Pretreatment of sugar cane bagasse hemicellulose hydrolyzate for ethanol production by yeast

Detoxification of a pyrolytic aqueous condensate from wheat straw for utilization as substrate in Aspergillus oryzae DSM 1863 cultivations

BACKGROUND

The pyrolytic aqueous condensate (PAC) formed during the fast pyrolysis of wheat straw contains a variety of organic carbons and might therefore potentially serve as an inexpensive substrate for microbial growth. One of its main components is acetic acid, which was recently shown to be a suitable carbon source for the filamentous fungus Aspergillus oryzae. However, the condensate also contains numerous toxic compounds that inhibit fungal growth and result in a tolerance of only about 1%. Therefore, to enable the use of the PAC as sole substrate for A. oryzae cultivations, a pretreatment seems to be necessary.

RESULTS

Various conditions for treatments with activated carbon, overliming, rotary evaporation and laccase were evaluated regarding fungal growth and the content of inhibitory model substances. Whereas the first three methods considerably increased the fungal tolerance to up to 1.625%, 12.5% and 30%, respectively, the enzymatic treatment did not result in any improvement. The optimum carbon load for the treatment with activated carbon was identified to be 10% (w/v) and overliming should ideally be performed at 100 °C and an initial pH of 12. The best detoxification results were achieved with rotary evaporation at 200 mbar as a complete removal of guaiacol and a strong reduction in the concentration of acetol, furfural, 2-cyclopenten-1-one and phenol by 84.9%, 95.4%, 97.7% and 86.2%, respectively, were observed. Subsequently, all possible combinations of the effective single methods were performed and rotary evaporation followed by overliming and activated carbon treatment proved to be most efficient as it enabled growth in 100% PAC shake-flask cultures and resulted in a maximum cell dry weight of 5.21 ± 0.46 g/L.

CONCLUSION

This study provides a comprehensive insight into the detoxification efficiency of a variety of treatment methods at multiple conditions. It was revealed that with a suitable combination of these methods, PAC toxicity can be reduced to such an extent that growth on pure condensate is possible. This can be considered as a first important step towards a microbial valorization of the pyrolytic side-stream with A. oryzae.

Effect of overliming and activated carbon detoxification on inhibitors removal and butanol fermentation of poplar prehydrolysates

BACKGROUND

Biomass prehydrolysates from dilute acid pretreatment contain a considerable amount of fermentable sugars for biofuels production. However, carbonyl degradation compounds present severe toxicity to fermentation microbes. Furans (such as furfural and hydroxymethylfurfural), aliphatic acids (such as acetic acid, formic acid and levulinic acid) and phenolic compounds (such as vanillin and syringaldehyde) have been suggested to be the main inhibitors in biomass prehydrolysates. However, no single compound has been determined as the dominant toxic inhibitor. The effects of various detoxification methods on inhibitors removal have not been fully understood.

RESULTS

The effects of overliming and activated carbon (AC) detoxification on the removal of inhibitors and butanol fermentation of the poplar prehydrolysates were investigated. Gas chromatography-mass spectrometry (GC/MS) was used to identify and quantify 46 carbonyl compounds as potential inhibitors. It was observed that overliming and AC treatment alone did not make the prehydrolysates fermentable with Clostridium saccharobutylicum. The sequential overliming and AC resulted in a remarkable fermentability and a high butanol yield at 0.22 g g-1 sugar. The inhibitor removal in the prehydrolysates treated by overliming and AC was also examined by GC/MS. Overliming removed 75.6% of furan derivatives and 68.1% of aromatic monomers. In comparison, AC (5.0% w/v) removed 77.9% of furan derivatives and 98.6% of aromatic monomers. In addition, overliming removed much more 2,5-furandicarboxyaldehyde, 5-ethylfuran-2-carbaldehyde and 2,5-hexanedione than AC did. On the contrary, AC could remove considerably more phenolic acids than overliming. In the sequential detoxification, both dialdehydes/diketones and phenolic acids were extensively removed. This could be the main reason why the sequential detoxification enabled a remarkable ABE fermentation for the prehydrolysates.

