitoring oil authenticity and freshness.

Related standards and references

• ISO 661 - Preparation of test sample (referenced in ISO 3656)
• ISO 5555 - Recommended sampling methods for fats and oils
• ISO 5725 - Precision of measurement methods (interlaboratory evaluation)
• Commission Regulation (EEC) No 2568/91 and IOC methods - olive oil UV criteria and ΔK usage

Keywords: ISO 3656:2011, ultraviolet absorbance, specific UV extinction, K232, K268, K270, ΔK, fats and oils testing, isooctane, cyclohexane, spectrophotometric method.