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 actsmainly 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]