SOP: Measurement of Colour of Transparent Products by Transmittance
GIFSQ — Global Institute for Food Safety & Quality
Principle
The methods for measuring colour by reflectance locate the colour of any food in colour space by determining the three attributes of colour. For products which are transparent — like lime juice cordial and clarified apple, guava or grape juice — or for a product which can be made transparent by extraction with suitable solvents and filtration or centrifuging, colour may be assessed in terms of transmitted light. The method is applied chiefly to prepared extracts of natural pigments, which may be closely related to visual evaluation of colour. A coloured solution transmits its own colour and absorbs the complementary colour; for example, a yellow-coloured extract of mango transmits the yellow colour but absorbs the complementary blue colour. The intensity of the colour of the extract is a measure of the concentration of the pigment in the solution: with the increase in the concentration of the colour in the solution, the complementary colour is absorbed more completely. If the instrument is adjusted to 100% transmittance with the colourless solvent as reference, the per cent transmittance decreases with increase in the concentration of the pigment. The blank in transmittance measurements is usually the sample cuvette filled with pure solvent; in reflectance measurement the same relationship R % = 100 I/I₀ holds good except that white magnesium oxide, magnesium carbonate or vitrolite is used as the blank.
Apparatus required
- Blender
- Colorimeter or spectrophotometer with sample cuvettes
- Centrifuge or filtration assembly
Reagents
- Suitable solvent for extraction of water-insoluble colouring matter
- Pure solvent for the blank (the sample cuvette filled with pure solvent)
Sample preparation
Blend a known weight of a representative sample in a blender. If the plant material contains water-insoluble colouring matter, extract with a suitable solvent. Make up to a known volume and centrifuge or filter.
Procedure
Blend a known weight of a representative sample in a blender
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If the plant material contains water-insoluble colouring matter, extract with a suitable solvent
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Make up to a known volume and centrifuge or filter
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Check that the extract is clear, since turbidity interferes with transmittance measurements
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Adjust the instrument to 100% transmittance with the colourless solvent (pure solvent in the sample cuvette) as reference
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Record the results as per cent transmittance at the wavelength of maximum absorbance
Calculation
T % = 100 I / I₀
Per cent transmittance may also be expressed in terms of absorbance (optical density) A as:
A = log₁₀(I₀ / I)
Where,
- T % = per cent transmittance
- I = intensity of the light transmitted by the sample
- I₀ = intensity of the light transmitted by the blank (the sample cuvette filled with pure solvent)
- A = absorbance (optical density)
- R % = per cent reflectance = 100 I/I₀, using a white standard as the blank
Example — absorption spectrum of the carotenoid pigments of Badami mango: maximum absorption is at 443 nm, but another absorption peak is seen at 468–470 nm. At 500 nm, there is a sharp decrease in absorption, which means a sharp increase in transmittance; practically most of the light is transmitted at 500 nm.
Precautions
- The extract should be clear, as turbidity interferes with transmittance measurements.
- In fruits and vegetables containing natural pigments, colour may be determined by this method; the methods for extraction, separation and measurement given for the plant pigments may prove useful.
References
Ranganna, S., Handbook of Analysis and Quality Control for Fruit and Vegetable Products, 2nd edition.
Kramer, A., Nat. Canners Ass. Inform. Lett., 1666, 49 (1958) — cited in the book.
Pomerantz, R., S.A. Goldblith and B.J. Proctor, Food Technol. (Chicago), 9, 478 (1955) — cited in the book.