Annual plants hemicelluloses

Aqueous alkaline extraction of wood was employed by Poumarede and Figuier48 in 1846 for the removal of a substance called wood gum. Similar very crude xylan or hemicellulose preparations were made by other workers of the early period. Extractions were made not only from wood sawdust, but from annual plant materials such as wheat straw, corn cob, etc.49-66... 

Unfortunately, pure xylan is an expensive carbon source for commercial-scale xylanase production. Therefore, several groups have tried to develop xylanase production on cheaper xylan-rich materials. The best candidates for the purpose appear to be water-soluble hemicellulose from steam-treated wood (63,69) and residues of annual plants like wheat bran (70). 

Hardwood xylans and xylans of annual plants may contain up to 7% O-bound acetyl groups. Seven out of ten xylose residues in native hardwood xylan are acetylated on C-2 and/or C-3 (10). Because of the possible migration of O-acetyl groups between 2- and 3-positions during and after isolation of hemicellulose components, it is difficult to determine their original distribution in nature (11). The ratios reported for 2-, 3-, and 2,3-positions of acetyl groups in birch xylan have been 2 4 1 (3) and 2 2 1 (10) and in bracatinga xylan 3 3 1 (12). 

Whistler, R. L., and Feather, M. S. (1965). Hemicellulose extraction from annual plants with alkaline solutions. Methods in Carbohydrate Chemistry, Vol. V, pp. 144-145. Academic Press, New York. 

In nature, hemicelluloses are found in the cell walls of woody and annual plants, together with cellulose and lignin.33 Hemicellulose is made up of a group of complex low-molecular-weight polysaccharides that are bound to the surface of cellulose microfibrils, but their structure prevents them from forming microfibrils by themselves. 

This chapter gives a general introduction to the book and describes briefly the context for which the editors established its contents and explains why certain topics were excluded from it. It covers the main raw materials based on vegetable resources, namely (i) wood and its main components cellulose, lignin, hemicelluloses, tannins, rosins and terpenes, as well as species-speciflc constituents, like natural rubber and suberin and (ii) annual plants as sources of starch, vegetable oils, hemicelluloses, mono and disaccharides and algae. Then, the main animal biomass constituents are briefly described, with particular emphasis on chitin, chitosan, proteins and cellulose whiskers from molluscs. Finally, bacterial polymers such as poly(hydroxyalkanoates) and bacterial cellulose are evoked. For each relevant renewable source, this survey alerts the reader to the corresponding chapter in the book. 

Animal biomass. Vegetal biomass. Wood, Cellulose, Lignins, Hemicelluloses, Natural rubber, Suberin, Tannins, Rosins, Terpenes, Annual plants. Starch, Vegetable oils, Hemicelluloses, Mono and disaccharides, Polylactic acid. Algae, Chitin, Chitosan, Proteins, Cellulose whiskers. Bacterial polymers. Poly (hydro xyalkanoates). Bacterial cellulose... 

The relevant contribution of the output of annual plants to the realm of polymer synthesis and applications stems, instead, from some specific products, namely starch as a polymer, vegetable oils as triglyceride oligomers and hemicelluloses and monosaccharides as potential monomers or precursors to furan derivatives. 

Annual plants produce a rich selection of hemicelluloses, often with quite different structures compared with those found in woods, although of course the basic chemical features are always those of polysaccharides. It follows that specific applications are associated with these different structural features, notably as food additives. The presence of charged monomer units is one of the most exploited characteristics, because of the ensuing rheological sensitivity to physical parameters. The properties and appbcations of plant and seaweed hemicelluloses are dealt with in Chapters 13 and 24, the latter dealing with the specific features related to polyelectrolytes. 

In annual plants, the main hemicelluloses are represented hy xylans (Fig. 13.3), which are more heterogeneous than the xylans from wood tissues [17]. They contain both glucuronic acid and/or its 4-(9-methyl-ether and arab-inose attached to C2 or C3 of the xylose units. Both xylan and glucomannans can be partly acetylated [18]. 

Fairly pure cellulose occurs in seed hairs (cotton) and the stalks or leaves of many plants (flax, hemp, China grass). Since only a mechanical separation is needed for commercial purposes, these sources were made use of thousands of years ago. More recently, cellulose has been obtained from both deciduous and coniferous trees and the stalks of annuals (plants) by nonmechanical separation processes. In these plants, about 40% cellulose is present together with noncellulose components in the wooden cell walls. The latter consist of about 70% lignin (cf. Section 32.3) and about 30% so-called hemicelluloses. Hemicelluloses are short-chain polysaccharides from nonglucose sugars (mannose, galactose, xylose, arabinose, uronic acids, etc.). 

Hemicelluloses from annual plants (cereals crops) are mainly of the xy-lan type mixed-linked j6-l,3-l,4 glucans are also found. The structure has been... 

Cotton, the most widely used natural fiber, is an annual plant. The cotton containing some linters (short fibers) swells out in thick white flocks when the ripe fruit shells burst open. The raw cotton is composed of cellulose (80—90%), waxes and fats (0.5—1.0%), proteins (0—1.5%), hemicelluloses and pectins (4—6%), ash (1—1.8%), and water (6—8%). The quality of cotton is graded by the basic properties, such as fiber length (10—50% mm), linear density (1.0—2.8% dtex), color, impurities (trash and dust), tenacity (2.5—5.0% cN/dtex), and breaking elongation (7—10%). 

Figure 4. Semi-industrial plant for steaming hardwoods (capacity 600 kg x h ) and annual plants, equipped with a counter-current extraction unit for hemicellulose recovery (FMTZ WERNER GmbH, Geisenheim/Germany), The horizontally operating reactor (background) is optionally fedfrom two pressured vessels in top or from an extruder, located parallel at the left side of the reactor. Steam is releasedfrom the blow tank in front of the extraction unit.

The MALDI-MS procedures described here, which were developed for wood and pulp hemicelluloses, should also be suitable for characterizing hemi-celluloses isolated from annual plant materials and such studies are currently being carried out in our laboratory. 

Hemicelluloses are among the world s most abundant biopolymers. They are heteropolysaccharides present in large quantities in wood and the majority of plant tissues. The total global production of lignocellulosic materials in forests is estimated at 140 billion tons per year. The assumption that about 25 % of the lignocellulosic materials are hemicelluloses gives an estimation of the annual hemicellulose biosynthesis of 35 billion metric tons. 

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