The regular and cross-linked cellulose chains form a very efficient barrier against penetration of the enzymes into the fibres. Swelling of the pores can be achieved by alkaline pretreatment through soaking of the material in an alkaline solution, such as NaOH, and then heating it for a certain time. The swelling causes an increase in the internal surface area, and a decrease in the degree of polymerization and crystallinity. Usually a major fraction of the lignin is solubilized together with some of the hemicellulose. A rather large fraction of the hemicellulose sugars are usually recovered as oligomers. Al­kaline pretreatment breaks the bonds between lignin and carbohydrates and disrupts the lignin structure, which makes the carbohydrates more accessible to enzymatic attack. As it acts mainly by delignification, it is more effective on agricultural residues and herbaceous crops than on wood materials, as these materials in general contain less lignin. For softwood species, which contain a large amount of lignin, a small or no effect has been observed. Pretreatment using lime instead of sodium hydroxide is another alkaline method, especially suited for agricultural residues, e. g. corn stover, or hardwood materials, such as poplar [24,25].

Dilute acid pretreatment is performed by soaking the material in dilute acid solution and then heating to temperatures between 140 and 200 °C for a certain time (from several minutes up to an hour). Sulphuric acid, at con­centrations usually below 4 wt %, has been of most interest in such studies as it is inexpensive and effective. The hemicellulose is hydrolysed and the main part is usually obtained as monomer sugars. It has been shown that materials that have been subjected to acid hydrolysis may be harder to ferment because of the presence of toxic substances [26-28].

Another approach is to use an organic or aqueous-organic solvent mixture with addition of an inorganic acid catalyst (H2SO4 or HCl), which is used to break the internal lignin and hemicellulose bonds. These methods are usu­ally referred to as organosolv processes [29]. In these cases the hydrolysed lignin is dissolved and recovered in the organophilic phase. It is important to thoroughly wash the material prior to enzymatic hydrolysis and fermenta­tion, as the solvents may act as inhibitors. Solvents that are used are typically methanol, ethanol, acetone, ethylene glycol, triethylene glycol and phenol. Some of these substances are explosive and highly inflammable and thus dif­ficult to handle.

3.3