###### Abstract  
The effect of drying on the enzymatic hydrolysis of cellulose was determined by analysis of porosity and crystallinity. Fiber hornification induced by drying produced an irreversible reduction in pore volume due to shrinkage and pore collapse, and the decrease in porosity inhibited enzymatic hydrolysis. The drying effect index (DEI) was defined as the difference in enzymatic digestibility between oven- and never-dried pulp, and it was determined that more enzymes caused a higher DEI at the initial stage of enzymatic hydrolysis and the highest DEI was also observed at the earlier stages with higher enzyme dosage. However, there was no significant difference in the DEI with less enzymes because cellulose conversion to sugars during hydrolysis did not enhance enzymatic hydrolysis due to the decrease in enzyme activity. The water retention value (WRV) and Simons' staining were used to measure pore volume and to investigate the cause of the decrease in enzymatic hydrolysis. A decrease in enzyme accessibility induced by the collapse of enzymes' accessible larger pores was determined and this decreased the enzymatic hydrolysis. However, drying once did not cause any irreversible change in the crystalline structure, thus it seems there is no correlation between enzymatic digestibility and crystalline structure. drying effect; cellulose; enzymatic hydrolysis; hornification; porosity 202020  
enzymatic digestibility [10]. Once cellulose has been dried, the dimensions of the cellulose are changed due to the collapse of pores and shrinkage of the internal volume [11; 12]. The structural change stiffens the cellulose via a process known as \"hornification\" [13]. The process of hornification is frequently explained as irreversible, or partially irreversible hydrogen bonding upon drying or water removal [14] due to the aggregation of the cellulose chains [15]. The cellulose chains are aggregated due to the removal of water by heating and then they are not able to fully open up to the next exposure to water [16]. The aggregation may collapse the pore structure, and thus a decrease in pore volume and surface area can be expected [17; 12]. Crystallinity in cellulose fiber has been considered as one of the major properties altered by the aggregation [18]. It was reported that several cycles of drying and rewetting caused irreversible change in crystalline structure, which could not be recovered due to the growth of crystalline domains through co-crystallization [19]. However, no change in crystalline index by the recycling of paper has been reported [20]. It has been reported that pore volume is significantly decreased by the process of repetitive drying and rewetting. Repetitive recycling of delignified and alkali-extracted pulp was carried out by rewetting and drying, and it decreased the Brunauer-Emmett-Teller (BET) surface area and pore volume of cellulose, as measured by the water retention value (WRV). In a five-times recycling procedure, the BET surface area and the WRV decreased by 22.9% and 35.9%, respectively, compared with non-dried and rewet pulp [17]. In addition, in previous studies on the effect of drying, it was reported that hornification induced by drying affects enzymatic hydrolysis [21; 22]. The enzymatic digestibility of dried substrate was significantly lower than that of never-dried substrate, and the reduction in enzymatic conversion was caused by a decrease in pore volume, which can be evaluated by WRV [22]. Hornification induced by drying caused the collapse of larger pores while increasing the number of smaller pores, which are not accessible to enzymes [21]. This suggests that the collapse or closure of larger pores could be a primary reason for a reduction in enzyme accessibility to cellulose, given the size of the cellulase core. Although numerous studies have been performed to investigate the drying effect on enzymatic hydrolysis [23], little research on the effect with regard to enzyme dosage and the kinetics of hydrolysis has been performed.