Biopolymers hemicelluloses

In the past, research activities in the field of hemicellulose were aimed mainly at utilizing plant biomass by conversion into sugars, chemicals, fuel and as sources of heat energy. However, hemicelluloses, due to their structural varieties and diversity are also attractive as biopolymers, which can be utilized in their native or modified forms in various areas, including food and non-food applications. 

Xylan-type polysaccharides are the main hemicellulose components of secondary cell walls constituting about 20-30% of the biomass of dicotyl plants (hardwoods and herbaceous plants). In some tissues of monocotyl plants (grasses and cereals) xylans occur up to 50% [6j. Xylans are thus available in huge and replenishable amoimts as by-products from forestry, the agriculture, wood, and pulp and paper industries. Nowadays, xylans of some seaweed represent a novel biopolymer resource [4j. The diversity and complexity of xylans suggest that many useful by-products can be potentially produced and, therefore, these polysaccharides are considered as possible biopolymer raw materials for various exploitations. As a renewable resource, xylans are... 

Wood is a composite material that is made, up basically of a mixture of three main constituents, cellulose, hemicellulose, and lignin (see Textbox 54), all of them biopolymers synthesized by the plants, which differ from one another in composition and structure (see Textbox 58). The physical properties of any type of wood are determined by the nature of the tree in which the wood grows, as well as on the environmental conditions in which the tree grows. Some of the properties, such as the density of wood from different types of trees, are extremely variable, as can be appreciated from the values listed in Table 71. No distinctions as to the nature of a wood, whether it is a hardwood or a softwood, for example, can be drawn from the value of its specific gravity. 

Pyrolysis of biomass is defined as the chemical degradation of the biopolymers (cellulose, lignin and hemicellulose) constituting the wood fuel which initially requires heat. As can be seen in Figure 51, all reaction pathways making up the pyrolysis are not endothermic, which implies that some of the pyrolysis reactions generate heat. However, overall the pyrolysis process is endothermic. 

Complex pyrolysis chemistry takes place in the conversion system of any conventional solid-fuel combustion system. The pyrolytic properties of biomass are controlled by the chemical composition of its major components, namely cellulose, hemicellulose, and lignin. Pyrolysis of these biopolymers proceeds through a series of complex, concurrent and consecutive reactions and provides a variety of products which can be divided into char, volatile (non-condensible) organic compounds (VOC), condensible organic compounds (tar), and permanent gases (water vapour, nitrogen oxides, carbon dioxide). The pyrolysis products should finally be completely oxidised in the combustion system (Figure 14). Emission problems arise as a consequence of bad control over the combustion system. 

Plants are wonderful chemical reactors that fabricate complex macromolecules. These compounds are located in the cell wall (e.g. cellulose, lignin, hemicelluloses and pectin) or they constitute the energy stocks (e.g. starch) and even they have specific functions (e.g. proteins). Most of these biopolymers are useful for making industrial biomaterials. 

The characteristics of the isolated biopolymers depend on their structure. Cellulose and amylose are linear polymers, whereas amylopectin, pectin and hemicelluloses are branched polymers. Pectin and amylopectin contain carboxylic groups, which make interactions with water molecules very important. Amylose has a helix structure, whereas the cellulose molecule looks like a ribbon. The interactions with water and other neighbouring molecules are therefore different. 

Lignocellulose denotes the mixture of the carbohydrate biopolymers cellulose and hemicellulose with the aromatic polymer lignin that is found in plants. Wooden raw materials consist mainly of cellulose (30-50 wt%), hemicellulose (10-40 wt%), and lignin (15-30 wt%). As the structure of cellulose (C6 carbohydrates) and hemicellulose (C5 carbohydrates) is quite similar, they will be discussed together in Section 2.2.2.1.1, followed by lignin, which has a very different composition (Section 2.2.2.1.2). 

Lignin is a high-energy content biopolymer rich in phenolic components. It provides structural integrity to plants. The combination of hemicellulose and lignin provide a protective sheath around the cellulose and this sheath must be modified or removed before efficient hydrolysis of cellulose can occur. 

