Extractives of Douglas-fir and Their Impacts for Biofuel Saccharification

Abstract

Forestry residues are being investigated as a potential biofuel feedstock since they are an abundant biomass that does not require major land-use change or compete with food resources. This biomass is the waste created from logging and forest thinning operations and consists of branches, treetops, and other unmerchantable timber. Douglas-fir residues have received special attention since Douglas-fir is the most abundant tree in the timberlands of the Western United States. Biofuel from this material has yet to hit full-scale production, but production processes are already being investigated at the pilot plant scale. Creating a biofuel industry from forestry residues is a complex challenge, and there is a need for research into secondary products and process optimization. The best way to go about finding improvements is to holistically connect the challenges observed in processing millions of tons of biomass to the molecular interactions at hand. With this in mind, this research begins by modeling a biofuel process in ASPEN to understand what molecules are present in processing. We then experimentally identify process inhibitors and explore the molecular origins of this inhibition. The specific focus of this research is on the “extractives” class of molecules in forestry residues. Extractives make up 5% to 25% of the dry weight for different tissues of Douglas-fir, but are overlooked in most biofuel studies. These molecules have been well studied in pulp and paper industry circles and this past knowledge can be used for inspiration for biofuel research. Extractives are both unconsidered secondary products and biofuel process inhibitors. This work first collates the known literature on Douglas-fir extractives into a representative chemical model of Douglas-fir forestry residues. This model is then used to predict which extractives end up in the various process streams of a simulation model of the Northwest Advanced Renewables Alliance (NARA)’s biofuel process. NARA’s biofuel process relies upon a process called saccharification, and the information derived from process modelling is used to begin studying this step. Saccharification is the key step in this process and involves enzymes breaking down biomass into soluble sugars for further conversion into fuels and chemicals. This work shows these enzymes are inhibited by the “tannins” class of extractives in Douglas-fir bark. Initial glucose production from crystalline cellulose is shown to be inhibited ~40%, and specific types of enzyme are individually inhibited ~15-20%. Molecular docking and molecular dynamics simulations were used to investigate how a characterized tannin in Douglas-fir binds onto the most abundantly produced cellulase from Trichoderma reesei, Cel7A. We find that the tannin can bind onto the cellulase and obstruct the catalytic tunnel of this enzyme in a variety of different ways with the tannin preferring to bind to sites that contain aromatic or charged amino acid residues. There is still much to learn and explore in this area. Extraction and collection of the tannins as secondary products may be the most appealing option to remove inhibition but may not be economical. Instead, bioengineered tannin resistant enzymes could be appealing, and the information presented here could provide the foundation for that work.