16.3.2.1 Liquid Hot Water (LHW)

Hot-water pretreatment changes the structure of lignocellulose and solubilizes the hemicellulose, thus keeping cellulose amenable for cellulolytic enzymic action [18, 29]. LHW penetrates inside the lignocellulose matrix and degrades hemicel — lulose into xylose and other accessory sugars with the least generation of inhibitors [17]. The required pressure was applied at high temperatures (160-240 °C) for the cell wall degradation [29]. Laser et al. [30] reported 80 % xylan recovery from SB after hot-water pretreatment (170-230 °C, 1-46 min) which showed 90 % ethanol conversion after simultaneous saccharification and fermentation (SSF).

16.3.2.2 Autohydrolysis

Autohydrolysis is a process that involves the use of steam with or without explosion, based on the selective depolymerization of hemicelluloses from SB/SL [14]. The hydrolysis of acetyl groups generates acetic acid, and hemicelluloses breakdown into its monomeric constituents. Mechanically, a quick reduction of pressure (0.69­4.83 MPa) at high temperature (160-260 °C) provokes an explosive decompression of lignocellulosic material effectively releasing hemicellulosic sugars, preserving the physical-chemical properties of the cellulose [14].

During autohydrolysis, due to depolymerization and repolymerization reactions lignin moieties are redistributed on the fiber surface promoting an enhancement of the pore size and surface area of SB/SL. Hemicellulose is solubilized under high temperature and short residence time (270 °C, 1 min) or lower temperature and longer residence time (190 °C, 10 min) [15, 31]. Dekker and Wallis [32] performed auto hydrolysis of SB at 200 °C for 4 min and observed 90 % solubilization of hemicellulose. The pretreated bagasse enzymatically hydrolysed, revealed 80 % saccharification after 24 h.