Ammonium Hydroxide Detoxification of Spruce Acid Hydrolysates

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.

Chromosomal integration of aldo-keto-reductase and short-chain dehydrogenase/reductase genes in Clostridium beijerinckii NCIMB 8052 enhanced tolerance to lignocellulose-derived microbial inhibitory compounds

In situ detoxification of lignocellulose-derived microbial inhibitory compounds is an economical strategy for the fermentation of lignocellulose-derived sugars to fuels and chemicals. In this study, we investigated homologous integration and constitutive expression of Cbei_3974 and Cbei_3904, which encode aldo-keto reductase and previously annotated short chain dehydrogenase/reductase, respectively, in Clostridium beijerinckii NCIMB 8052 (Cb), resulting in two strains: Cb_3974 and Cb_3904. Expression of Cbei_3974 led to 2-fold increase in furfural detoxification relative to Cb_3904 and Cb_wild type. Correspondingly, butanol production was up to 1.2-fold greater in furfural-challenged cultures of Cb_3974 relative to Cb_3904 and Cb_wild type. With 4-hydroxybezaldehyde and syringaldehyde supplementation, Cb_3974 showed up to 2.4-fold increase in butanol concentration when compared to Cb_3904 and Cb_wild type. Syringic and vanillic acids were considerably less deleterious to all three strains of Cb tested. Overall, Cb_3974 showed greater tolerance to furfural, 4-hydroxybezaldehyde, and syringaldehyde with improved capacity for butanol production. Hence, development of Cb_3974 represents a significant progress towards engineering solventogenic Clostridium species that are tolerant to lignocellulosic biomass hydrolysates as substrates for ABE fermentation.

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.

Microbial Enzyme Production Using Lignocellulosic Food Industry Wastes as Feedstock: A Review

Enzymes are of great importance in the industry due to their substrate and product specificity, moderate reaction conditions, minimal by-product formation and high yield. They are important ingredients in several products and production processes. Up to 30% of the total production cost of enzymes is attributed to the raw materials costs. The food industry expels copious amounts of processing waste annually, which is mostly lignocellulosic in nature. Upon proper treatment, lignocellulose can replace conventional carbon sources in media preparations for industrial microbial processes, such as enzyme production. However, wild strains of microorganisms that produce industrially important enzymes show low yield and cannot thrive on artificial substrates. The application of recombinant DNA technology and metabolic engineering has enabled researchers to develop superior strains that can not only withstand harsh environmental conditions within a bioreactor but also ensure timely delivery of optimal results. This article gives an overview of the current complications encountered in enzyme production and how accumulating food processing waste can emerge as an environment-friendly and economically feasible solution for a choice of raw material. It also substantiates the latest techniques that have emerged in enzyme purification and recovery over the past four years.

Detoxification of biomass hydrolysates with nucleophilic amino acids enhances alcoholic fermentation

Carbonyl compounds generated in biomass pretreatment hinder the biochemical conversion of biomass hydrolysates to biofuels. A novel approach of detoxifying hydrolysates with amino acids for ethanol production was developed. Among the 20 amino acids assessed for their detoxification efficiency and nucleophilicity, cysteine was the most effective one. It increased both ethanol productivity and final yield of biomass hydrolysates from 0.18 (untreated) to 1.77 g/L/h and from 0.02 to 0.42 g/g, respectively. Detoxification efficiency was followed by histidine and it increased the final yield to 0.42 g/g, then by lysine, tryptophan and asparagine. It was observed all five effective amino acids contained reactive side-chain functional groups, which played important roles in the amino acid detoxification reaction. The study further showed cysteine and glycine detoxifications were temperature and pH dependent. The mechanistic study using mass spectrometry revealed thiazolidine carboxylic acid, a Schiff base, was formed by condensation of aldehyde and cysteine.

Techno-economics of integrating bioethanol production from spent sulfite liquor for reduction of greenhouse gas emissions from sulfite pulping mills

BACKGROUND

Flow sheet options for integrating ethanol production from spent sulfite liquor (SSL) into the acid-based sulfite pulping process at the Sappi Saiccor mill (Umkomaas, South Africa) were investigated, including options for generation of thermal and electrical energy from onsite bio-wastes, such as bark. Processes were simulated with Aspen Plus® for mass- and energy-balances, followed by an estimation of the economic viability and environmental impacts. Various concentration levels of the total dissolved solids in magnesium oxide-based SSL, which currently fuels a recovery boiler, prior to fermentation was considered, together with return of the fermentation residues (distillation bottoms) to the recovery boiler after ethanol separation. The generation of renewable thermal and electrical energy from onsite bio-wastes were also included in the energy balance of the combined pulping-ethanol process, in order to partially replace coal consumption. The bio-energy supplementations included the combustion of bark for heat and electricity generation and the bio-digestion of the calcium oxide SSL to produce methane as additional energy source.

