Substrate factors limiting the enzymatic hydrolysis of softwood substrates
Creators: name: Mooney, Catherine Ann
Publisher: name: University of British Columbia
Year: 2009

description: Past work has shown that softwood substrates are inherently more difficult to pretreat and enzymatically hydrolyse than are hardwoods or agricultural residues. While the recalcitrance of lignocellulosic substrates to enzymatic hydrolysis after pretreatment has been the subject of considerable research, there is a dearth of information regarding the specific factors which influence softwood hydrolysis. It has been shown that the lignin content and distribution as well as overall accessible surface area, are two of the major factors that influence the hydrolysis rates and yields of many lignocellulosic substrates. The first part of this study investigated the effect of these two factors on the accessibility of cellulases to the cellulose component of a Douglas-fir kraft pulp and three pulps derived from a Douglas-fir refiner mechanical pulp. The refiner mechanical pulp was pretreated to selectively remove the lignin and to also increase the swelling of the pulp without removing the lignin. This resulted in four pulps, a kraft pulp with little lignin, a refiner mechanical pulp with a high amount of lignin, a delignified refiner mechanical pulp, and a refiner mechanical pulp in which the lignin had been sulphonated. The accessibility of cellulose within the four pulps was investigated by measuring the cellulase adsorption capacity, the pore volume and the Simons stain accessibility of each substrate. It was found that, while the adsorption capacity was significantly increased with the more swollen substrate, there was still only approximately 50% hydrolysis of the substrate. It was apparent that lignin removal was necessary in order to facilitate complete hydrolysis. The pore volume of the substrates as well as the Simons stain accessibility was significantly higher with the pulps which contained little lignin reflecting the higher accessibility of cellulose within these substrates. The second part of this study investigated the effect of fiber size on the adsorption capacity and hydrolysability of three Douglas-fir kraft pulps with different fiber size distributions. It was found that, by decreasing the average fiber size of the kraft pulp, the adsorption capacity and the hydrolysis yields could be greatly increased. The average fiber size was reduced by increasing the fines content from approximately 5% to 20%, which resulted in a significantly increased adsorption capacity with up 96% of added enzyme adsorbed. In contrast, a maximum of approximately 75% of the added protein was adsorbed to the untreated kraft pulp. It was also found that the enzyme loading required to achieve maximum hydrolysis of the kraft pulp could be decreased significantly by decreasing the average fiber size. By observing the changes in fiber size distribution and fiber coarseness, it was determined that the smaller fibers and fines were hydrolysed preferentially resulting in the traditionally observed rapid initial hydrolysis rates. In contrast, if the initial substrate consisted entirely of larger coarser fibers, the rate was greatly influenced by the time taken to disintegrate the fibers during the initial phase of hydrolysis.