Fuel ethanol production from corn fiber current status and technical prospects

The Xylose Metabolizing Yeast Spathaspora passalidarum is a Promising Genetic Treasure for Improving Bioethanol Production

Currently, the fermentation technology for recycling agriculture waste for generation of alternative renewable biofuels is getting more and more attention because of the environmental merits of biofuels for decreasing the rapid rise of greenhouse gas effects compared to petrochemical, keeping in mind the increase of petrol cost and the exhaustion of limited petroleum resources. One of widely used biofuels is bioethanol, and the use of yeasts for commercial fermentation of cellulosic and hemicellulosic agricultural biomasses is one of the growing biotechnological trends for bioethanol production. Effective fermentation and assimilation of xylose, the major pentose sugar element of plant cell walls and the second most abundant carbohydrate, is a bottleneck step towards a robust biofuel production from agricultural waste materials. Hence, several attempts were implemented to engineer the conventional Saccharomyces cerevisiae yeast to transport and ferment xylose because naturally it does not use xylose, using genetic materials of Pichia stipitis, the pioneer native xylose fermenting yeast. Recently, the nonconventional yeast Spathaspora passalidarum appeared as a founder member of a new small group of yeasts that, like Pichia stipitis, can utilize and ferment xylose. Therefore, the understanding of the molecular mechanisms regulating the xylose assimilation in such pentose fermenting yeasts will enable us to eliminate the obstacles in the biofuels pipeline, and to develop industrial strains by means of genetic engineering to increase the availability of renewable biofuel products from agricultural biomass. In this review, we will highlight the recent advances in the field of native xylose metabolizing yeasts, with special emphasis on S. passalidarum for improving bioethanol production.

Identification and characterization of GH11 xylanase and GH43 xylosidase from the chytridiomycetous fungus, Rhizophlyctis rosea

The early-lineage, aerobic, zoosporic fungi from the Chytridiomycota constitute less than 1% of the described fungi and can use diverse sources of nutrition from plant or animal products. One of the ancestral sources of fungal nutrition could be products following enzymatic degradation of plant material. However, carbohydrate-active enzymes from these ancient fungi have been less studied. A GH11 xylanase (RrXyn11A) (EC 3.2.1.8) and a GH43 xylosidase (RrXyl43A) (EC 3.2.1.37) were identified from an early-lineage aerobic zoosporic fungus, Rhizophlyctis rosea NBRC 105426. Both genes were heterologously expressed in Pichia pastoris and the recombinant enzymes were purified and characterized. The optimal pH for recombinant RrXyn11A and RrXyl43A was pH 7. RrXyn11A had high stability over a wide range of pH (4–8) and temperature (25–70 °C). RrXyn11A also showed high substrate specificity on both azurine-cross-linked (AZCL) arabinoxylan and AZCL xylan. RrXyl43A had β-xylosidase and minor α-l-arabinofuranosidase activity. This enzyme showed low product inhibition and retained 51% activity in the presence of 100 mM xylose. A combination of RrXyn11A and RrXyl43A exhibited significantly higher hydrolytic and polymer degradation capability and xylose release on wheat bran and beechwood xylan compared to treatment with commercial enzymes. This study was the first to heterologously express and characterize the GH11 xylanase (RrXyn11A) and GH43 xylosidase (RrXyl43A) from the ancient fungus, R. rosea. Meanwhile, this study also demonstrated that the enzymes from the ancient fungus R. rosea can be easily handled and heterologously expressed in Pichia, which presents a promising path to a new source of enzymes for biomass degradation.

Cellulose conversion of corn pericarp without pretreatment

We report enzyme hydrolysis of cellulose in unpretreated pericarp at a cellulase loading of 0.25FPU/g pericarp solids using a phenol tolerant Aspergillus niger pectinase preparation. The overall protein added was 5mg/g and gave 98% cellulose conversion in 72h. However, for double the amount of enzyme from Trichoderma reesei, which is significantly less tolerant to phenols, conversion was only 16%. The key to achieving high conversion without pretreatment is combining phenol inhibition-resistant enzymes (such as from A. niger) with unground pericarp from which release of phenols is minimal. Size reduction of the pericarp, which is typically carried out in a corn-to-ethanol process, where corn is first ground to a fine powder, causes release of highly inhibitory phenols that interfere with cellulase enzyme activity. This work demonstrates hydrolysis without pretreatment of large particulate pericarp is a viable pathway for directly producing cellulose ethanol in corn ethanol plants.

