Hemicelluloses
Hydrolyses of hemicelluloses yield mixtures of glucose, glucuronic acid, xylose, arabinose, galactose, galacturonic acid, mannose, and rhamnose. Some common polymers of pentoses, also known as pentosans, are xylan, galactan, araban, and others. Pentosans are found in large amounts (20–40%) in cereal straws and in brans. Large-scale industrial preparations of furfural, for instance, are based on these materials. Xylan, one of the better-known hemicelluloses, is a component of plant cell membranes. This pentosan occurs in association with cellulose. The structure of xylan was shown to be1,4-polyxylose :
Another hemicellulose, Galactan, is a minor component of some coniferous and deciduous woods. Larch wood was shown to contain about 8% of this polymer [4]. Araban, or polyarabinose is found in plant saps. All pectins also belong to the family of hemicelluloses. These are gelatinizing substances that are found in many plants, particularly in fruit juices. Crude pectins contain pentosans, galactosans, and similar materials. Purified pectins yield on hydrolysis galacturonic acid and methanol. These high molecular weight polymers are believed to consists to a good extent of poly (galacturonic acid), partially esterified with methyl alcohol. In addition, the polymers contain galactose and arabinose molecules. The polymer is probably linear [5–10] with a 1,4-glucosidal linkage between monomers. The relative amount of various components depends upon the source of the pectin. Citrus pectin, for instance, is rich in galacturonic acid but poor in galactose and arabinose. Plant gums and mucilages are high molecular weight polysaccharides composed of hexoses and pentoses. They also contain some uronic acid units. Among the gums there is gum arabic, gum tragacanth, and many others.
