Lignocellulosic biomass recalcitrance

Plant cell walls contain cellulose, which is synthesized in the plasma membrane and hemicelluloses, synthesized, and secreted from the Golgi apparatus (Ding and Himmel, 2006). The cellulose microfibril typically contains 36 hydrogen-bonded cellulose chains arranged parallel to one another (Somerville, 2006). However, recent studies revealed this to be a rough approximation, showing that the number of hydrogen-bonded cellulose chains may exceed 36 (Herth, 1983 Fernandes et al., 2011). In most plant cell walls, the synthesis of cellulosic microfibrils is enhanced 

FIGURE 3.3 (a) Interactions between cellulose microfibrils, hemicelluloses, pectin, and structural proteins in plant cells, with (b) showing formation of an amorphous region from untwisted cellulose microfibrils, when completely dried. The crystalline region becomes fragile, turns amorphous, and further degradation occurs. 

FIGURE 34 Arrangement of the monoclinic (thin lines) and triclinic (bold lines) structures in a single unit cell. The orientation of cellulose in the monochnic phase, and its bond angle and bond distance (Sugiyama et al., 1991 El Oudiani et al., 2011). 

Cellulose oligomers containing less than ten monomers are water-soluble (Atalla et al., 2008). It is expected that the cellulose chains are hydrated at the elementary nanofibril level and that cellulose processing at high temperature reduces the hydration. Thus, an increase in temperature changes the state of aggregation of native celluloses. Water aids in the relative motion of nanofibrils, while removal of water causes dryness and stimulates formation of hydrogen bonds between nanofibrils. This 

---

Due to the complex structure of lignocelluloses, the release of fermentable sugars is a hurdle for the industrialization of this renewable resource (Zhao et al., 2012a). The complex structure of plants functions as protection against microbial and enzymatic attacks and is called biomass recalcitrance (Himmel and Picataggio,... 

Taherzadeh MJ, Jeihanipour A. (2012). Recalcitrance of lignocellulosic biomass to anaerobic digestion. In Mudhoo A, editor. Biogas Production Pretreatment Methods in Anaerobic Digestion. Wiley Online Library, pp. 27-54. 

Zhao X, Zhang L, Liu D. (2012a). Biomass recalcitrance. Parti the chemical compositions and physical structures affecting the enzymatic hydrolysis of lignocellulose. Biofuels, Bioprvd Biorefin, 6, 465-482. 

Lignocellulose waste hydrolysis to fermentable sugars still brings many challenges, because the degradation of cellulose and hemicellulose are hampered by the structure and composition of lignocellulosic biomass (Himmel et al., 2007). Due to its structural recalcitrance, a chemical and physicochemical pretreatment is a required stage to obtain potentially fermentable... 

Separation of biomass components is the very important initial step for the upcoming efficient utilization of biomass sources. Several physical and chemical separation methods are currently employed to overcome the recalcitrance of lignocelluloses. These methods include acid hydrolysis, alkaline hydrolysis, ammonia fiber expansion, hot water, organic solvent, and ionic liquid separation technologies. In this section of the chapter, all of the above separation methods are discussed. 

---