Biochemical Engineering Journal 43 (2009) 92–97 Contents lists available at ScienceDirect Biochemical Engineering Journal journal homepage: www.elsevier.com/locate/bej Differential effects of mineral and organic acids on the kinetics of arabinose degradation under lignocellulose pretreatment conditions A. Maarten J. Kootstra a,c,∗ , Nathan S. Mosier b , Elinor L. Scott a , Hendrik H. Beeftink c , Johan P.M. Sanders a a Valorisation of Plant Production Chains, Wageningen University, P.O. Box 17, 6700 AA Wageningen, The Netherlands b Laboratory of Renewable Resource Engineering, 500 Central Drive, Purdue University, West Lafayette, IN 47907, USA c Food and Bioprocess Engineering Group, Wageningen University, P.O. Box 8129, 6700 EV Wageningen, The Netherlands article info Article history: Received 16 July 2008 Received in revised form 3 September 2008 Accepted 4 September 2008 Keywords: Arabinose degradation Furfural Cellulosic ethanol Pretreatment Fumaric acid Maleic acid abstract Sugar degradation occurs during acid-catalyzed pretreatment of lignocellulosic biomass at elevated tem- peratures, resulting in degradation products that inhibit microbial fermentation in the ethanol production process. Arabinose, the second most abundant pentose in grasses like corn stover and wheat straw, degrades into furfural. This paper focuses on the first-order rate constants of arabinose (5g/L) degra- dation to furfural at 150 and 170 ◦ C in the presence of sulfuric, fumaric, and maleic acid and water alone. The calculated degradation rate constants (k d ) showed a correlation with the acid dissociation constant (pK a ), meaning that the stronger the acid, the higher the arabinose degradation rate. However, de-ionized water alone showed a catalytic power exceeding that of 50 mM fumaric acid and equaling that of 50 mM maleic acid. This cannot be explained by specific acid catalysis and the shift in pK w of water at elevated temperatures. These results suggest application of maleic and fumaric acid in the pretreatment of ligno- cellulosic plant biomass may be preferred over sulfuric acid. Lastly, the degradation rate constants found in this study suggest that arabinose is somewhat more stable than its stereoisomer xylose under the tested conditions. © 2008 Elsevier B.V. All rights reserved. 1. Introduction Future oil shortages, increasing oil prices and international agreements are reasons for increased research on alternative routes to produce chemicals and transportation fuels. Fermenta- tion technology can produce such liquid fuels, but the feedstock (fermentable sugars) and processing costs need to be sufficiently low to compete economically with oil-derived fuels. In current first generation bioethanol production, relatively expensive sugar and starch derived from sugar cane and maize are used as feedstock. However, second generation processes will use relatively cheap and more abundant renewable lignocellulosic raw material, such as agricultural residues like corn stover, wheat straw, or forestry by-products. Using these by-product streams also results in less competition for high-quality edible carbohydrates. Lignocellulosic biomass requires pretreatment to facilitate the hydrolysis of cell wall polysaccharides to fermentable sugars [1]. ∗ Corresponding author at: Valorisation of Plant Production Chains, Wageningen University, P.O. Box 17, 6700 AA Wageningen, The Netherlands. Tel.: +31 317 481315; fax: +31 317 483011. E-mail address: maarten.kootstra@wur.nl (A.M.J. Kootstra). Pretreatment usually combines a catalyst (acid or base) in water with thermal treatment. For example, sulfuric acid pretreatment is used at 50–300 mM at 100–200 ◦ C to hydrolyze hemicellulose, disrupt lignin, and render the residual cellulose more reactive when exposed to cellulolytic enzymes [1–4]. During the acid pre- treatment at elevated temperature, degradation of the fermentable sugars occurs. Degradation products like furfural from pentoses and 5-hydroxymethylfurfural (HMF) from hexoses are inhibitory to yeasts in subsequent sugar-to-ethanol fermentation processes, which results in a lower efficiency of the ethanol production process [5–8]. At elevated temperatures, furfural degrades further into formic acid [9], while HMF degrades into both formic and levulinic acid [5,6]. In warm season grasses like wheat and maize, the hemicel- lulose fraction of the structural polysaccharides largely consists of arabinoxylan or glucuronoarabinoxylan (GAX) [10–12]. Thus, ara- binose is the second most abundant pentose present in biomass like corn stover and wheat straw. While lignocellulosic mate- rials contain much less l-arabinose than d-xylose, the relative amounts of the sugars strongly depend on the raw material. For example, on a dry matter basis corn stover contains of 15% xylan and 3% arabinan, wheat straw contains 19% xylan and 2% arabi- nan, whereas wheat bran contains 19% xylan and 15% arabinan 1369-703X/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.bej.2008.09.004