CONCLUSIONS

This study indicated that the effect of overliming and AC treatment on inhibitors removal was related to their chemical structures. Overliming removed more dialdehydes and diketones than AC treatment, while AC removed more phenolic acids than overliming. Sequential overliming and AC treatment were required to make the prehydrolysates fermentable with C. saccharobutylicum. The study also suggested different detoxification method was needed for ABE fermentation of the prehydrolysate as compared to ethanol fermentation.

Analysis of biodegradation performance of furfural and 5-hydroxymethylfurfural by Amorphotheca resinae ZN1

BACKGROUND

Furfural and 5-hydroxymethylfurfural (HMF) are the degradation products of lignocellulose during pretreatment operations and significantly inhibit the consequent enzymatic hydrolysis and fermentation processes. The biodetoxification fungus Amorphotheca resinae ZN1 had demonstrated its excellent capacity on degrading lignocellulose derived inhibitors and helped the fermentation processes to achieve high yield of ethanol and biochemicals. Analysis of the biological degradation performance of furfural and HMF by A. resinae ZN1 will provide essential information for their fast and complete removal from the pretreated lignocellulose materials and facilitate the consequent ethanol fermentation.

RESULTS

The degradation performance of furfural and HMF by A. resinae ZN1 was investigated by capturing intermediate metabolic products at various culture conditions. A. resinae ZN1 converts furfural/HMF into furfuryl/HMF alcohols and furoic/HMF acids simultaneously at aerobic condition, and only the corresponding furfuryl/HMF alcohols are obtained at anaerobic condition. The existence of glucose accelerates the degradation rate of furfural and HMF by A. resinae ZN1 and the cell mass growth rate aerobically. Remarkably, glucose is not consumed before furfural or HMF is degraded to a low threshold concentration. The finding suggests that furfural or HMF has a substrate priority of utilization by A. resinae ZN1 than glucose. This property may help the detoxification of furfural and HMF to be operated without consuming glucose.

CONCLUSIONS

The biological degradation performance of furfural and HMF by A. resinae ZN1 was investigated experimentally. Oxygen supply is important on the complete biodegradation of furfural and HMF by A. resinae ZN1. Furfural or HMF has the priority of substrate utilization than glucose by A. resinae ZN1. This study provided important information for detoxification enhancement and strain modification.

Detoxification and fermentation of pyrolytic sugar for ethanol production

The sugars present in bio-oil produced by fast pyrolysis can potentially be fermented by microbial organisms to produce cellulosic ethanol. This study shows the potential for microbial digestion of the aqueous fraction of bio-oil in an enrichment medium to consume glucose and produce ethanol. In addition to glucose, inhibitors such as furans and phenols are present in the bio-oil. A pure glucose enrichment medium of 20 g/l was used as a standard to compare with glucose and aqueous fraction mixtures for digestion. Thirty percent by volume of aqueous fraction in media was the maximum additive amount that could be consumed and converted to ethanol. Inhibitors were removed by extraction, activated carbon, air stripping, and microbial methods. After economic analysis, the cost of ethanol using an inexpensive fermentation medium in a large scale plant is approximately $14 per gallon.

Effects of pretreatment methods for hazelnut shell hydrolysate fermentation with Pichia Stipitis to ethanol