Towards enzymatic breakdown of complex plant xylan structures: State of the art

Significant progress over the past few years has been achieved in the enzymology of microbial degradation and saccharification of plant xylan, after cellulose being the most abundant natural renewable polysaccharide. Several new types of xylan depolymerizing and debranching enzymes have been described in microorganisms. Despite the increasing variety of known glycoside hydrolases and carbohydrate esterases, some xylan structures still appear quite recalcitrant. This review focuses on the mode of action of different types of depolymerizing endoxylanases and their cooperation with β-xylosidase and accessory enzymes in breakdown of complex highly branched xylan structures. Emphasis is placed on the enzymatic hydrolysis of alkali-extracted deesterified polysaccharide as well as acetylated xylan isolated from plant cell walls under non-alkaline conditions. It is also shown how the combination of selected endoxylanases and debranching enzymes can determine the nature of prebiotic xylooligosaccharides or lead to complete hydrolysis of the polysaccharide. The article also highlights the possibility for discovery of novel xylanolytic enzymes, construction of multifunctional chimeric enzymes and xylanosomes in parallel with increasing knowledge on the fine structure of the polysaccharide.

Lipid production by Cryptococcus curvatus on hydrolysates derived from corn fiber and sweet sorghum bagasse following dilute acid pretreatment

Corn fiber and sweet sorghum bagasse (SSB) are both pre-processed lignocellulosic materials that can be used to produce liquid biofuels. Pretreatment using dilute sulfuric acid at a severity factor of 1.06 and 1.02 released 83.2 and 86.5 % of theoretically available sugars out of corn fiber and SSB, respectively. The resulting hydrolysates derived from pretreatment of SSB at SF of 1.02 supported growth of Cryptococcus curvatus well. In 6 days, the dry cell density reached 10.8 g/l with a lipid content of 40 % (w/w). Hydrolysates from corn fiber, however, did not lead to any significant cell growth even with addition of nutrients. In addition to consuming glucose, xylose, and arabinose, C. curvatus also utilized formic acid, acetic acid, 4-hydroxymethylfurfural, and levulinic acid for growth. Thus, C. curvatus appeared to be an excellent yeast strain for producing lipids from hydrolysates developed from lignocellulosic feedstocks.

Ethanol production via in situ fungal saccharification and fermentation of mild alkali and steam pretreated corn fiber

The effect of mild alkali and steam pretreatments on fungal saccharification and sequential simultaneous-saccharification and fermentation (SSF) of corn fiber to ethanol was studied. The corn fiber was pretreated with: (i) 2% NaOH (w/w) at 30 degrees C for 2h and (ii) steaming at 100 degrees C for 2h. Ethanol yields were 2.6g, 2.9g and 5.5g ethanol/100g of corn fiber, respectively, for Phanerochaete chrysosporium, Gloeophyllum trabeum and Trichoderma reesei saccharification and sequential SSFs. SSF with commercial cellulase enzyme - Spezyme-CP had 7.7g ethanol/100g corn fiber. Mild alkali pretreatment resulted in higher glucose yields following fungal saccharification of corn fiber. However, the ethanol yields were comparatively similar for untreated and mild alkali pretreated corn fiber. Solid-substrate fermentation of corn fiber with fungi can be improved to either eliminate or reduce the dosage of commercial cellulase enzymes during SSF.

Enzymatic hydrolysis of cellulosic municipal wastewater treatment process residuals as feedstocks for the recovery of simple sugars

This study examined the hydrolysis of lignocellulose extracted from municipal wastewater treatment process residuals for the purpose of investigating low-cost feedstocks for ethanol production, while providing an alternative solid waste management strategy. Primary and thickened waste activated sludges and anaerobically digested biosolids underwent various pre-treatments to enhance subsequent enzymatic hydrolysis. Half of the pre-treated samples were dried and grinded, while the other half were used as is (wet). The wet primary sludge yielded the highest reducing sugar conversions. When wet primary sludge without pre-treatment was hydrolyzed at 40 degrees C and an enzyme loading of 800 U/g substrate, 31.1+/-2.7% was converted to reducing sugars in 24 h. This increased to 54.2+/-4.0% when HCl and KOH pre-treatments were applied. FTIR analyses were used to examine differences in the sludge compositions. These indicated that the cellulose content in the primary sludge was higher than that of the thickened waste activated sludge and biosolids, which was consistent with the higher reducing sugar yields observed in the primary sludge.

