Enzymatic hydrolysis can be facilitated by chipping, milling and grinding the biomass into a fine powder to increase the surface area of the cellulose. In most cases the power consumption is forbiddingly high to reach high digestibility in the enzymatic hydrolysis. It can be even higher than the theor­etical energy content that is available in the biomass [21]. However, physical treatment in an extruder combined with heating and addition of chemicals could be an interesting option [22]. Another method that has been suggested is irradiation of cellulose by gamma rays, which cleaves the ^-1,4-glycosidic bonds, thus giving a larger surface area and a lower crystallinity [23]. This method is, however, far too expensive to be used in a full-scale process. It is also doubtful that it can be used in combination with technologies supposed to be environmentally friendly.

3.2