16.3.3.1 Acid Pretreatment

Among different types of pretreatments, the acid hydrolysis is one of the most com­monly used methods for SB/SL. This method is usually employed to solubilize the hemicelluloses fraction of the cell wall which eventually aids the accessibility of cellulolytic enzymes action [12, 29,44]. In the acid pretreatment, the hemicellulose can be hydrolyzed keeping the biomass in contact with diluted acid or concentrated acid under high temperature and low temperature respectively [10, 45]. Figure 16.3 shows the mechanistic demonstration of acidic pretreatment applied to SB/SL.

The hemicellulose fraction of SB/SL is depolymerized primarily into pentose sugars (xylose and arabinose) and hexose sugars (glucose, galactose, mannose, etc) along with inhibitory compounds [12, 46,47]. Recently, Moutta et al. [46] reported

56.5 g/L total reducing sugars from the hemicellulosic fraction of SL under the opti­mized set of conditions (130 °C, 2.9 % w/vH2SO4, 1:4 solid:liquid ratio, and 30 min of residence time).

Multiple methods showed their feasibility to eliminate the inhibitors from the hydrolysates prior to fermentation [47, 48]. The hemicellulosic hydrolysate after detoxification can be converted into value-added products such as xylitol, ethanol, lactic acid, etc [2, 49]. The advantage of the dilute acid hydrolysis process is the low operation and energy costs [14, 15,49]. Concentrated acid hydrolysis of SB/SL leads to the issues such as equipment corrosion and expensive costs of maintenance [29,45]. The diluted acid pretreated SB/SL is subsequently hydrolysed with cellulase to depolymerize the cellulosics fraction into glucose [44, 37].