In this study, we investigated the use of hazelnut shell as a renewable and low cost lignocellulosic material for bioethanol production for the first time. High lignin content of hazelnut shell is an important obstacle for such a biotransformation. Biomass hydrolysis with acids yields reducing sugar with several inhibitors which limit the fermentability of sugars. The various conditioning methods for biomass and hydrolysate were performed to overcome the toxicity and their effects on the subsequent fermentation of hazelnut shell hydrolysate by Pichia stipitis were evaluated with shaking flasks experiments. Hazelnut shells hydrolysis with 0.7M H(2)SO(4) yielded 49 gl(-1) total reducing sugars and fermentation inhibitors in untreated hydrolysate. First, it was shown that several hydrolysate detoxification methods were solely inefficient in achieving cell growth and ethanol production in the fermentation of hazelnut shell hydrolysates derived from non-delignified biomass. Next, different pretreatments of hazelnut shells were considered for delignification and employed before hydrolysis in conjunction with hydrolysate detoxification to improve alcohol fermentation. Among six delignification methods, the most effective pretreatment regarding to ethanol concentration includes the treatment of shells with 3% (w/v) NaOH at room temperature, which was integrated with sequential hydrolysate detoxification by overliming and then treatment with charcoal twice at 60 degrees C. This treatment brought about a total reduction of 97% in furans and 88.4% in phenolics. Almost all trialed treatments caused significant sugar loss. Under the best assayed conditions, ethanol concentration of 16.79gl(-1) was reached from a hazelnut shell hyrolysate containing initial 50g total reducing sugar l(-1) after partial synthetic xylose supplementation. This value is equal to 91.25% of ethanol concentration that was obtained from synthetic d-xylose under same conditions. The present study demonstrates that Pichia stipitis is able to grow and ferment sugars to ethanol in detoxified hazelnut hydrolysate derived from delignified biomass.

Optimizing dilute-acid pretreatment of rapeseed straw for extraction of hemicellulose

Biological conversion of biomass into fuels and chemicals requires hydrolysis of the polysaccharide fraction into monomeric sugars prior to fermentation. Hydrolysis can be performed enzymatically or with mineral acids. In this study, dilute sulfuric acid was used as a catalyst for the pretreatment of rapeseed straw. The purpose of this study is to optimize the pretreatment process in a 15-mL bomb tube reactor and investigate the effects of the acid concentration, temperature, and reaction time. These parameters influence hemicellulose removal and production of sugars (xylose, glucose, and arabinose) in the hydrolyzate as well as the formation of by-products (furfural, 5-hydroxymethylfurfural, and acetic acid). Statistical analysis was based on a model composition corresponding to a 3(3) orthogonal factorial design and employed the response surface methodology to optimize the pretreatment conditions, aiming to attain maximum xylan, mannan, and galactan (XMG) extraction from hemicellulose of rapeseed straw. The obtained optimum conditions were: H2SO4 concentration of 1.76% and temperature of 152.6 degrees C with a reaction time of 21 min. Under these optimal conditions, 85.5% of the total sugar was recovered after acid hydrolysis (78.9% XMG and 6.6% glucan). The hydrolyzate contained 1.60 g/L glucose, 0.61 g/L arabinose, 10.49 g/L xylose, mannose, and galactose, 0.39 g/L cellobiose, 0.94 g/L fructose, 0.02 g/L 1,6-anhydro-glucose, 1.17 g/L formic acid, 2.94 g/L acetic acid, 0.04 g/L levulinic acid, 0.04 g/L 5-hydroxymethylfurfural, and 0.98 g/L furfural.

Complex media from processing of agricultural crops for microbial fermentation

This mini-review describes the concept of the green biorefinery and lists a number of suitable agricultural by-products, which can be used for production of bioenergy and/or biochemicals. A process, in which one possible agricultural by-product from the green crop drying industry, brown juice, is converted to a basic, universal fermentation medium by lactic acid fermentation, is outlined. The resulting all-round fermentation medium can be used for the production of many useful fermentation products when added a carbohydrate source, which could possibly be another agricultural by-product. Two examples of such products-polylactic acid and L-lysine-are given. A cost calculation shows that this fermentation medium can be produced at a very low cost approximately 1.7 Euro cent/kg, when taking into account that the green crop industry has expenses amounting to 270,000 Euro/year for disposal of the brown juice. A newly built lysine factory in Esbjerg, Denmark, can benefit from this process by buying a low price medium for the fermentation process instead of more expensive traditional fermentation liquids such as corn steep liquor.

