SOP: Determination of Colour of Sugar
GIFSQ — Global Institute for Food Safety & Quality
Principle
The colouring matter in sugar is measured by its absorption at 420 nm in a spectrophotometer against a distilled-water reference of zero colour. Earlier procedures based on the difference between the spectral absorption curves of colouring matter and turbidity at two wavelengths were not entirely successful; the ICUMSA and the ISI procedures, which differ slightly in procedural details, both involve measurement of colour at 420 nm.
Apparatus required
- Spectrophotometer
- Absorption cells (cell length chosen to give a reading between 10 and 90% transmittancy)
- Sintered-glass funnel of porosity F (pores of 0.45 µm) with suction (ICUMSA method)
- Filter paper (for dark solutions, ICUMSA method)
- Centrifuge (ISI method)
- 50-ml volumetric flask (ISI method)
Reagents
a. Distilled water (reference standard of zero colour).
b. Filter aid such as Super-cel, Celite or Kieselguhr at 1% on solids content (for dark solutions, ICUMSA method).
Procedure — ICUMSA method
Prepare a solution of white sugar of 50% solids (for darker sugar, prepare a solution of as high a concentration as possible, consistent with a reasonable filtration rate and cell depths)
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Filter the white sugar solution and light-coloured liquors through a sintered-glass funnel of porosity F (pores of 0.45 µm) under suction, without adding filter aid; for dark solutions, add a filter aid (such as Super-cel, Celite or Kieselguhr) at 1% on solids content and filter under suction using a filter paper; discard the first portion of the filtrate if cloudy
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Measure the colour at 420 nm in a spectrophotometer using distilled water as the reference standard of zero colour, with a cell length that gives a reading between 10 and 90% transmittancy
Calculation — ICUMSA method
Colour = (a*)₄₂₀ × 1000
Where,
- (a*)₄₂₀ = attenuation index at 420 nm = A_b / (b × c)
- A_b = attenuancy = −log T_s
- T_s = transmittancy
- b = cell length in cm
- c = concentration of total solids as g/ml
Note: the printed forms (“a* = Ab C8 —log Ts”, “(ae)420 5 ie”) are garbled in the extraction; rendered here as the standard relationships the passage defines in words.
Procedure — ISI method
Weigh accurately 20 g of the sample, dissolve in water and make up to 50 ml in a volumetric flask
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Centrifuge the solution at 1000–1200 rpm for 15 min
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Measure the absorbance of the supernatant at 420 nm in a spectrophotometer using distilled water as blank
Calculation — ISI method
Extinction coefficient = (absorbance at 420 nm) / (concentration of sugar in g/ml × length of the absorption column in cm)
Where,
- Absorbance at 420 nm = measured absorbance of the supernatant
- Concentration of sugar = g/ml in the prepared solution
- Length of the absorption column = cell length in cm
Note: the printed layout (“Extinction _ Absorbence at 420 nm / Concentration of sugar in g/ml × Length of the absorption column in cm”) is ambiguous in the extraction; rendered here as absorbance divided by the product of concentration and path length, the relationship the passage defines.
Precautions
- Discard the first portion of the filtrate if it is cloudy.
- Choose a cell length that gives a reading between 10 and 90% transmittancy.
References
Ranganna, S., Handbook of Analysis and Quality Control for Fruit and Vegetable Products, 2nd edition.
DeWhalley, H.C.S., ICUMSA Methods of Sugar Analysis, Elsevier Publishing Co., New York, 1964 — cited in the book.
Specification for Sugar used in Food Preservation Industry IS: 1679-1960, Indian Standards Institution, New Delhi — cited in the book.