In order to obtain crude fibre from cabbage, certain procedures must be strictly followed. For a sample of finely grated cabbage (12g), 200ml of boiling sulphuric acid will be added. An anti-foaming agent is added if necessary. This will continue to boil for 30 minutes and filtered with the use of a Buchner funnel and a filter cloth. Boiling water is then used to wash the organic matter. This is then placed in the flask and 200ml of boiling sodium hydroxide added to it and is allowed to boil for 30 minutes as seen with the acid above. This is then filtered and the insoluble matter washed with boiling water, hydrochloric acid and boiling water again. A nitric acid test solution is done and then the matter washed with alcohol. The matter is then dried at 100°C and placed in a muffle furnace for 2 hours. This is then allowed to cool in a desiccator and the crude fibre content determined by the difference between the weight of the ash obtained and the increase of weight on the crucible due to the insoluble material.
In order to achieve the fibre from the organic matter, several other extractions have to be done. These steps as seen in the procedure were to make sure that at the end of the experiment, only the crude fibre will be obtained. The addition of the boiling sulphuric acid to the cabbage begins the process of acid digestion. This form of digestion is used to remove the carbohydrates that are present in the cabbage. An anti-foaming may be used if effervescence is seen in the solution with the sulphuric acid and the cabbage. Note that foaming is the dispersion of gas caused by the presence of fats. In the event that this occurs, solvents such as petroleum or ether may be used to be rid of the fats present. The subsequent rinse with boiling water after filtration is to ensure that all the acid is removed from the organic matter. The addition of boiling sodium hydroxide begins the process of alkali digestion. In this digestion, proteins and some carbohydrates are removed. The process also removes some of the hemicelluloses and lignin. As with the acid digestion boiling water is used to wash the cabbage after filtration.
However, hydrochloric acid is now introduced in order to get rid of all sodium hydroxide solution that may still be present in the organic matter. It must be noted that the reaction between sodium hydroxide and hydrochloric acid is a neutralization reaction. The additional wash with boiling water is to ensure the hydrochloric acid is totally removed from the cabbage. In order however, to be sure if all acid was removed, the nitric acid test solution is used to identify the chloride ion. If present, a white precipitate will be seen. To get rid of the precipitate, alcohol is used to wash the organic matter. After the drying period, the cabbage is placed in a muffle furnace where it is burnt and the ash collected. The % crude fibre can be obtained by the following equation:
(weight of crucible + dried residue) - (weight of crucible + ashed residue) x 100
Weight of sample
Crude fibre can be defined as the insoluble residue that remains after a defatted plant food stuff has been extracted first with boiling dilute acid and then with boiling dilute alkali, treatment that solubilizes almost all but the cellulose and lignin (Coultate, T.P. , 1989). Crude fibre as mentioned above is mainly obtained from plant material and would consist of polysacharrides such as cellulose which is the major component and minor components such as hemicelluloses and lignin. These microfibrils are found in the cell wall of plants. The human body is able to hydrolyze the α glycosidic bonds in starch but not the β glycosidic bonds in cellulose. Human cannot digest fiber because they do not have the enzymes to break the bonds in the fibre molecules.
In the extraction of crude fibre from any plant material, a series of processes must be carried out to get the cellulose remains. These processes include treatment of the plant material with sulphuric acid, sodium hydroxide and petroleum or ether if necessary. The use of sulphuric acid and sodium hydroxide are for acid and alkali digestion and the petroleum and ether, antifoaming agents. The end result after the extraction process yields a mixture of cellulose, hemicelluloses and lignin. As mentioned above, cellulose is the major component in crude fiber because it cannot be digested. Hemicelluloses and lignin however can be digested to certain extents. Hemicellulose consists of sugar monomers which can be easily hydrolyzed and lignin has hydrophilic properties. These properties largely explain why they are in so little amount in crude fibre.