Table 2.

Lignocellulosic biomass structural/chemical properties and their recalcitrant effects on pretreatment and enzymatic hydrolysis.

Biomass Property	Effects on Pretreatment and Enzymatic Hydrolysis	Reference
Cellulose crystallinity	The intramolecular and intermolecular chemical linkages such as hydrogen bonding in the linear cellulose chains increase the feedstock recalcitrance, enzyme loading, and pretreatment severe condition. The high cellulose crystallinity contributes to the feedstock recalcitrance, and subsequently decreases the cellulose conversion.	[37,38,39,40]
Degree of polymerization (DP)	Cellulose DP is normally in the range of 800–10,000 (up to 17,000). Since the high DP structure has less reducing sugar ends that could affect feedstock disobedience and enzyme catalyst, the reduction of DP is required for effective cellulose conversion	[41,42,43,44,45]
Lignin	Lignin plays a key role in the lignocellulosic materials as a biological glue and secondary cell wall. Both lignin and its roles have negative effects on pretreatment, enzyme usage, cellulose conversion, and total costs. Delignification and/or reduction of lignin content using pretreatments, genetic/system engineering, and feedstock selection/modification are required to improve the final conversion yield and productivity.	[33,34,36,46,47,48,49,50,51,52,53]
Hemicellulose	Xyan, the most plentiful hemicellulose in plants, forms a coating layer with cellulose by hydrogen bonding and covalently links with lignin to protect the plant cells. Primary role of the pretreatment is to solubilize the hemicellulose components, and it could improve the cellulose digestibility and hydrolysis.	[54,55,56,57]