Crude Fiber Determination: Principle and Method

The human body cannot absorb fiber, yet research shows that a certain daily intake of fiber is still beneficial — which is why food production sets standards for the fiber content of food. Crude fiber is concentrated in cereal bran, the skins of fruits and vegetables, and fibrous tissue. It is not a precise chemical entity: crude fiber as determined by the classical method is mainly cellulose, together with small amounts of hemicellulose and lignin.

Dietary fiber refers to the fibrous substances in food that cannot be digested and utilized, most of them from plant material in the diet. They belong to the polysaccharides among carbohydrates — cellulose, dextran, hemicellulose, pectin, gums and others. They are digested and absorbed only to a very small extent in the body, but they promote intestinal motility; pectin in particular absorbs water and swells, increasing stool volume and aiding excretion. They are therefore a necessary component of the diet for human physiological activity.

Crude-fiber foods are those containing more than 2 g of crude fiber per 100 g of food. The main consumers of crude-fiber foods are adults and the elderly, and intake should follow the nutritional recommendation of 20–35 g per person per day — eating more actually reduces the utilization of other nutrients.

The determination principle

Among the methods for determining crude fiber, the fiber determinator is one of the simpler ones, and it delivers high determination accuracy. Because fiber is insoluble in water and must be separated by dissolving the surrounding organic matter, the operating principles applied are consistent. The principle of crude fiber determination is as follows:

1. Hot dilute-acid treatment — removes starch, pectic substances and part of the cellulose from the sample.
2. Hot sodium-hydroxide treatment — removes protein, part of the hemicellulose and part of the lignin, and saponifies fat for removal.
3. Ethanol or diethyl-ether washing — removes tannins, pigments, residual fat, waxes, part of the protein and pentoses.
4. Ashing — removes the ash (metal oxides).

The fiber determinator operates on exactly this principle, and its test results conform to the national standards GB/T 5515 and GB/T 6434. Compared with traditional measurement methods, the fiber determinator needs much less time per measurement while delivering comparatively accurate results. Sonnen's F22 is the fully automated fiber determinator in this line.