Clearly, the hydrolysis step is affected by the type of pretreatment and the quality of this process — particularly by the accessibility of the lignocellulose.

Lignoculluloses can be solubilised by enzymatic or chemical hydrolysis (mainly with acids). Both the pretreatment and hydrolysis are performed in a single step during acid hydrolysis. Two types of acid hydrololysis are usually applied: concentrated and dilute acid hydrolysis (Wyman et al., 2004, Gray et al., 2006, Hendriks & Zeeman, 2009).

Cellulase enzymes from diverse fungi (e. g. like Trichoderma, Aspergillus) (Dashtban et al., Sanchez, 2009) and bacteria (e. g Clostridium, Bacillus) (Sun & Cheng, 2002) can release sugar from lignocellulose at moderate temperatures (45-50°C) with long reaction times (one to several days) (reviewed in Brethauer & Wyman, 2010; Balat, 2011).

Three different enzymes work synergistically — the endo-|3-1,4-glucanases (EC 3.1.2.4), exo-|3- 1,4-glucanases (EC 3.2.1.91) and P-glucosidase (EC 3.2.1.21) — to generate glucose molecules from cellulose (Lynd et al., 2002). In addition, enzymes like hemicellulases and ligninases improve the hydrolysis rate and raise the content of the fermentable sugar (Palonen & Viikari, 2004; Berlin et al., 2005).

Diverse factors inhibit the activity of the cellulase and thereby decrease the rate of hydrolysis and the effectiveness of the hydrolysis step: end-product inhibition, easily degradable ends of molecules are depleted, deactivation of the enzymes, binding of enzymes in small pores of the cellulose and to lignin (Brethauer & Wyman, 2010; Balat, 2011).

Hemicellulose is a highly complex molecule and multi-enzyme systems are needed like endoxylanase, exoxylanase, |3-xylanase, a-arabinofuranosidase, a-glucoronidase, acetyl xylan esterase and ferulic acid esterase (all produced by diverse fungi e. g. Aspergillus and bacteria e. g. Bacillus) for the enzymatic hydrolysis (reviewed in Balat, 2011).