The cellulose and hemicellulose part of lignocellulosic feedstock needs to be detached from the lignin part and this is possible either by physical and/or chemical and/or biological pre-treatment. In the physical pre-treatment technique no chemicals are involved. Physical pre-treatments can include: communition i. e. dry, wet and vibratory ball milling; irradiation i. e. electron beam irradiation, or microwave heating and also steam explosion (Wyman, 1996).

In the irradiation pre-treatment method electron beam irradiation or microwave assisted depolymerisation are used to separate cellulose or hemicelluloses from lignin. Electron beam irradiation has some effect on fatty and resin acids in the wood material and changes the physical properties of sawdust (Finell, Arshadi, Gref, Knolle, and Lestander, 2009). However, the irradiation pre-treatment method for ethanol production is not commercial.

In the physical steam explosion pre-treatment, the chipped lignocellulosic materials such as hardwood is treated by high pressure saturated steam (autohydrolyses) and then by reducing the pressure quickly, the material undergoes an explosive decompression. This results in hemicellulose degradation and some changes in polymeric lignin structure with the cellulose becoming more accessible for hydrolysis.

The addition of dilute acid in the steam explosion method (so-called acid catalysed steam explosion) may improve enzymatic hydrolysis of the cellulose and facilitate removal of the hemicellulose. Steam explosion requires less energy than mechanical pre-treatment methods (communition) and is the most effective pre-treatment method for hardwoods and agricultural lignocellulosic products.

One disadvantage of the steam explosion pre-treatment method is the formation of some inhibitory compounds for enzymatic hydrolysis in the next step in ethanol production (Sun and Cheng, 2002).

There are other physical-chemical pre-treatment methods such as ammonia fibre explosion (AFEX) where lignocellulosic feedstock is exposed to liquid ammonia at high temperature and pressure over a period of time and then the pressure suddenly lowered. This method can be used for many different materials including corn stover, wheat straw, softwood newspaper, switch grass, alfalfa, etc. In contrast to acid catalysis steam explosion, the AFEX pre-treatment does not significantly solubilise hemicellulose and high hydrolysis of hemicellulose and cellulose has been obtained after this pre-treatment method. But the superheated ammonia vapour must be recovered to protect the environment (Sun and Cheng, 2002).

In the chemical pre-treatment method some chemical/s are added to the feedstock, e. g. concentrated acids, dilute acids, alkaline solutions. It is possible to use concentrated acids such as H2SO4 and HCl for pre-treatment (acid hydrolysis) of lignocellulosic feedstock. The acids effectively hydrolyse the cellulose. However the concentrated acids are toxic, corrosive and must be recovered after the process. Therefore, dilute acid hydrolysis (e. g. sulphuric acid, hydrochloric acid) as a pre-treatment has been used instead in many applications (softwoods, hardwoods, agricultural residues) with a high reaction rate and effective cellulose hydrolysis. But a neutral pH is necessary for enzymatic hydrolysis or fermentation. One advantage of dilute acid hydrolysis as a pre-treatment method in comparison to the steam explosion method is that the xyloses in hemicellulose remain intact with high yields (Wyman, 1996). These xylans can be utilised in value-added products.

Alkaline pre-treatment with sodium hydroxide alone or in combination with other chemicals like peroxide are most effective for agricultural residues rather than wood feedstock. By this method the lignin is effectively removed and some of the hemicellulose solubilised as well (Wyman, 1996). In the biological pre­treatment technique, microorganisms degrade the lignin (lignin solubilising microorganism) by producing lignin-degrading enzymes and no chemicals are needed. The method is slow which makes it less economical and sometimes consumes hemicellulose as well but it does not require a high energy input and only needs mild environmental conditions (Wyman, 1996).

Recently, Lignol Innovations Corporation has developed a method based on an ethanol-based organosolv pretreatment (i. e. delignification by extraction of lignin from the lignocellulosic biomass with organic solvents or their aqueous solutions) to separate lignin, hemicellulose components (e. g. xylose) and extractives from the cellulose part of the woody biomass (Arato, Kendall, and Gjennestad, 2005). In a review article the prospects and evaluation of different organosolv methods and mechanisms have been presented recently (Zhao, Cheng, and Liu, 2009)

In general, an optimal pre-treatment stage should improve the enzyme/ hydrolysis accessibility, should avoid the degradation of carbohydrates and should avoid the formation of by-products which may have inhibitory effects on the hydrolysis and fermentation steps. In another words, any pre-treatment method must be tailored to the specific lignocellulosic material with different chemical and structural compositions. The economic aspects are also very important in large scale industrial bio-ethanol production.