Sunday, April 29, 2007

Recycling free cellulases during the hydrolysis of lignocellulosic substrates

Maobing Tu1, Richard Chandra2, and Jack Saddler2. (1) Pulp and Paper Research Institute of Canada (Paprican), 570 Boul. St-Jean, Pointe Claire, QC H9R 3J9, Canada, (2) Wood Science, University of British Columbia, 2424 Main Mall, Vancouver, BC V6T 1Z4, Canada

The recycling of cellulase enzymes is one potential strategy for reducing the cost of the enzymatic hydrolysis step during the bioconversion of lignocellulosics to ethanol. To determine the influence of lignin on the post-hydrolysis distribution of cellulase enzymes between the liquid and solid phases, the hydrolysis of Avicel was compared to an organosolv pretreated Douglas fir substrate with a lignin content of 3%. After the hydrolysis of Avicel, 90% of the added cellulases (including β-glucosidases) remained “free” in the liquid phase compared to only 65% in the case of the hydrolysis of the organosolv pretreated Douglas fir substrate. The re-adsorption of free cellulases by supplementing the hydrolysis reaction with fresh substrate was explored as a potential means of recovering the free cellulases that remain in the liquid phase after hydrolysis. The Langmuir adsorption isotherm was used to develop a model predicting that approximately 82% of the free cellulases could be recovered via re-adsorption onto fresh substrates during the hydrolysis of an ethanol pretreated mixed softwood substrate with a lignin content of 6%. Recoverable free cellulase values of 85% and 88% based on cellulase activity and protein content respectively were obtained after experimental verification of the model. The readsorption of free cellulases onto fresh lignocellulosic substrates was shown to be an effective method for free enzyme recovery.