Pyrolysis in inert atmosphere between 400 and yOO C produces water vapour, CO2, combustible gases CO, H2, CH and a multitude of organic vapours from the biopolymers cellulose (C6(H 0)s), hemicellulose (Cj(H20)4) and lignin. An impression of the complex product spectrum especially of the condensable organic vapours is given in Fig. 6. The remainder is a black char, mainly consisting of carbon and inorganic ash oxides. 

The wastes were milled and screen-sieved. Wastes composition was determined in terms of the major constituent biopolymers, holocellulose (cellulose + hemicellulose) and lignin, and extractive components that are soluble in ethanol-benzene. Lignin and extractives were isolated according to TAPPI standard methods. Holocellulose was obtained following the procedure described elsewhere (9). 

Tobacco is a complex plant material containing small organic and inorganic molecules and biopolymers. The biopolymers consist of cellulose, hemicellulose, pectin, lignin, proteins and peptides, nucleic acids, etc. [43]. Tobacco leaf and stem composition for flue-cured and burley tobacco [44] is summarized in Table 16.2.2. 

Despite the variety of sources, all lignocellulosic material is composed primarily of cellulose, hemicellulose and lignin [22], Agricultural wastes such as bagasse, com stover and wheat straw are thus a relatively cheap source of these three biopolymers. The major challenge to using lignocellulosic biomass as a feedstock is the development of cost-effective methods to separate, refine and transform it into chemicals and fuels [20],... 

Abstract Cellulose is the most important biopolymer in Nature and is used in preparation of new compounds. Molecular structure of cellulose is a repeating unit of p-D-glucopyranose molecules forming a linear chain that can have a crystallographic or an amorphous form. Cellulose is insoluble in water, but can dissolve in ionic liquids. Hemicelluloses are the second most abundant polysaccharides in Nature, in which xylan is one of the major constituents of this polymer. There are several sources of cellulose and hemicelluloses, but the most important source is wood. Typical chemical modifications are esterifications and etherifications of hydroxyl groups. TEMPO-mediated oxidation is a good method to promote oxidation of primary hydroxyl groups to aldehyde and carboxylic acids, selectively. Modified cellulose can be used in the pharmaceutical industry as a metal adsorbent. It is used in the preparation of cellulosic fibers and biocomposites such as nanofibrils and as biofuels. 

Cellulosic fiber reinforced polymeric composites find applications in many fields ranging from the construction industry to the automotive industry. The reinforcing efficiency of natural fiber is related to the namre of cellulose and its crystallinity. The main components of natural fibers are cellulose (a-cellulose), hemicelluloses, lignin, pectins, and waxes. For example, biopolymers or synthetic polymers reinforced with natural or biofibers (termed biocomposites) are a viable alternative to glass fiber composites. The term biocomposite is now being applied to a staggering range of materials derived wholly or in part from renewable biomass resources [23]. 

The biopolymers in decreasing order of stability are lignin, pectin, a-cellulose, and hemicellulose. Hedges et al. (4) suggested that the inferred order of polysaccharide preservation may be related to the ultrastructure of the wood cells and not to intrinsic chemical stability. 

To summarize, the analytical information of archaeological or ancient wood shows variability in chemical changes and losses that may result from the burial environment, the wood species, sapwood or heartwood, outer or inner wood, anomalies in growth, and certainly the methods of analysis. The stability of wood biopolymers has been found to be (in decreasing order) lignin, pectin, cellulose, and hemicellulose. As a rule, increase in moisture content indicates increase in degradation. 

Lignin makes up to 15-35% of fresh wood, with 60-80% of the lignin located in the secondary wall. The middle lamella-primary wall complex has the higher concentration (0.6-0.9 g/g), as compared to the secondary wall (0.2-0.3 g/g). In the cell wall, lignin, hemicellulose, and pectin fill the interstices between the cellulose microfibrils. Lignin may be bound to hemi-celluloses, the most unstable of the biopolymers in wood, and thus hemicellulose loss would expose the lignin to chemical changes. 

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