Hardwood Hemicellulose Research Articles

Fermentation of hardwood hemicellulose hydrolysate byPachysolen tannophilus, candida shehatae andPichia stipitis

Hardwood hemicellulose hydrolysate has been utilized as a substrate for ethanol production. Among the three different yeasts tested, the best performances have been obtained, in decreasing order, usingPachysolen tannophilus, Candida shehatae andPichia stipitis. Several pretreatments of this raw material have been studied to improve ethanol yields; in one such pretreatment a strain ofP. tannophilus produced ethanol with a yield of 0.29 gethanol/gsugars (gP/gS); which is only 15% less than the values observed with synthetic media. Neither aeration nor acetone addition improved the fermentation of this substrate; in fact, only a marked stimulation of biomass growth has been observed at the expense of both ethanol and xylitol production.

Effect of transient acid diffusion on pretreatment/hydrolysis of hardwood hemicellulose

Diffusivities of sulfuric acid in Aspen wood were experimentally determined at various diffusional temperatures and fitted to Arrhenius equations. These equations were subsequently incorporated into a theoretical model to establish the effect of transient acid concentration gradients within a solid substrate during acid-catalyzed hydrolysis of hemicellulose. Total xylose yield was found to decrease for increasing chip size, and this effect was intensified by increasing reaction temperature. Quantitative criteria were established for assessment of transient acid concentration effects upon xylose yield and reaction time at various reaction conditions.

An Engineering Analysis of the Production of Xylose by Dilute Acid Hydrolysis of Hardwood Hemicellulose

Numerical simulations of various reactors for the production of xylose from hardwood hemicellulose by dilute sulfuric acid hydrolysis have been developed to analyze the effects on reactor performance of heat and mass transfer as well as reaction kinetics. An economic objective function representing the incremental cost of producing a 10% xylose solution for fermentation to ethanol was calculated from the results of the reactor simulations to identify the operating conditions that minimize production costs for each reactor type. Lower production costs were estimated for percolation and continuous counter-current reactors; the cost for xylose production in a continuous co-current reactor is significantly higher. Production of ethanol from hardwood hemicellulose is not economical with any of the reactors considered, but the models developed here may be used to analyze other process alternatives for use of hemicellulose via production of xylose.

Evaluation of D-xylose fermenting yeasts for utilization of a wood-derived hemicellulose hydrolysate

Seven of 37 strains of yeast, recently identified for their ability to ferment D-xylose to ethanol, produced this alcohol from crude hardwood hemicellulose hydrolysate in batch culture, with variable amounts of xylitol as a by-product. Of these, Candida tropicalis showed the greatest potential for ethanol production. The crude acid hydrolysate was found to be inhibitory to all of the yeasts, even at dilute hydrolysate concentrations. Significant improvements in the utilization of the substrate were achieved through both strain acclimatization and chemical pretreatment of the hydrolysate.

THE HEMICELLULOSE OF WHITE ELM (ULMUS AMERICANA) II. CONSTITUTION OF THE METHYL GLUCURONO-XYLAN

The wood of white elm (Ulmusamericana) has been found to contain less xylan than most other hardwoods. Partial hydrolysis of the hemicellulose component yielded an aldotriouronic acid, a glucuronosyl xylose, 2-O-(4-O-methyl-α-D-glucopyranosyluronic acid)-D-xylopyranose, 4-O-methyl-D-glucuronic acid, and galacturonic acid.Hydrolysis of the fully methylated polysaccharide gave a mixture of 2-O- and 3-O-methyl-D-xylose, 2,3-di-O-methyl-D-xylose, 2,3,4-tri-O-methyl-D-xylose, and 2-O-(2,3,4-tri-O-methyl-α-D-glucopyranosyluronic acid)-3-O-methyl-D-xylopyranose in a molar ratio of 6:124:1:14. The number-average degree of polymerization of the methylated hemicellulose was 133 and that of the native polymer 185. On the basis of this and other evidence it is suggested that the xylan is linear and contains 185 1,4-linked β-D-xylopyranose residues, every seventh of which carries a single terminal side chain of 4-O-methyl-D-glucuronic acid attached by an α-glycosidic bond through C2 of the xylose. The number of acid groups is larger than in other hardwood hemicelluloses so far studied.