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Vegetable fibres contain various impurities in different quatities. A few important impurities are discussed below
Impurities in Vegetable Fibres
Hemicellulose
Hemicellulose is a substance with the general properties of carbohydrates and is a mixture of a variety of compounds, the proportions of which vary according to the source of origin. It is soluble in 18% caustic soda solution and this suggests that it may probably have a much lower degree of polymerisation than cellulose.
Mectic Acid and Pectin
Pectic acid occurs in vegetables as calcium/mahnesium salt or pectin (i.e. methyl pectate) They are abundant in many fruits as apples, pears and sugar beet. Pectic acid is a long chain polymer similar to polysaccharide having one carboxyl group for every sixth carbon atom. It consists of a polygalacturonic acid backbone, which may be partially methylated and often han rhamnose, arabinose, galactose and xylose and other sugar bonded as side chains. It is insoluble in water but soluble in alkaline solutions.
Lignin
This substance is absent in cotton but occurs in considerable amounts in bast fibres and is responsible for their yellowing. It is associated with woody tissues in plants as cementing materials. The constitution is not well established , its low hydrogen content in relation to carbon suggests that it may be composed of aromatic nuclei 3,3 dihydroxyphenylpropane is a degradation derivative and is probably one of its basic structural units. It is soluble in sodium hypochlorite or sodium chlorite solution.
Fats and Waxes
Th chemical nature of fats and waxes has never been investigated exhaustively, because the actual amounts present in the fibres are very small. Waxes are the product of monohydric alcohols, such as ceryl alcohol, gossipyl alcohol, montanyl alcohol etc. While the oils can be made water-soluble by saponification, waxes are not saponifiable but can be removed by emulsification with soap at high temperature.
Nitrogenous Compounds
These impurities are degradation products of protoplasm, which are contained in the cell when it was still living. They are basically protein and polypeptides. Though they are present in small amounts, they can produce undesirable effect in finished materials. They are readily soluble in boiling alkali.
Mineral Matters
The quantity and composition of water-soluble mineral matters vary according to the nature of soil on which cotton is cultivated. Silicon is always present. The metallic salts commonly present are of iron, aluminium, calcium and magnesium. All these salts are convertes into respective carbonates when burnt.
Natural Colouring Matters
The yellow or brown colour of cotton still remains even after scouring. These natural colouring matters can be effectively destroyed or made colourless by oxidising bleaching agents. They are present in traces and are probably related to the flavone pigments of cotton flowers.
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