Methodology for Carbohydrates

Estimation of crude fiber


Crude fiber consists largely of cellulose and lignin (97%) plus some mineral matter. It represents only 60% to 80% of the cellulose and 4% to 6% of the lignin. The crude fiber content is commonly used as a measure of the nutritive value of poultry and livestock feeds and also in the analysis of various foods and food products to detect adulteration, quality and quantity.





Principle

During the acid and subsequent alkali treatment, oxidative hydrolytic degradation of the native cellulose and considerable degradation of lignin occur. The residue obtained after final filteration is weighed, incinerated, cooled and weighed again. The loss in weight gives the crude fiber content.





Materials

» Sulphuric acid solution (0.255 ±0.005N) : 1.25g concentrated sulphuric acid diluted to 100mL (concentration must be checked by titration)
» Sodium hydroxide solution (0.313 ±0.005N0 : 1.25g sodium hydroxide in 100mL disyilled water (concentration must be checed by titration with standard acid)


Procedure
1.
Extract 2g of ground material with ether or petroleum ether to remove fat (Initial boiling temperature 35 -38°C and final temperature 52°C). if fat content is below 1%, extraction may be omitted.
2.
After extraction with ether boil 2g of dried material with 200mL of sulphuric acid for 30min with bumping chips.
3.
Filter through muslin and wash with boiling water until washing are no longer acidic.
4.
Boil with 200mL of sodium hydroxide solution for 30min.
5.
Filter through muslin cloth again and wash with 25mL of boiling 1.25% H2SO4, three 50mL portions of water and 25mL alcohol.
6.
Remove the residue and transfer to ashing dish (preweighed dish W1).
7.
Dry the residue for 2h at 130 ±2°C. Cool the dish in a desiccator and weigh (W2).
8.
Ignite for 30min at 600 ±15°C.
9.
Cool in a desiccator and reweigh (W3).




Calculation

% crude fiber in ground sample =
Loss in weight on ignition (W2 - W1) – (W3 - W1)
x 100
Weight of the sample




Reading

1. Maynard, A J (Ed) (1970) Methods in Food Analysis Academic Press New York p 176.