Hydrolysis is the process by which water splits a larger molecule into two smaller molecules. In the case of the hydrolysis of polysaccharides to soluble sugars this is called “saccharifica­tion”. The goal of this process is the de-polymerization of cellulose and hemicelluloses into soluble monomer sugars (hexoses and pentoses). This can be accomplished by two different processes: (1) acid hydrolysis with a variety of low acid-high temperature or high acid-low temperature conditions being suitable to both the breakdown of the structure of the biomass and the release of free sugars, and (2) enzymatic hydrolysis after some sort of pretreat­ment which allows enzymatic attack of the polymers [22, 23]. The C6 dominated cellulose

can be enzymatically hydrolyzed by cellulases; for the C5 dominated hemicellulose the hemicellulases (such as xylanase) can be used.

Acid (sulfuric or hydrochloric) can serve both for disruption and hydrolysis of the cellulosic polymers and is currently seen as the most technologically mature method of sugar release from biomass. A major disadvantage of acid hydrolysis is the potential degradation of the released monosaccharides that leads to reduced sugar yields [13, 23]. Other drawbacks are the cost of acid, the requirement to neutralize the acid after treatment and the production of inhibitory by-products such as furfural and hydroxymetyl furfural [22, 24, 25].

Enzymatic degradation of lignocellulosic biomass on the other hand is very specific and side reactions, such as degeneration of sugars, do not occur. High yields are therefore possible. In addition, the mild conversion conditions lower the maintenance costs of the production plant [23]. High temperature and low pH tolerant enzymes are preferred for the hydrolysis due to the fact that most current pretreatment strategies rely on acid and heat [26]. In addition, thermostable enzymes have several advantages, including higher specific activity and higher stability, which improve the overall hydrolytic performance [27]. Ultimately, improvement in catalytic efficiencies of enzymes will reduce the cost of hydrolysis by enabling lower enzyme dosages [7].

Although acid hydrolysis methods have long industrial histories and are, therefore, more mature, enzymatic hydrolysis is seen as the most economically promising method for redu­cing costs while improving yields and a key to cost-effective production of monosaccha­rides [28]. Research is focusing on advanced screening processes of natural enzymes and developed man-made enzymes to increase the efficiency and improve enzymatic hydrolysis [29].