The main reaction that occurs during acid pretreatment is the hydrolysis of hemicel — lulose. Hemicellulose mainly xylan is hydrolyzed to fermentable sugars during DAP [59]. Solubilized hemicelluloses (oligomers) can be subjected to hydrolytic reactions producing monomers, furfural, HMF, and other (volatile) products in acidic environ­ments [60, 61]. Recently, Sannigrahi et al. [62] have demonstrated that pseudo-lignin can be generated solely from carbohydrates without significant contribution from lignin during DAP especially under high severity pretreatment conditions. Further analysis indicates that pseudo-lignin is in spherical droplet form and has carbonyl, aromatic, methoxy, and aliphatic structures.

During DAP, it is generally accepted that the majority of the hemicellulose are removed initially, followed by the hydrolyzation of cellulose and subsequently some solubilization of Glu through the course of DAP [63-65]. Foston et al. [65] stated that cellulose degradation pathway can be viewed as acid catalyzed, thermally accelerated polysaccharide hydrolysis by chain scission within the fibril structure from either a crystalline or amorphous region of cellulose. This process consists of two major stages: The initial stage was regarded as rapid hydrolytic attack on the amorphous chain segments while the latter stage takes place on the crystal surfaces [66, 67]. Sannigrahi et al. [68] observed an increase in the relative proportion of cellulose Ip accompanied by a decrease in the relative proportion of both cellulose Ia and para- crystalline region from dilute acid pretreated Loblolly pine. This suggested that the types of lignocellulosic materials and pretreatment conditions influence cellulose crystalline allomorphs and para-crystalline contents during DAP.

DAP does not lead to significant delignification. Recent studies revealed an in­crease in the degree of condensation of lignin, during the DAP. The increase in degree of condensation is accompanied by a decrease in в-О-4 linkages which are frag­mented and subsequently recondensed during the high-temperature acid-catalyzed reactions [68, 69]. In addition, studies also indicated that lignin balls (or lignin droplets) were formed during DAP. These lignin droplets originated from lignins and possible lignin carbohydrates complexes [70, 71].