Hemicelluloses specific types

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. 

Dietary fiber has been suggested to play a protective role against chemically-induced toxicity (1) and against colon cancer (2). However, the mechanism(s) by which dietary fiber modulates chemical toxicity or colon cancer has not been well studied. The fiber fraction of the diet is resistant to mammalian digestive enzymes and consequently dietary fiber is not absorbed from the small intestine (3). However, certain types of dietary fiber specifically fermentable fibers, including the pectic substances and hemicelluloses, are readily digested by the intestinal microflora (4,5). Pectic... 

The most commercially feasible hydrolysis method is the initial hydrolysis at 140°C for 30 minutes. This method hydrolyzes most of the starch and 72% of the hemicellulose. This leaves the cellulose and a portion of the hemicellulose to act as a carrier for the corn steep liquor and stillage to make corn gluten feed. The additional acid hydrolysis step creates a large amount of degradation products, which would be inhibitory to the ethanol fermentation, without giving a greater monosaccharide concentration. The enzyme hydrolysis step is not feasible without enzymes that contain activities specifically for the corn fiber hemicellulose matrix. These types of enzymes are not commercially available, or would be prohibitively expensive, therefore, enzyme hydrolysis of corn fiber is not currently commercially feasible. 

It can be concluded that kraft pulping in particular offers attractive possibilities to manufacture alternative bioproducts in addition to the traditional market of pulp and extractives (turpentine and tall oil). However, the specific utilization of various hemicellulose- and lignin-derived fractions for chemical and fuel purposes is complex, and their manufacturing depends on several factors, such as the type of feedstock, available conversion technologies, production scale, energy and pulp prices, as well as pollution problems. For example, besides a straightforward combustion of... 

In contrast to glycanases, most esterases are known to be rather unspecific enzymes. Therefore some esterases are able to act on several different hemicelluloses, as shown in Table V, whereas others are more specific and act only on one type of hemicellulose (ii). Thus, one esterase may be useful in the treatment of several different hemicelluloses. 

The degradation of hemicelluloses also proceeds via endo- and exohydrolases. The substrate specificity depends on the monosaccharide building blocks and on the type of binding, e. g., endo-1,4-P-D-xylanase, endo-l,5-a-L-arabinase. These enzymes occur in plants and microorganisms, frequently together with cellulases. 

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