Lignin hemicellulose-cellulose complex

Typically, the raw material for cellulose processing is a lignin-hemicellulose-cellulose (LHC) complex that is not very amenable to hydrolysis. Various pretreatments involving size reduction, separation of constituents of the complex, and processes to increase the accessibility of cellulose to hydrolytic agents may be required. These activities convert a relatively intractable raw material into a cellulosic substrate. 

The raw material that reaches the factory is a far cry from Avicel or Solka Floe that are used by many experimenters to simulate cellulosic products (Table I). In addition to the fact that the cellulose is tied up in a lignin-hemicellulose-cellulose (LHC) complex, it should be recognized that the terms lignin and hemicellulose are catchall terms covering a range of compositions and molecular weights. 

Lignin is a complex phenolic cell wall polymer that is chemically cross-linked with hemicellulose and cell wall proteins. Most of the methods to determine lignin content are based on the removal of all other cell wall constituents, typically through acid hydrolysis, which will readily remove hemicellulose under mild conditions, and non-crystalline cellulose under more severe conditions. Several different methods will be discussed below. The different methods have also been extensively reviewed and compared by Hatfield et al. (1994), Brinkmann et al. (2002), Fukushima and Hatfield (2004), and Hatfield and Fukushima (2005). 

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 native structure of lignocellulose, which is very complex, renders it resistant to enzymatic hydrolysis. The major component is cellulose, a pi -> 4 polymer of glucose (see Fig. 8.2 a), which accounts for 35-50% of the mass. Lignocellulose also contains 20-35% hemicellulose, a complex polymer of pentoses and hexoses and 10-25% lignin [10, 12]. Techniques to convert lignocellulose into a fermentable sugar mixture have been under development for over 20 years and are now entering use. 

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. 

Purified plant fibers for dietary purposes are combinations of cellulose, lignin, hemicellulose, gums and pectinaceous substances that exert chemical and physical actions in the gastrointestinal tract. Dietary fiber complexes such as wheat bran contain not only the fiber components but also other organic compounds that contribute to nutrition. In comparison to wheat bran, the deproteinized and decolorized fiber complexes of tobacco and alfalfa have favorable chemical composition as dietary fibers. Their low concentrations in starch and lipids... 

Xylanases depolymerize hemicellulose and breaks the covalent bond between lignin and cellulose. The depolymerized hemicellulose and separated lignin may be removed exposing the celulosic fiber and increasing cellulose content. Cellulases are enzyme complexes including enzymes that attack the cellulose chains at random, that hydrolyze the cellulose chains from the end and that hydrolyze the cellulobiose into glucose [12]. Yilmaz [13] detected stronger effect of xylanases on fiber properties compared to that of cellulase enzymes. 

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