Resume

Introduction ഀ
Alkali and oxidative treatments of lignocellulose are well-established for delignification in chemical pulping and bleaching. However, their application for pretreatment technologies optimized for liberating carbohydrates as a substrate for biofuels production is different in several regards and requires further research before these technologies are suitable for cellulosic biofuels processes. This work will present research results on alkaline hydrogen peroxide (AHP) pretreatment on several topics including: (1) Scale-up of high-solids pretreatment; (2) Optimized utilization of input chemicals for an industrially relevant process for a standalone pretreatment or delignifying ″post-treatment″ of herbaceous plants; (3) Characterization of soluble inhibitors to enzymatic hydrolysis and fermentation that are released during pretreatment; (4) Characterization of structural and chemical changes in the plant cell wall associated with pretreatment; (5) Integration of glucose and xylose fermentation with a delignifying oxidative pretreatment. ഀ ഀ

Results and Conclusionsഀ
An understanding of the relationship between the pretreatment conditions, the changes associated with the plant cell walls, and the enzymatic digestibility is developed. Both alkali and hydrogen peroxide contribute to the degradation of plant cell walls to improve digestibility. We found that alkali alone can solubilize up to 50% of the plant cell wall material in several commelinid monocots as polymeric hemicellulose, lignin, and extractives. Minimal glucan is lost during pretreatment and hemicellulose is preserved as soluble polymers. We validated that pretreatment at pH 11.5 can result in near-theoretical glucose yields from switchgrass. Rather than improving digestibility, higher loadings of NaOH on biomass at the same hydrogen peroxide loading decreased the digestibility, indicating that the mechanism of pretreatment is likely due to reactions of OOH- and ^.OH with lignin rather than ^.O₂-, which is favored at high pH. Challenges for AHP pretreatment at higher solids include increasing concentrations of soluble fermentation inhibitors. ഀ ഀ

Hydrolysates from AHP pretreated biomass contain inorganics (Na⁺ and SO₄^2- used for pH adjustment) polymeric hemicellulose and lignin, as well as acetate, aromatics, and aliphatic and aromatic oxidation products. Molecular weight distributions of solubilized hemicellulose and lignin were determine by size exclusion chromatography for both alkali-only extraction and AHP pretreatment. These results identified either lignin or lignin carbohydrate complexes with a MW range of approximately 2000-20000 Da and hemicellulose aggregates with an apparent DP range of approximately 100-1000 units. The identification and quantification of potential inhibitors was performed using LC-MS and, among others, ferulate and p-coumarate were identified as the primary soluble aromatics. ഀ ഀ

AHP-pretreated switchgrass (Panicum virgatum, cv. Cave-in-Rock) hydrolysate was tested for ethanol fermentation by using xylose-fermenting yeasts including wild-type Pichia stipitis CBS 6054 and several engineered and adapted Saccharomyces cerevisiae strains expressing heterologous xylose fermentation pathways. Without detoxification or Na+ removal, these yeast strains are capable of growth and glucose and fermentation on hydrolyzates, however xylose utilization is particularly slow. Removal of aromatics in the hydrolyzate by detoxification with activated carbon significantly improved the rate of growth, glucose, and xylose fermentation indicating aromatics were the primary contributor to inhibition. Process integration challenges for AHP pretreatment at higher solids loadings which are necessary for generating suitably high ethanol titers include increasing concentrations of these soluble fermentation inhibitors. High-solids AHP pretreatment and enzymatic hydrolysis was capable of yielding the promising results of more than 100 g/L of fermentable sugars with [Na⁺] < 0.5 M.ഀ ഀ

Publicationsഀ
Banerjee G, Car S, Scott-Craig JS, Hodge DB, Walton JD (in press). Alkaline peroxide pretreatment of corn stover: Effects of biomass, peroxide, and enzyme loading and composition on yields of glucose and xylose. Biotechnol Biofuels.ഀ ഀ

Helmerius J, Vinblad von Walter J, Rova U, Berglund KA, Hodge DB (2010). Impact of hemicellulose pre-extraction for bioconversion on birch Kraft pulp properties. BioresTechnol 101:5996-6005.ഀ ഀ

Berglund KA, Rova U, Hodge DB (2010). Microbial Metabolites as Building Blocks for Renewable Resource Based Surfactants. In: Surfactants from Renewable Resources. M. Kjellin, Ed. John Wiley & Sons. ഀ ഀ

Hodge DB, Andersson CA, Berglund KA, Rova U (2009). Detoxification Requirements for Bioconversion of Softwood Dilute Acid Hydrolyzates to Succinic Acid. Enz Microb Technol.44:309-316. ഀ ഀ

Andersson CA, Helmerius J, Hodge DB, Berglund K, Rova U (2009). Inhibition of Succinic Acid Production in Metabolically Engineered Escherichia coli by Neutralizing Agent, Organic Acids, and Osmolarity. Biotech Prog. 25(1):116-123.ഀ ഀ

Hodge DB, Karim MN, Schell DJ, McMillan JD (2009). Model-Based Fed-Batch for High-Solids Enzymatic Cellulose Hydrolysis. Appl Biochem Biotech. 152(1):88-107. ഀ