Compared to agricultural biomass (i. e. wheat straw), woody biomass has a higher density, higher cellulose and lignin content and lower hemicelluloses content, which means it is more resistant to biological conversion. Enzymatic hydrolysis and digestibility of lignocellulose are the key steps in biological conversion and they depend strongly on the feedstock composition and structure, pre-processing of the material and dosage and efficiency of the enzymes used for hydrolysis.

Biological conversion of lignocellulose is enhanced by mechanical comminution that reduces the particle size as well as the degree of polymerization of the cellulose. For biochemical conversion the biomass should have a loose structure that can easily be penetrated by enzymes. Therefore additional pre-treatment is needed to improve accessibility of the cellulose and increase the digestibility to above 50 % for enzymatic hydrolysis (Vidal et al. 2011). High crystallinity of the cellulose and a high degree of polymerization limit enzymatic hydrolysis, mainly the initial hydrolysis rate.

For biochemical conversion feedstock with a high moisture content is preferred and drying reduces the accessibility of biomass to chemicals, steam and enzymes (Stephen et al. 2010; Liu et al. 2002). Special care must therefore be taken to avoid drying and the associated “hornification” phenomenon. Freshly harvested wood chips are the optimal woody feedstock for bioconversion of lignocellulose.

The disruption of the lignocellulose structure by pre-treatments can significantly reduce the recalcitrance of lignocellulose to biological degradation. Most of the pretreatments alter not only the chemical composition but also the physical structure of the biomass by increasing the accessible surface area and pore volume, thereby enhancing cellulase attack.

An increase in lignin content reduces the digestibility of biomass, thereby decreasing the rate and extent of enzymatic hydrolysis. Lignin with a high syringyl content (typical for hardwoods) can be easier degraded by pre-treatments (diluted acid, alkali, hydrogen peroxide, etc.). Apart from being a physical barrier to hydrolysis, lignin can adsorb irreversibly to the enzymes, thereby reducing the yield and increasing time required for effective conversion. The adsorption capacity of lignin depends on the type of lignin and pretreatment applied. Unlike in the pulping process, where the objective is to maintain the fiber integrity, these pretreat­ments aim for maximum digestibility by removing lignin and loosening the fibre structure.

The type of hemicelluloses also affects the choice of pre-treatment and the enzymes used for the fermentation process. For example, the acetyl content in hemicelluloses from hardwoods is involved in autohydrolysis reactions during thermochemical pretreatments, enhancing the cellulose accessibility. On the other hand, residual acetyl groups in pre-treated material constitute a steric hindrance for the enzymatic hydrolysis.

However, the pretreatment of lignocellulose to improve digestibility may also result in the production of sugar and lignin degradation compounds, which may lead to hydrolysis/fermentation inhibition.

Biomass with a low ash content is preferred for biological conversion, not only because it maximizes the availability of carbohydrates and lignin for the conversion process, but also because the buffering capacity of ash may increase the chemicals requirement in acid-catalysed pretreatments for biological conversion.