The effect of initial pore volume and lignin content on the enzymatic hydrolysis of softwoods

Four Douglas-fir pulps, a refiner mechanical pulp (RMP), sulphonated RMP, delignified RMP and a kraft pulp were used to determine whether the lignin content and initial pore volume affected cellulase adsorption and substrate hydrolysis. When compared on the basis of lignin content, it was apparent from the cellulase treatment of the sulphonated RMP that the proportion of lignin did not affect enzyme adsorption when the fibres were sufficiently swollen. However, it was observed that the initial adsorption of cellulase does not always translate to fast and complete hydrolysis. Pore volume data revealed that although modification of lignin resulted in a dramatically increased fibre saturation point the median pore width was not increased accordingly. In contrast, the delignified RMP had a higher median pore width and was hydrolyzed more completely, suggesting that steric hindrance from the residual lignin may be the rate limiting characteristic in this situation. Hydrolysis of the kraft pulp indicated that the larger average particle size of this substrate might have been an inhibiting factor, since it was hydrolyzed more slowly than the delignified RMP despite having a higher median pore width and lower lignin content.