Determination of crude fiber (CF) in food sample

Crude Fiber definition-

After fat extraction from a food sample, we get a fat-free sample. Now when we treat that fat-free sample with any acidic or alkaline media, some part of the fat-free sample is not soluble in them. that part is called Crude fiber.

Crude fiber mainly contains the indigestible lignin, cellulose, pentosans, etc. present in the food sample.

So, crude fiber is also called the Insoluble carbohydrate or the non-soluble carbohydrates.

If we eat these crude fibers, they are not digested by our digestive juices, so they do not get degraded by the hydrochloric acids that are secreted by our gastrointestinal mechanism.

EXAMPLE- If we add weak acid and weak base to foods like grains, legumes, seeds or okra(having roughage content in it), we will observe that their sugars and proteins are dissolved in the solution, but few components like cellulose, aligning and hemicellulose are not dissolved. These undissolved components are collectively called Crude Fiber.

Okra (Lady’s finger) has more roughage i.e. Crude fiber. Roughage is a fibrous and indigestible material helps in the passing of the food and other digested food via the digestive tract


Importance of Dietary Fiber estimation in foods-

1. Humans do not have Cellulase enzyme, so they cannot digest the crude fibers, for example, cellulose.

2. Sometimes intentional adulteration in foods is done. So, to confirm the composition and quality, estimating crude fiber can be very useful.

3. We can also guess the freshness of fruits and vegetables if we estimate the crude fiber. More mature and aged fruit or vegetable means more percentage of fiber in it.



The food should be fat-free, i.e. the food should be firstly passed via the soxhlet apparatus for fat estimation and removal.

Then that fat-free sample is ground well and passed through a 1mm sieve. Now we use that filtrate as the sample source.



1. Weigh 3g of the fat-free sample. (Sample amount may differ according to food products and institution manuals)

2. Now place the sample in a spoutless beaker. Spoutless beakers have no spout which provide easy flow to the liquids. See images here.

3. Add 100ml of 1.25% H2SO4 into the spoutless beaker.

4. Then boil the content for half an hour

5. Filter out the content

6. Now scrap the unfiltered content from the filter paper and put it into the same beaker.

7. Then add 100ml of 1.25% NaOH and boil for half an hour

8. Filter the content and scrap the unfiltered part again. Place the content in crucibles which are already oven-dried and weighed.

9. Place those crucibles at 130C for 3 hours and weigh again.

10. Now place these crucibles in a muffle furnace for 3 hours at 550C and weigh again.




So, we obtained the crude fiber content as 1.8866%.

We can also note that crude fibers are also greater than the dietary fibers which is written on the food packs.

Hope this article was useful.

These articles can also be useful-

Determine crude fat in a food sample by using Soxhlet Apparatus

Determine moisture content in a food sample by using Hot air oven


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