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Some physico-chemical characteristics of dietary fiber Lapsley, Karen Gail


Particle size distribution, chemical composition, water binding and water holding capacities, microstructure, and Theological properties of dietary fiber residues of American Association of Cereal Chemists certified food grade wheat bran, apple mesocarp and pomace, cabbage and carrot (edible portion) were determined. The semiauto-mated Tecator FiberTec system and the extraction techniques of D.A.T. Southgate and of P.J. Van Soest were used for digestion and collection of dietary fiber residues. The ability of various fiber residues, of different size ranges, to interact with water was assessed by two methods. Water holding capacity was estimated as the amount of water remaining within a hydrated sample subjected to compaction by a standard centrifugation technique. This method closely resembles conditions in the body, but is an inaccurate technique, since supernatant water is not easily separated from the sample. A new method, called filtrafugation, was developed to obtain information on water binding which Mas defined as the amount of water adhering to the fiber particle when water is allowed to drain from the sample during centrifugation. The amounts of water remaining with the fiber in centrifugation and filtrafugation tests differed significantly for each fiber source, the difference being the water held within the fiber matrix interstices. Although the fruit and vegetable fiber sources had greater water binding and holding capacities on a dried weight basis, wheat bran held and bound more water on a fresh weight basis because of higher fiber content and percent dry matter. Scanning electron microscopy of the different fiber sources and their fiber residues confirmed the progressively erosive action of the solvent extraction treatments and elucidated subsequent differences in water binding and holding capacities. Since bran is a senescent and lignified plant tissue, in vitro digestion changed the particle appearance, but the structural matrix, and the water binding and holding capacities were maintained throughout. Micrographs of the fruit and vegetable fiber sources and the fiber residues confirmed that the initial structures were able to bind and hold large amounts of water but with digestion the delicate structures were totally disrupted, thus providing a weak compact matrix characterized by low water binding and holding capacities. Dispersions of the fiber residues were evaluated viscometrically at human body temperature in order to elucidate their behavior in fluid systems. When aqueous dispersions were prepared at low concentrations the solid materials gradually settled except for the apple pomace dispersions. Steady shear flow behavior studies using 60% w/w sucrose solution as the continuous phase revealed large viscosity increases at higher concentrations of the dietary fiber residue.

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