What Is Fiber?
What Is Fiber?
Dietary fibers come only from plants. They are the chemical substances in the cell walls that give plants structure and stability. Fibers in elude cellulose, polysaccharides, hemicelluloses, pectins, gums, mucilages, and lignin.
Different kinds of plants contain different fibers. Even within a species, the fiber content may vary according to growing conditions and maturity at harvest. Bran is almost entirely cellulose; apples, grapes, and some other fruits are high in pectin.
Fibers are not digested by human digestive enzymes. However, many are partially or completely digested by bacteria that reside in the gut, resulting in the production of gases.
Some fibers, such as cellulose, hemicellulose, and lignin, are not soluble in water, although most can absorb several times their weight in water. Insoluble fibers predominate in grains. Others, such as pectin (used to make jelly), gums, and mucilages, are water-soluble and can leach out into cooking water. They are found naturally in fruits and vegetables, beans, and whole oats and are used as additives in processed foods.
Old methods of analyzing for “crude fiber” greatly underestimate the actual amount of dietary fiber present in a food. Newer, more accurate – but still imperfect – methods of analysis are quite cumbersome and not yet widely used by the food industry, although a few researchers have used them to determine “total dietary fiber” for a number of fresh and processed foods. Crude fiber listings on food labels are considered useless by fiber experts; some food packages now also list “dietary fiber.”