Improved Bioethanol Production Using Activated Carbon-treated Acid Hydrolysate from Corn Hull in Pachysolen tannophilus

To optimally convert corn hull, a byproduct from corn processing, into bioethanol using Pachysolen tannophlius, we investigated the optimal conditions for hydrolysis and removal of toxic substances in the hydrolysate via activated carbon treatment as well as the effects of this detoxification process on the kinetic parameters of bioethanol production. Maximum monosaccharide concentrations were obtained in hydrolysates in which 20 g of corn hull was hydrolyzed in 4% (v/v) H2SO4. Activated carbon treatment removed 92.3% of phenolic compounds from the hydrolysate. When untreated hydrolysate was used, the monosaccharides were not completely consumed, even at 480 h of culture. When activated carbon-treated hydrolysate was used, the monosaccharides were mostly consumed at 192 h of culture. In particular, when activated carbon-treated hydrolysate was used, bioethanol productivity (P) and specific bioethanol production rate (Qp) were 2.4 times and 3.4 times greater, respectively, compared to untreated hydrolysate. This was due to sustained bioethanol production during the period of xylose/arabinose utilization, which occurred only when activated carbon-treated hydrolysate was used.

Fermentative production of succinic acid from straw hydrolysate by Actinobacillus succinogenes

In this work, straw hydrolysates were used to produce succinic acid by Actinobacillus succinogenes CGMCC1593 for the first time. Results indicated that both glucose and xylose in the straw hydrolysates were utilized in succinic acid production, and the hydrolysates of corn straw was better than that of rice or wheat straw in anaerobic fermentation of succinic acid. However, cell growth and succinic acid production were inhibited when the initial concentration of sugar, which was from corn straw hydrolysate (CSH), was higher than 60 g l(-1). In batch fermentation, 45.5 g l(-1) succinic acid concentration and 80.7% yield were attained after 48 h incubation with 58 g l(-1) of initial sugar from corn straw hydrolysate in a 5-l stirred bioreactor. While in fed-batch fermentation, concentration of succinic acid achieved 53.2 g l(-1) at a rate of 1.21 g l(-1) h(-1) after 44 h of fermentation. Our work suggested that corn straw could be utilized for the economical production of succinic acid by A. succinogenes.

Dilute Acid Pretreatment, Enzymatic Saccharification, and Fermentation of Rice Hulls to Ethanol

Rice hulls, a complex lignocellulosic material with high lignin (15.38 +/- 0.2%) and ash (18.71 +/- 0.01%) content, contain 35.62 +/- 0.12% cellulose and 11.96 +/- 0.73% hemicellulose and has the potential to serve as a low-cost feedstock for production of ethanol. Dilute H2SO4 pretreatments at varied temperature (120-190 degrees C) and enzymatic saccharification (45 degrees C, pH 5.0) were evaluated for conversion of rice hull cellulose and hemicellulose to monomeric sugars. The maximum yield of monomeric sugars from rice hulls (15%, w/v) by dilute H2SO4 (1.0%, v/v) pretreatment and enzymatic saccharification (45 degrees C, pH 5.0, 72 h) using cellulase, beta-glucosidase, xylanase, esterase, and Tween 20 was 287 +/- 3 mg/g (60% yield based on total carbohydrate content). Under this condition, no furfural and hydroxymethyl furfural were produced. The yield of ethanol per L by the mixed sugar utilizing recombinant Escherichia colistrain FBR 5 from rice hull hydrolyzate containing 43.6 +/- 3.0 g fermentable sugars (glucose, 18.2 +/- 1.4 g; xylose, 21.4 +/- 1.1 g; arabinose, 2.4 +/- 0.3 g; galactose, 1.6 +/- 0.2 g) was 18.7 +/- 0.6 g (0.43 +/- 0.02 g/g sugars obtained; 0.13 +/- 0.01 g/g rice hulls) at pH 6.5 and 35 degrees C. Detoxification of the acid- and enzyme-treated rice hull hydrolyzate by overliming (pH 10.5, 90 degrees C, 30 min) reduced the time required for maximum ethanol production (17 +/- 0.2 g from 42.0 +/- 0.7 g sugars per L) by the E. coli strain from 64 to 39 h in the case of separate hydrolysis and fermentation and increased the maximum ethanol yield (per L) from 7.1 +/- 2.3 g in 140 h to 9.1 +/- 0.7 g in 112 h in the case of simultaneous saccharification and fermentation.

Contaminant occurrence, identification and control in a pilot-scale corn fiber to ethanol conversion process

While interest in bioethanol production from lignocellulosic feedstocks is increasing, there is still relatively little pilot-plant data and operating experience available for this emerging industry. A series of batch and continuous fermentation runs were performed in a pilot-plant, some lasting up to six weeks, in which corn fiber-derived sugars were fermented to ethanol using glucose-fermenting and recombinant glucose/xylose-fermenting yeasts. However, contamination by Lactobacillus bacteria was a common occurrence during these runs. These contaminating microorganisms were found to readily consume arabinose, a sugar not utilized by the yeast, producing acetic and lactic acids that had a detrimental effect on fermentation performance. The infections were ultimately controlled with the antibiotic virginiamycin, but routine use of antibiotics is cost prohibitive. The severity of the problem encountered during this work is probably due to use of a highly contaminated feedstock. Lignocellulosic conversion facilities will not employ aseptic designs. Instead, techniques similar to those employed in the corn-based fuel ethanol industry to control infections will be used. Effective control may also be possible by using fermentative microorganisms that consume all biomass-derived sugars.