RESULTS

Ethanol production from SSL at the highest substrate concentration was the most economically feasible when coal was used for process energy. However this solution did not provide any savings in greenhouse gas (GHG) emissions for the concentration-fermentation-distillation process. Maximizing the use of renewable energy sources to partially replace coal consumption yielded a satisfactory economic performance, with a minimum ethanol selling price of 0.83 US$/l , and a drastic reduction in the overall greenhouse gas emissions for the entire facility.

CONCLUSION

High substrate concentrations and conventional distillation should be used when considering integrating ethanol production at sulfite pulping mills. Bio-wastes generated onsite should be utilized at their maximum potential for energy generation in order to maximize the GHG emissions reduction.

Comparison of methods for detoxification of spruce hydrolysate for bacterial cellulose production

Background

Bacterial cellulose (BC) is a nanostructured material with unique properties and wide applicability. In order to decrease the production cost of bacterial cellulose, lignocellulose-based media have considerable potential as alternative cost-effective feedstocks. However, pretreatment and enzymatic hydrolysis of lignocellulose to sugars also generate fermentation inhibitors. Detoxification of lignocellulosic hydrolysates is needed to achieve efficient production of BC. In this investigation, different methods for detoxification of spruce hydrolysate prior to production of BC were compared with respect to effects on potential inhibitors and fermentable sugars, sugar consumption, BC yield, and cell viability. The objectives were to identify efficient detoxification methods and to achieve a better understanding of the role played by different inhibitors in lignocellulosic hydrolysates.

Results

In a first series of experiments, the detoxification methods investigated included treatments with activated charcoal, alkali [sodium hydroxide, calcium hydroxide (overliming), and ammonium hydroxide], anion and cation ion-exchange resins, and reducing agents (sodium sulfite and sodium dithionite). A second series of detoxification experiments included enzymatic treatments (laccase and peroxidase). The potential inhibitors studied included aliphatic acids, furan aldehydes, and phenolic compounds. The best effects in the first series of detoxification experiments were achieved with activated charcoal and anion exchanger. After detoxification with activated charcoal the BC yield was 8.2 g/L, while it was 7.5 g/L in a reference medium without inhibitors. Treatments with anion exchanger at pH 10 and pH 5.5 gave a BC yield of 7.9 g/L and 6.3 g/L, respectively. The first series of experiments suggested that there was a relationship between the BC yield and phenolic inhibitors. Therefore, the second series of detoxification experiments focused on treatments with phenol-oxidizing enzymes. The BC yield in the laccase-detoxified hydrolysate reached 5.0-5.5 g/L after 14 days cultivation, which demonstrated the important inhibitory role played by phenolic compounds.

Conclusions

The investigation shows that detoxification methods that efficiently remove phenolics benefit bacterial growth and BC production. Negative effects of salts could not be excluded and the osmotolerance of Gluconacetobacter xylinus needs to be further investigated in the future. Combinations of detoxification methods that efficiently decrease the concentration of inhibitors remain as an interesting option.

Effect of salts on the Co-fermentation of glucose and xylose by a genetically engineered strain of Saccharomyces cerevisiae

BACKGROUND

A challenge currently facing the cellulosic biofuel industry is the efficient fermentation of both C5 and C6 sugars in the presence of inhibitors. To overcome this challenge, microorganisms that are capable of mixed-sugar fermentation need to be further developed for increased inhibitor tolerance. However, this requires an understanding of the physiological impact of inhibitors on the microorganism. This paper investigates the effect of salts on Saccharomyces cerevisiae 424A(LNH-ST), a yeast strain capable of effectively co-fermenting glucose and xylose.

RESULTS

In this study, we show that salts can be significant inhibitors of S. cerevisiae. All 6 pairs of anions (chloride and sulfate) and cations (sodium, potassium, and ammonium) tested resulted in reduced cell growth rate, glucose consumption rate, and ethanol production rate. In addition, the data showed that the xylose consumption is more strongly affected by salts than glucose consumption at all concentrations. At a NaCl concentration of 0.5M, the xylose consumption rate was reduced by 64.5% compared to the control. A metabolomics study found a shift in metabolism to increased glycerol production during xylose fermentation when salt was present, which was confirmed by an increase in extracellular glycerol titers by 4 fold. There were significant differences between the different cations. The salts with potassium cations were the least inhibitory. Surprisingly, although salts of sulfate produced twice the concentration of cations as compared to salts of chloride, the degree of inhibition was the same with one exception. Potassium salts of sulfate were less inhibitory than potassium paired with chloride, suggesting that chloride is more inhibitory than sulfate.

CONCLUSIONS

When developing microorganisms and processes for cellulosic ethanol production, it is important to consider salt concentrations as it has a significant negative impact on yeast performance, especially with regards to xylose fermentation.

