An outline of 'fiber' analysis techniques

A major distinction that must be made when discussing 'fiber' is that between 'crude fiber' and 'dietary fiber'. Crude fiber is simply an analytical term describing the inert residue which remains after laboratory analysis, normally an acid-alkali extraction. Dietary fiber, on the other hand, is usually considered to be any polymeric substance resistant to mammalian digestive enzymes. Crude fiber is composed primarily of lignin and cellulose, whereas dietary fiber also includes hemicelluloses, pectins, mucilages and gums (there is some botanical criticism that fiber must represent the plant cell wall and exclude plant storage compounds thus excluding gums and crystalline starches that reach the large colon). Values for crude fiber may be only one-fifth to one-seventh the total dietary fiber of a given food. Older food composition tables, based on crude fiber determinations, are incomplete and of limited value when prescribing high or low fiber diets. This is but one example illustrating the need for a full and complete understanding of the analytical procedures involved in 'fiber' determinations. The purpose of this paper is to review some of the more common procedures used in the analysis and fractionation of food and feed fiber.