Proximate analysis: Crude fiber

Principle and Scope

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Two boiling processes simulate the pH conditions of the digestive tract, acidic in the stomach and alkaline in the small intestine, but nothing more. However, the enzymatic digestion is not simulated.


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  • Sulfuric acid (H2SO4) 1.25% - 0.255 ± 0.005 N. 12, 5g, 98% concentrated to 1000 ml with distilled water. Control the concentration by titration.
  • Potassium hydroxide (KOH) 1.25% - 0.223 ± 0.005 N, free from carbonate. 12.5 g to 1000 ml with distilled water. Control the concentration by titration.
  • n-octanol as antifoam.
  • Anhydrous acetone

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  1. Determine separately the sample moisture by heating in an oven at 105 °C to constant weight. Cool in a desiccator.
  2. Weight accurately 1 g about of grinded sample (1 mm about) approximately with 1 mg. ==> W1
  3. Add 1.25% sulfuric acid up to the 150 ml notch, after preheating by the hot plate in order to reduce the time required for boiling.
  4. Add 3-5 drops of n-octanol as antifoam agent.
  5. Boil 30 minutes exactly from the onset of boiling.
  6. Connect to vacuum for draining sulfuric acid.
  7. Wash three times with 30 ml (crucible filled up to the top) of hot deionized water, connecting each time to compressed air for stirring the content of crucible.
  8. After draining the last wash, add 150 ml of preheated potassium hydroxide (KOH) 1.25% and 3-5 drops of antifoam.
  9. Boil 30 minutes.
  10. Filter and wash as point 7.
  11. Perform a last washing with cold deionized water aimed to cool the crucibles and then wash three times the crucible content with 25 ml of acetone, stirring each time by compressed air.
  12. Remove the crucibles and determine the dry weight after drying in an oven at 105 °C for an hour or up to constant weight. Let cool in a desiccator. This weight (W2)represents the crude fiber plus ash content in comparison to initial weight.

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Calculate the percentage crude fiber (wet weight basis)as follows:

(W2 - W1)
% Crude fiber (wet) =
x 100
Possible errors

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This is the most unsatisfactory principle of the Proximate Analysis. Major problem:

  1. acid and base solubilize some of the true fiber (particularly hemicellulose, pectin and lignin).
  2. Cellulose too is partially lost. Hence, crude fiber underestimates true fiber.

The number or value obtained in this procedure, therefore, is practically meaningless. Most laboratories have phased out the CF term and replaced it with the Van Soest "Detergent Fiber" determination.