Overview
ISO 5566:1982 specifies a standardized spectrophotometric method to determine the colouring power of turmeric. The procedure quantifies curcuminoid content and expresses the result as curcumin percentage by mass. The method is based on pigment extraction with hot ethanol, controlled dilution, and absorbance measurement at the wavelength of maximum absorption (425 nm).
This ISO method is intended for laboratories performing routine quality control, specification testing and comparative assessment of turmeric and turmeric-containing products.
Key Topics
- Sample preparation: Sampling and grinding are performed in accordance with related ISO sampling and analysis guidance; a typical test portion is about 0.5 g of finely ground turmeric.
- Extraction: Pigments are extracted using 96% (v/v) ethanol under reflux. The standard describes a reflux period (the published method indicates 2.5 h) and quantitative transfer of the cooled extract to a volumetric flask.
- Dilution and measurement: A defined aliquot of the extract is further diluted and placed in matched 1 cm path-length silica cuvettes. Absorbance is measured at 425 nm using ethanol as the reference.
- Conversion to curcumin %: The absorbance result is converted to % curcumin by mass using the standard's calculation, which incorporates the dilution factor, test portion mass and a specified specific absorbance (E1% at 425 nm = 1607 for curcumin). The method also provides a moisture-correction option so results can be reported on a dry-matter basis.
- Equipment and reagents: Required items include a reflux condenser and 100 ml round-bottom flask, 5 ml pipette, 100 ml and 250 ml volumetric flasks, matched silica cuvettes (1 cm), analytical balance and a spectrophotometer capable of 425 nm measurements.
Applications
This method is primarily used for:
- Quality control in spice processing and supply chains to verify turmeric colouring strength.
- Specification compliance when setting purchase or grade criteria based on curcumin content.
- Comparative testing between cultivars, batches or suppliers.
- Laboratory validation and method transfer, as the ISO procedure provides detailed operational steps and reporting requirements.
Benefits include reproducibility across labs, a clear conversion to an industry-understood index (% curcumin), and a moisture-correction mechanism for consistent reporting.
Related Standards
- ISO 948 - Spices (sampling and general considerations noted by ISO 5566)
- ISO 2325 - Spices - Preparation of ground samples for analysis (sample preparation referenced by ISO 5566)
For testing laboratories and technical managers, ISO 5566:1982 provides a concise, replicable spectrophotometric protocol for assessing turmeric colouring power and expressing results in actionable terms for quality assurance and specification control.