Structural features lead to two-phase enzymatic hydrolysis for mild pretreated biomass

The limiting factors that affect enzymatic hydrolysis of biomass have been traditionally divided into two groups: substrate-based and enzyme based. The substrate-based factors mainly involve chemical structural features such as the compositions of cellulose, hemicellulose, lignin, and side groups bound to hemicellulose and physical structural features that consist of accessible surface area, crystallinity, the physical distribution of lignin in the biomass matrix, degree of polymerization, pore volume, and biomass particle size.

For mild pretreated biomass, the inital rate of enzymatic hydrolysis will be influenced by the transport of enzymes and the dominant hydrolysis will be on amorphous cellulose. At this phase, the hydrolysis of hemicellulose will correlate with the slow increase of glucose yield. When the hemicellulose removal to some point, say 50% removal, a slight hemicellulose hydrolysis will lead to significant increase in cellulose hydrolysis, i.e. crystal cellulose hydrolysis: A transition point occurs, demonstrating the main or 2nd phase hydrolysis.

At 1st phase, lignin content and side groups in hemicellulose play an important role. Any treatments to remove lignin and peel off side groups and break down hemicellulose will improve enzyme transport though opened channels; At the 2nd phase,cellulose crystallility become the limiting factor, any destruction of cellulose crystallility will speed up the hydrolysis and increase the yield.