Fuels and Chemicals from Hemicellulose Sugars - Biotechnology in China: Biofuels and Bioenergy - page 203

Fig. 2 Schematic representation of lignocellulosic biomass pretreatment (modified from [ 18 , 19 ]).
Pretreatment of lignocellulose by different methods removes hemicellulose and lignin from this
matrix of polymers in which cellulose exists before hydrolysis

Table 2 Methods for pretreatment of lignocellulosic biomass (adapted from [ 20 ])

Method Example

Thermo-mechanical Grinding, milling, shearing, extruder

Autohydrolysis Steam pressure, steam explosion, supercritical carbon dioxide

explosion

Acid treatment Dilute acid (H 2 SO 4 , HCl), concentrated acid (H 2 SO 4 , HCl)

Alkali treatment Sodium hydroxide, ammonia, alkaline hydrogen peroxide

Organic solvent
treatment

Methanol, ethanol, butanol, phenol

Apart from the chemical hydrolysis of hemicelluloses to monomer sugars, the
most promising method is using enzymes, i.e., hemicellulases. Moreover, hemicel-
lulases can facilitate cellulose hydrolysis by exposing the cellulose fibres, thus
making them more accessible [ 25 ]. Because hemicelluloses are heterogeneous, with
different constituents linked by different types of bonds, the enzymatic hydrolysis
requires several enzymes. Each enzyme attacks one or more types of bonds and all are
identified as hemicellulases (Fig. 3 )[ 18 , 26 ]. A wide range of microorganisms
produce different types of hemicellulases in response to the different types of
hemicellulose in their environments [ 25 ]. In general, microorganisms can be divided
into three groups according to their strategies for hydrolyzing hemicellulose [ 25 ]:
(1) complete hydrolysis to monosaccharides and disaccharides by several and syn-
ergic extracellular hemicellulases; (2) partial extracellular hydrolysis to oligosac-
charides further hydrolyzed by cell-associated or intracellular hemicellulases;
(3) hydrolysis by cellulosomes, extracellular cell-associated multienzyme com-
plexes, harbouring cellulases and hemicellulases. In combination, the hemicellulase
enzymes that work synergistically, and, in some cases, are organized in complexes,