Q. Determine the crude fibre content in the given of flour?

To determine the crude fibre content in the given sample of flour.

After undertaking this activity, you will be able to:

• assess the crude fibre content the given sample of flour,

• check the given sample for conformance to the standard for crude fibre content, and

• determine the indigestible residue for dry samples.

Principle

Crude fibre is the organic residue which remains after the food sample has been treated with boiling dilute sulphuric acid, boiling dilute sodium hydroxide solution and alcohol. The crude fibre consists of cellulose together with a little lignin. Crude fibre estimation is of great value in judging the quality of wheat products, particularly flours. High crude fibre foods are low in nutritional value. It also reflects the efficiency of milling and separation of bran from the starchy endosperm. Further, crude fibre is a more direct index of flour purity than ash or colour.

For determining crude fibre, the material is passed through the soxhlet's extractor for dissolving the fat so that the defatted residue is more receptive to the alkali or acid treatment.

On refluxing with 1.25% sulphuric acid (H2SO4), the fat present within the cell walls gets dislodged. The starches are hydrolyzed to soluble saccharides. The soluble minerals are washed out and some of the proteins are hydrolyzed to soluble forms. Treatment with 1.25% sodium hydroxide (NaOH) results in the hydrolysis of remaining proteins. This treatment removes tannins and fats. The fats form soaps and are washed out during hot water washings. The unreacted alkali and the sodium soaps must be completely washed off otherwise, after washing, free alkali gets converted to its oxide leaving black residue even after ignition in the muffle furnace.

Materials Required

Collect the following material for conducting the activity:

• Sample of flour

• Flat-bottom dish - of stainless steel, porcelain, silica or platinum.

• Muffle Furnace-maintained at 550 ± 10°C.

• Desiccator

• Weighing balance

• 1.25% H₂SO₄

• 1.25 % NaOH

• Reflux condenser

• Funnel

• Filter paper

Procedure

Now carry out the practical step-by-step as enumerated herewith:

1) Weigh accurately about 3-5 g of the sample in flask of the reflux condenser.

2) Add 200 ml boiling 1.25% H₂SO₄ solution in the flask.

3) Reflux* for at least half an hour.

*Refluxing means to avoid the loss of volatile components during the process of heating  on low flame. This is made possible by use of reflux condenser. However, if the condenser is not available, then a conical flask with cotton plug may be used. It should be noted that the latter procedure might result in some losses and thus cannot be regarded as the standard procedure.

4)  Remove from the flame and add 50 ml of cold water to the solution and filter it through the filter paper.  

5)  Wash the residue on the filter paper with boiling water until the washings are no longer acid to litmus (acid turns red litmus to blue).

6)  Return the residue to the condenser/conical flask.

7)  Add 200 ml boiling 1.25% NaOH solution.

8)  Reflux for at least half an hour.

9)  Remove from the flame and add 50 ml of cold water to the solution and filter it through the filter paper.

10)  Wash the residue on the filter paper with boiling water until the washings are no longer alkaline to litmus (alkaline medium turns blue litmus to red).