## 1 Introduction  
The development of second-generation biofuels has been lately receiving a lot of attention due to the limitations of using crops for producing first-generation bioethanol [1; 2]. Alternative feedstocks, such as lignocellulosic biomass, have been used to substitute crop feedstocks [2; 3; 4; 5].  
The difficulty of producing bioethanol from lignocellulosic biomass lies mainly in its recalcitrance and this is a major barrier to the economic development of bio-based fuels and products [6; 7]. Although cellulose and hemicellulose from lignocellulosic biomass are hydrolysate polysaccharides that give way to carbohydrates whose fermentation results in ethanol, the cell walls of lignocellulosic materials form a barrier that has a natural resistance to microbial and enzymatic deconstruction [6; 7; 8; 9].  
Several factors such as lignin content, lignin distribution and its structure, cellulose crystallinity and degree of polymerization, pore size and internal porosity, end-product inhibition, need for synergism, and non-productive enzyme adsorption on lignin have been suggested to account for the recalcitrance of lignocellulosic materials to pretreatment and notably to enzymatic hydrolysis [3; 6; 10; 11; 12; 13; 14]. Several different approaches are being developed to pretreat lignocellulosic materials so that its complex architecture can be disrupted, thereby making its cellulosic components more accessible to enzymes [6; 7; 9; 10; 15].  
Apart from delignification, lignocellulosic biomass pretreatments can cause structural changes in the fiber cell wall in the form of delamination and the development of pores of different sizes [16]. While mechanical treatments of the fibers contribute to an increase in the pores size in the cell wall due to the repeated bending, cracking, and compressing, chemical treatments, on the other hand, create new pores through the removal of lignin and polysaccharides [16; 17]. Regarding lignin, apart from the physical barrier, the non-productive binding of enzymes on lignin is the critical issue [6; 18]. The results from SPORL (Sulfite Pretreatment to Overcome Recalcitrance of Lignocelloses) pretreatment of lignocellulosic biomasses [6] suggest that sulfonated lignin can remain in the solid residue without drastically impairing the enzymatic hydrolysis. Another prerequisite for an effective enzymatic hydrolysis is enzyme accessibility [19; 20; 21]. [PERSON] et al. reported that the formation of micro- and macropores in the pulp fibers during mechanical shredding correlates with the susceptibility of the fibers to enzymatic hydrolysis [19]. Moreover, the formation of pores in the pulp fibers with a minimum size similar to that of the enzyme's hydrodynamic radii during mechanical and chemical treatments correlates with the susceptibility of the fibers to enzymatic hydrolysis [17; 19]. Other authors [20; 21] reported on the role of the cellulose accessibility, using more specific methodologies.  
In a previous work, using high sulfite charges, we observed a very good enzymatic hydrolysis of pretreated _E. globulus_ biomass, even with reduced enzymes charges [22]. In the present work, we have widened the range of the sulfite charges to lower values aiming to reduce the pretreatment cost. The effect of different pretreatment conditions on the carbohydrates and byproducts released into the pretreatment hydrolysates and the enzymatic digestibility of the corresponding solid residues was studied. The effect of the global pore volume and volumetric fraction of the solid residues' pores correspondent to the hydrodynamic radius of a catalytic unit of a typical cellulase enzyme on the enzymatic hydrolysis performance was also studied.