Polysaccharides, alkaline degradation hemicellulose

Chemical pulps are prepared by the digestion of wood chips with chemicals at high temperature (170-180°C) until much of the lignin has been removed. There is a concomitant loss of hemicelluloses, because of polysaccharide degradation due to chemical attack. Therefore yields are low, from 45-50%. Chemical pulping falls into two main classes, those based on pulping with sulphite liquors at various pH levels, and alkaline pulping of which the kraft process is by far the most important. 

G. has two hydroxy groups in the cis-position and therefore forms gels with borax in aqueous, alkaline solution. It liquifies again when pH is dropped below 7. Similar gels are obtained by addition of polyvalent cations (e. g., Ca". Aqueous solutions of g. are degraded rapidly by acids and enzymes. Stability in alkaline media is rather good. Chemically, g. is a polysaccharide of the galacto-mannan type - hemicelluloses. 

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