Compositional analysis of lignocellulosic materials: evaluation of methods used for sugar analysis of waste paper and straw

To determine the overall efficiency of processes designed to convert lignocellulosic polysaccharides to ethanol, it is first necessary to determine the composition of the lignocellulosic substrates. Three standard methods routinely referenced in the literature for this purpose are monoethanolamine, trifluoroacetic acid and concentrated sulphuric acid-based methods. However, in the course of our studies, the suitability of these standard methods for analysis of wastepaper and wheat straw came into question. This paper details our investigations in this area, together with recommendations for appropriate modifications to one of the standard methods for reproducible and representative lignocellulosic compositional analysis of waste paper and cereal straw.

Enriched arabinoxylan in corn fiber for value-added products

A two-step process is evaluated to separate the hexose component in wet milling corn fibers from the pentose component for production of value-added products. Corn fibers were first pretreated with hot water at 121 degrees C for 1 h followed by glucoamylase hydrolysis to remove starch. The remaining solid was then treated with hot water at 140-170 degrees C followed by an enzymatic hydrolysis to further separate the hexose and pentose components. After the second pretreatment, the enzymatic digestibility of cellulose was much better than that of arabinoxylan. As a result, up to 90% arabinoxylan in corn fibers was retained in a solid form after the enzyme hydrolysis, while most of the hexose components were removed.

Genome sequence of the lignocellulose-bioconverting and xylose-fermenting yeast Pichia stipitis

Xylose is a major constituent of plant lignocellulose, and its fermentation is important for the bioconversion of plant biomass to fuels and chemicals. Pichia stipitis is a well-studied, native xylose-fermenting yeast. The mechanism and regulation of xylose metabolism in P. stipitis have been characterized and genes from P. stipitis have been used to engineer xylose metabolism in Saccharomyces cerevisiae. We have sequenced and assembled the complete genome of P. stipitis. The sequence data have revealed unusual aspects of genome organization, numerous genes for bioconversion, a preliminary insight into regulation of central metabolic pathways and several examples of colocalized genes with related functions. The genome sequence provides insight into how P. stipitis regulates its redox balance while very efficiently fermenting xylose under microaerobic conditions.

The influence of aeration and hemicellulosic sugars on xylitol production by Candida tropicalis

The influence of other hemicellulosic sugars (arabinose, galactose, mannose and glucose), oxygen limitation, and initial xylose concentration on the fermentation of xylose to xylitol was investigated using experimental design methodology. Oxygen limitation and initial xylose concentration had considerable influences on xylitol production by Canadida tropicalis ATCC 96745. Under semiaerobic conditions, the maximum xylitol yield was 0.62 g/g substrate, while under aerobic conditions, the maximum volumetric productivity was 0.90 g/l h. In the presence of glucose, xylose utilization was strongly repressed and sequential sugar utilization was observed. Ethanol produced from the glucose caused 50% reduction in xylitol yield when its concentration exceeded 30 g/l. When complex synthetic hemicellulosic sugars were fermented, glucose was initially consumed followed by a simultaneous uptake of the other sugars. The maximum xylitol yield (0.84 g/g) and volumetric productivity (0.49 g/l h) were obtained for substrates containing high arabinose and low glucose and mannose contents.

α-l-Arabinofuranosidases

Interest in the alpha-L-arabinofuranosidases has increased in recent years because of their application in the conversion of various hemicellulosic substrates to fermentable sugars for subsequent production of fuel alcohol. Xylanases, in conjunction with alpha-L-arabinofuranosidases and other accessory enzymes, act synergistically to degrade xylan to component sugars. The induction of alpha-L-arabinofuranosidase production, physico-chemical characteristics, substrate specificity, and molecular biology of the enzyme are described. The current state of research and development of the arabinofuranosidases and their role in biotechnology are presented.

Ethanol production from renewable resources

Vast amounts of renewable biomass are available for conversion to liquid fuel, ethanol. In order to convert biomass to ethanol, the efficient utilization of both cellulose-derived and hemicellulose-derived carbohydrates is essential. Six-carbon sugars are readily utilized for this purpose. Pentoses, on the other hand, are more difficult to convert. Several metabolic factors limit the efficient utilization of pentoses (xylose and arabinose). Recent developments in the improvement of microbial cultures provide the versatility of conversion of both hexoses and pentoses to ethanol more efficiently. In addition, novel bioprocess technologies offer a promising prospective for the efficient conversion of biomass and recovery of ethanol.