Inhibition of ethanol-producing yeast and bacteria by degradation products produced during pre-treatment of biomass

An overview of the different inhibitors formed by pre-treatment of lignocellulosic materials and their inhibition of ethanol production in yeast and bacteria is given. Different high temperature physical pre-treatment methods are available to render the carbohydrates in lignocellulose accessible for ethanol fermentation. The resulting hydrolyzsates contain substances inhibitory to fermentation-depending on both the raw material (biomass) and the pre-treatment applied. An overview of the inhibitory effect on ethanol production by yeast and bacteria is presented. Apart from furans formed by sugar degradation, phenol monomers from lignin degradation are important co-factors in hydrolysate inhibition, and inhibitory effects of these aromatic compounds on different ethanol producing microorganisms is reviewed. The furans and phenols generally inhibited growth and ethanol production rate (Q(EtOH)) but not the ethanol yields (Y(EtOH)) in Saccharomyces cerevisiae. Within the same phenol functional group (aldehyde, ketone, and acid) the inhibition of volumetric ethanol productivity was found to depend on the amount of methoxyl substituents and hence hydrophobicity (log P). Many pentose-utilizing strains Escherichia coli, Pichia stipititis, and Zymomonas mobilis produce ethanol in concentrated hemicellulose liquors but detoxification by overliming is needed. Thermoanaerobacter mathranii A3M3 can grow on pentoses and produce ethanol in hydrolysate without any need for detoxification.

Effect of wood ash treatment on improving the fermentability of wood hydrolysate

Softwood hydrolysates were overlimed with wood ash to improve the fermentability of hydrolysates. It could be demonstrated in fermentation tests that wood ash treatment increases fermentability compared to the hydrolysates untreated and treated with alkaline compounds such as Ca(OH)(2), NaOH, and KOH, which are commonly used for overliming. The enhanced fermentability of the hydrolysate treated with wood ash is due to the reduction of the inhibitors of the fermentation such as furan and phenolic compounds and to nutrient effects of some inorganic components from the wood ash on the fermentation.

The effect of overliming on the toxicity of dilute acid pretreated lignocellulosics: the role of inorganics, uronic acids and ether-soluble organics

Although the treatment of dilute acid pretreated lignocellulosics with calcium hydroxide or carbonate (overliming) is known to improve the fermentability of carbohydrate-rich hydrolyzate streams, a firm understanding of the chemistry behind the process is lacking. Quantitative evaluation of inorganics, uronic acids, and non-polar organics indicates that only a portion of the improvement can be ascribed to these materials. Upon overliming the concentrations of inorganics either increase (Ca, Mg), decrease (Fe, P, Zn, K) or remain relatively the same (Al, Na). Furthermore, organic compounds that are not extractable with tert-butyl methyl ether (MTBE) are toxic to Zymomonas mobilis CP4(pZB5). Overliming and direct neutralization are somewhat effective in removing sulfate anions, although sulfate toxicity is considerably less than that of acetic acid. Uronic acids were found to be non-toxic under pH controlled conditions.

Pretreatment of sugar cane bagasse hemicellulose hydrolysate for xylitol production by yeast

A total of six known xylitol-producing yeast strains were screened for production of xylitol from xylose. Candida sp. 11-2 proved to be the best producer. It was chosen to study its ability to produce xylitol from hemicellulose hydrolysate derived from sugar cane bagasse. The hydrolysate was prepared by dilute sulfuric acid (2-3% [w/v]) hydrolysis, with a high-solid, low-liquid ratio followed by leaching. Owing to the inhibitors present in the hydrolysate, different treatments were studied to overcome its effect. In order to reach higher xylitol productivity, treated hydrolysates were concentrated by vacuum evaporation in rotavapor to provide a higher initial xylose concentration. After treatment, Candida sp. 11-2 was able to ferment xylose in hemicellulose hydrolysate to produce xylitol.