The hemicelluloses constitute the second polysaccharidic constituent of wood. They are present in higher proportions in hardwoods (25-35%) than softwoods (15-25%). In hardwoods, pentosans (mainly xylans) are the dominant polyoses, whilst most of the softwood hemicelluloses are hexosans (mainly mannans). The xylans of hardwoods are more easily extracted and hydrolysed than the softwood mannans. This property is exploited in the pre-hydrolysis process in wood saccharification, in dissolving pulp manufacture, and in steaming processes. The arabinogalactans of larch, which are easily water extractable, have found application in the production of emulsions and as tablet binders, with research into their therapeutic properties actively being pursued. At present, the main potential for hemicelluloses for the production of chemicals is based upon the constituent sugars. Sugar mixtures from hardwood hemicelluloses contain xylose as the main sugar, with mannose the main sugar from softwood hemicelluloses. Lesser amounts of glucose, galactose, arabinose, uronic and aldonic acids are also present in the hydrolysates as well as acetic acid. Liquors from the sulphite pulping process contain hemicellulosic sugars, which are used as substrates for fermentation processes, glucose producing ethanol, with other sugars used for yeast production. The yeast thus produced has a high vitamin and protein content and can be used for cattle feed. Xylose can be converted to xylitol by catalytic hydrogenation. Xylitol is a non-cariogenic sweetener, with comparable sweetness to glucose. Furfural can be obtained by the dehydrogenation of xylose. This process was first developed commercially by the Quaker Oats Company in 1922. Furfural is used as an industrial solvent, disinfectant or preservative, in solvent refining of petroleum oils, and as a reactive solvent in phenolic resin production. Furfural can be converted to furfuryl alcohol, which can be polymerised to yield furan resins, used to modify wood in the Kebony process. Trees produce a wide range of non-cellulose and non-starch polysaccharides, that are often referred to as gums. These occur within the wood cell wall (hemicelluloses and pectins); the seed endosperm (e.g. guar gum, locust bean gum); or as exudates (e.g. gum Arabic from Acacia Senegal, gum karaya from Sterculia spp., gum tragacanth from Astralagus spp.). Many of these are used in the food industry as thickeners and stabilisers. Several non-food applications exist; for example, the galactomannan guar gum (and it’s hydroxypropyl derivative) is used by the oil-drilling industry as an ameliorating agent in drilling muds and fluids. Such gums are also used in paper, mining, textile coating and sizing, printing ink, paint, adhesive, personal care, and pharmaceutical applications.
Contributor: Dr Callum Hill FIMMM