Impact of Ammonia Fiber Explosion (AFEX) Pretreatment on the Lignin Carbohydrate Complex Structure
The mode of action of ammonia fiber explosion (AFEX) pretreatment involves ammonia reaction with lignin, which causes cellulose swelling resulting in cleavage of lignin-carbohydrate linkages and cellulose phase change from cellulose I to cellulose III (Lin et al.). During the AFEX process, lignocellulosic biomass is pretreated with liquid ammonia at relatively moderate temperatures and higher pressures for given residence times. At the end of the reaction, the pressure is suddenly released, literally expanding the fibrous biomass. It was reported that after the aqueous ammonia pretreatment of Miscanthus, the S-unit to G-unit ratio was increased (Le Ngoc Huyen et al. 2010) which implied that during AFEX pretreatment, cleavages of G-unit lignin were preferred reactions than S-unit lignin. AFEX pretreatment has shown to accelerate the enzymatic digestibility by reducing the crystallinity during pretreatment (Kassim and El-Shahed 1986). According to Mikhail and Adriaan (2012), AFEX proved to be efficient in opening up the cell wall structure in agricultural residues. Mendonca et al. (2004) reported that compared to acid pretreatment, AFEX is more effective in reducing the crystallinity of the cellulose and hemicellu- lose solubilization by deacetylation of acetyl groups attached on the xylan backbone chain and cleaving the lignin carbohydrate bonds with the formation of lesser degradation products. Substantial increase in enzymatic hydrolysis of cellulose to glucose after AFEX pretreatment of lignocellulosic biomass was also reported by the above studies. Ammonia fiber explosion pretreatment is well suited for herbaceous and agricultural residues; however it has shown no success in producing high sugar yields from woody biomasses (McMillan et al., Kim et al. 2000).