ISO 16624:2020 PDF

Overview - ISO 16624:2020 (wheat flour and durum wheat semolina colour)

ISO 16624:2020 specifies a laboratory method for the determination of colour in durum wheat semolina and wheat flour using diffuse reflectance colorimetry. The method applies to industrial semolina and flour and can also be used for flour from experimental mills. Colour is measured directly on granular samples by a reflectance colorimeter and reported in the CIELAB metric (L*, a*, b*), with emphasis on the yellow index (b*).

Key topics and technical requirements

• Measurement principle: Diffuse reflectance colorimetry on semolina/flour surfaces; pigments (xanthophylls, carotenoids) determine perceived colour.
• Instrument requirements:
  • Reflectance colorimeter with pulsed xenon lamp, geometry d/0° (diffuse illumination, 0° viewing).
  • Measurement in CIE 1976 Lab* space using illuminant D (≈ D65 daylight).
  • Measurement time ~1 s, circular measurement area, calibration capability and repeatability within ΔE* ≈ 0.6 on reference plate.
• Sample cell and size:
  • Granular sample cell: external Ø 60 mm, inner hole Ø 22 mm, thickness 9 mm.
  • Minimum homogeneous sample ≈ 100 g; measure each determination twice, mixing and emptying the cell between replicates.
• Calibration and procedure:
  • Calibrate before each series using an opaque reference plate (porcelain/glaze) and optionally a reference material close to sample colour.
  • Clean and store reference plate carefully; verify reference coordinates before use.
• Result expression:
  • Report L*, a*, b* values (two decimal places). Primary reporting often uses b* (yellow index); 100 − L* indicates darkening; a* indicates red/green shift.
• Precision & uncertainty:
  • Repeatability (r), reproducibility (R), critical differences and expanded uncertainty are defined based on interlaboratory testing and ISO 5725 methods. The document supplies statistical models and limits for typical L*, a*, b* ranges.

Applications and who uses it

• Quality control in milling and cereal processing: routine monitoring of flour/semolina colour for grading and product consistency.
• Product specification and supplier acceptance testing.
• Research and experimental mills: comparing processing effects on pigment retention and colour.
• Food testing laboratories, regulatory bodies and certification labs requiring standardized, traceable colour measurement.
• Instrument manufacturers and method developers validating colorimeters for cereal products.

Related standards

• ISO 24333 - recommended sampling methods for flour (referenced for sampling guidance).
• ISO 5725 series - methods for assessing repeatability, reproducibility and measurement uncertainty (used for precision statistics).

Keywords: ISO 16624:2020, wheat flour colour, durum wheat semolina, diffuse reflectance colorimetry, CIELAB, L* a* b*, yellow index, colorimeter, repeatability, reproducibility.