Pretreatment to Break the Lignin Recalcitrance
Pretreatment is applied to overcome the recalcitrance of the lignocellulosic biomass which is mainly derived from the cellulose crystallinity and complex lignin structure, before the sugars can be accessed by enzymes (Yang and Wyman 2008). The mode of action of pretreatment involves disrupting hydrogen bonds in crystalline cellulose, solubilizing the hemicellulose and lignin, or physically disrupting the cell wall to actually break down the lignin and, thereby, increase the porosity and surface area of the biomass (Ahring et al. 1996; Ahring and Thomsen 2003; Ahring and Westermann 2007; Agbor et al. 2011) and make the sugars available to the enzymes as shown in Fig. 1.3.
In another words, the pretreatment opens the structure of the biomass such that the enzymes can access the polysaccharides and convert them into monomeric sugars at an accelerated rate. Pretreatment is an essential first step in the biomass to biofuel conversion process (Fig. 1.4), because, in the absence of pretreatment, the rate of enzymatic hydrolysis of cellulose to glucose would reduce drastically as the sugars present in the biomass would be protected within the lignin-carbohydrate complex against the enzymatic attack. Sugar degradation products generated during the pretreatment of lignocellulosic biomass are as follows:
A. Acetic acid: Acetic acid is produced by the hydrolysis of acetyl groups that are generally associated with hemicellulose and result in lowering the pH of the pretreated slurry and acts to catalyze the biomass fractionation during pretreatment.
Fig. 1.4 Pretreatment followed by enzymatic hydrolysis to release the sugars from lignocellulosic biomass
B. Furfural: Furfural is the degradation product generated by the dehydration of pentose fractions within hemicellulose, such as arabinose and xylose.
C. Hydroxymethylfurfural (HMF): HMF is the component generated by acid dehydration of hexoses such as glucose.