Vegetable fibre: its physical properties

2XUT h e physical properties exerted by dietary fibre are individual to the age and aiiatomy of the fruit or vegetable eaten.The physical properties will be determined by the chemistry of the constituent polysaccharides and lignins, I n the plant these rnacromolecules arc systematically intermeshed for anatomical and physiological function.This intimate admixture is lost during digestion but the macerated polysaccharides and lignin will retain some of their functional characteristics.These characteristics are important in the function of non-ruminant colon and can be anticipated from the original physiological and anatomical role in the plant.Following cell division the primary cell wall consists of a watery pectin layer and cellulose.The secondary cell wall, which develops at a much later stage of growth, is formed by fibres running through a structureless matrix.Microfibrils of crystalline or partly crystalline cellulose are distributed with axes at different angles.T h e interspaces are filled with lignin, hemicelluloses and pectic substances held in a network (Rees, 1967). Po1,ysarrharidesCellulose is essentially a straight chain polymer and in the plant the polymers align side by side, stabilised by hydrogen bonds between hydroxyl groups forming strong intermolecular bonds.This has strength and insolubility and the way in which the chains are packed gives shape to the structure (Liang & Marchessault, Pectin is found in the primary cell walls and in intercellular layers.It is a polyuronic acid polymer, which tends to be branched in growing tissues and less branched when acting in a supporting role.It forins a gel, cross-linked to withstand osmotic rcpulsions.The association between neighbouring molecules is influenced by the proportion of methyl esters and presence of calcium which neutralises negative charges and influences hydrogen bonds (Could, Rees, Richardson & Steele, 1965).Pectic substances are hydrophilic and adhesive.When a fruit ripens the pectin changes from the insoluble material of the mature plant to a much morc watersoluble substance (Worth, 1967).Hemirelhloses are a mixture of linear and highly-branched polysaccharides containing various sugar residues, e.g.xylose, arabinose, glucose and mannose, and 496 of the residues are uronic acid.Hemicelluloses act as plasticizers.They are intertwined with lignin and laid down around cellulose fibres with a physical