The chemistry and estimation of fibre.

Crude fibre determination is commonly used to estimate the quality of foods of plant origin on the premise that it constitutes their least digestible fraction. Therefore, a criterion for evaluating fibre methods is thc recovery of indigestible plant residues. ‘I’he so-called Weende method for fibre estimation was not developed at Weende, but at Moglin after 1806 by Heinrich Einhof, who assumed that the fibrous residue left after sequential extraction with solvent, dilute aqueous acid and dilute alkali, represented the indigestible matter. Binhof calculated estimates of nutritive value of vegetables and forages on this basis (von Thaer, 1809). The deficiencies of Einhof’s method became apparent in the nineteenth century (Henneberg, 1859). Over the years many systems of analysis have been proposed for the replacement of crude fibre, but none have been successful in dislodging it as the official method. The crude fibre method as an estimate of the amount of fibre or plant cell wall in foods has many defects. On average, SooX of the hemicellulose or pentosans and from 50 to 90y0 of the lignin are removed by the acid and alkaline sequential extraction, while cellulose rccovery is j0-800/,. Thus, much of the hemicelliilose and lignin appears in the nitrogen-free-extract (NFE) to be counted as available carbohydrate (Maynard, 1940). The XFE of straws and grasses may contain as milch as 90‘;(, of these substances (Nordfeldt, Svanberg & Claesson, 1949 ; Stallcup, Davis & Ward, 1964; Kim, Gillingharn & T,oadholt, 1967). Because of the failure of the crude fibre method to recover indigestible substances, SI;E appears less digestible than crude fibre in significant numbers of cases (Cranipton & >‘laynard, 1938). In the case of vegetables and cereals the error is lcss because of the relatively lower content of hemicelluloses and lignin. IIowever, it may be substantial. There have been various attempts to replace the crude fibre method with a system of analysis which gives a better characterization of the less nutritive fraction of food. Such attempts face several problems, not all of them necessarily analytical. The problems are ( I ) conflicting concepts o f what constitutes fibre; ( 2 ) the definition of lignin, cellulose and hemicellulose ; (3) achieving separation of lignin from interfering matter; (4) the isolation of indigestible fibre and its relation to the true fibre of the food; (j) the failure of hemicellulose, cellulose and lignin to be biologically or chemically similar in different plant materials.