Role of pretreatment and conditioning processes on toxicity of lignocellulosic biomass hydrolysates

Abstract

The Department of Energy's Office of the Biomass Program has set goals of making ethanol cost competitive by 2012 and replacing 30% of 2004 transportation supply with biofuels by 2030. Both goals require improvements in conversions of cellulosic biomass to sugars as well as improvements in fermentation rates and yields. Current best pretreatment processes are reasonably efficient at making the cellulose/hemicellulose/lignin matrix amenable to enzymatic hydrolysis and fermentation, but they release a number of toxic compounds into the hydrolysate which inhibit the growth and ethanol productivity of fermentation organisms. Conditioning methods designed to reduce the toxicity of hydrolysates are effective, but add to process costs and tend to reduce sugar yields, thus adding significantly to the final cost of production. Reducing the cost of cellulosic ethanol production will likely require enhanced understanding of the source and mode of action of hydrolysate toxic compounds, the means by which some organisms resist the actions of these compounds, and the methodology and mechanisms for conditioning hydrolysate to reduce toxicity. This review will provide an update on the state of knowledge in these areas and can provide insights useful for the crafting of hypotheses for improvements in pretreatment, conditioning, and fermentation organisms.

Ethanol Production Efficiency of an Anaerobic Hemicellulolytic Thermophilic Bacterium, Strain NTOU1, Isolated from a Marine Shallow Hydrothermal Vent in Taiwan

“…Inhibitory compounds released from lignocellulose after acid hydrolysis could limit efficient utilization of the hydrolysates for ethanol production by fermentation (12,27,30,41). In our case probably because combined concentrations of glucose and arabinose were less than 0.1%, and concentrations of 5-HMF and acetate in the hemicellulose hydrolysate were less than the effective concentration, ethanol production by the hemicellulose hydrolysate containing 0.5% xylose was not affected by the inhibitors or other sugars.…”

“…Dilute acid hydrolysis is a widely used pretreatment, but in addition to a mixture of pentoses, hexoses, and uronic acids, a wide range of phenolic compounds such as furfural, 5-hydroxymethyl furfural (5-HMF), inhibitory to microorganisms are formed or released during the process (27).…”

Chemical Pretreatments of Wood Chips Prior to Alkaline Pulping - A Review of Pretreatment Alternatives, Chemical Aspects of the Resulting Liquors, and Pulping Outcomes

“…As many of these compounds are potentially inhibitory during fermentation processes and because the purity requirements of XOs used in functional foods are high, these components must be fractionated from the hydrolysates. Fractionation can be performed by various common laboratory techniques, such as liquidliquid extraction performed with suitable organic solvents (i.e., ethyl acetate, diethyl ether, various alcohols, and alkenes) or with various adsorption resins (Converti et al 2000;Klinke et al 2004;Zautsen et al 2009;Pienkos and Zhang 2009;Soto et al 2011). However, the non-carbohydrate phenolic component fraction has been shown to contain certain beneficial properties, including antioxidant, antimicrobial, and biological activities, which makes them potentially suitable for food or cosmetic applications (Barclay et al 1997;Lu et al 1998;Cruz et al 2001;Garrote et al 2004aGarrote et al , 2007González et al 2004;Dong et al 2011;Soto et al 2011).…”

Olive tree pruning as an agricultural residue for ethanol production. Fermentation of hydrolysates from dilute acid pretreatment

“…The time evolution of acetic acid, formic acid, furfural and HMF was followed throughout the fermentation experiments because all of them have been described as potential inhibitors of yeast growth (Pienkos & Zhang, 2009). As an example, Fig.…”

“…For example, in addition to sugars, hydrolysates contain sugar degradation products and lignin-derived products, some of which (e.g., furfural, hydroxymethylfurfural [HMF], etc.) have been described as potentially inhibitors for yeast growth (Cara et al, 2008;Pienkos & Zhang, 2009). Sometimes, a detoxification step must be included, for eliminating or reducing inhibitory compounds.…”