Bioconversion of lignocellulose: inhibitors and detoxification

Bioconversion of lignocellulose by microbial fermentation is typically preceded by an acidic thermochemical pretreatment step designed to facilitate enzymatic hydrolysis of cellulose. Substances formed during the pretreatment of the lignocellulosic feedstock inhibit enzymatic hydrolysis as well as microbial fermentation steps. This review focuses on inhibitors from lignocellulosic feedstocks and how conditioning of slurries and hydrolysates can be used to alleviate inhibition problems. Novel developments in the area include chemical in-situ detoxification by using reducing agents, and methods that improve the performance of both enzymatic and microbial biocatalysts.

Membrane extraction for detoxification of biomass hydrolysates

Membrane extraction was used for the removal of sulfuric acid, acetic acid, 5-hydroxymethyl furfural and furfural from corn stover hydrolyzed with dilute sulfuric acid. Microporous polypropylene hollow fiber membranes were used. The organic extractant consisted of 15% Alamine 336 in: octanol, a 50:50 mixture of oleyl alcohol:octanol or oleyl alcohol. Rapid removal of sulfuric acid, 5-hydroxymethyl and furfural was observed. The rate of acetic acid removal decreased as the pH of the hydrolysate increased. Regeneration of the organic extractant was achieved by back extraction into an aqueous phase containing NaOH and ethanol. A cleaning protocol consisting of flushing the hydrolysate compartment with NaOH and the organic phase compartment with pure organic phase enabled regeneration and reuse of the module. Ethanol yields from hydrolysates detoxified by membrane extraction using 15% Alamine 336 in oleyl alcohol were about 10% higher than those from hydrolysates detoxified using ammonium hydroxide treatment.

Fermentation of reactive-membrane-extracted and ammonium-hydroxide-conditioned dilute-acid-pretreated corn stover

Acid-pretreated biomass contains various compounds (acetic acid, etc.) that are inhibitory to fermentative microorganisms. Removing or deactivating these compounds using detoxification methods such as overliming or ammonium hydroxide conditioning (AHC) improves sugar-to-ethanol yields. In this study, we treated the liquor fraction of dilute-acid-pretreated corn stover using AHC and a new reactive membrane extraction technique, both separately and in combination, and then the sugars in the treated liquors were fermented to ethanol with the glucose-xylose-fermenting bacterium, Zymomonas mobilis 8b. We performed reactive extraction with mixtures of octanol/Alamine 336 or oleyl alcohol/Alamine 336. The best ethanol yields and rates were achieved for oleyl alcohol-extracted hydrolysates followed by AHC hydrolysates, while octanol-extracted hydrolysates were unfermentable because highly toxic octanol was found in the hydrolysate. Adding olive oil significantly improved yields for octanol-extracted hydrolysate. Additional work is underway to determine if this technology is a cost-effective alternative to traditional hydrolysate conditioning processes.

Improving the fermentability of enzymatic hydrolysates of lignocellulose through chemical in-situ detoxification with reducing agents

Inhibitory lignocellulose hydrolysates were treated with the reducing agents dithionite and sulfite to achieve improved fermentability. Addition of these reducing agents (in the concentration range 5.0-17.5 mM) to enzymatic hydrolysates of spruce wood or sugarcane bagasse improved processes based on both SHF (simultaneous hydrolysis and fermentation) and SSF (simultaneous saccharification and fermentation). The approach was exemplified in ethanolic fermentations with Saccharomyces cerevisiae and by using hydrolysates with sugar concentrations>100 g/L (for SHF) and with 10% dry-matter content (for SSF). In the SHF experiments, treatments with dithionite raised the ethanol productivities of the spruce hydrolysate from 0.2 to 2.5 g×L(-1)×h(-1) and of the bagasse hydrolysate from 0.9 to 3.9 g×L(-1)×h(-1), values even higher than those of fermentations with reference sugar solutions without inhibitors. Benefits of the approach include that the addition of the reducing agent can be made in-situ directly in the fermentation vessel, that the treatment can be performed at a temperature and pH suitable for fermentation, and that the treatment results in dramatically improved fermentability without degradation of fermentable sugars. The many benefits and the simplicity of the approach offer a new way to achieve more efficient manufacture of fermentation products from lignocellulose hydrolysates.

Conditioning of dilute-acid pretreated corn stover hydrolysate liquors by treatment with lime or ammonium hydroxide to improve conversion of sugars to ethanol

Dilute-acid pretreatment of lignocellulosic biomass enhances the ability of enzymes to hydrolyze cellulose to glucose, but produces many toxic compounds that inhibit fermentation of sugars to ethanol. The objective of this study was to compare the effectiveness of treating hydrolysate liquor with Ca(OH)2 and NH4OH for improving ethanol yields. Corn stover was pretreated in a pilot-scale reactor and then the liquor fraction (hydrolysate) was extracted and treated with various amounts of Ca(OH)2 or NH4OH at several temperatures. Glucose and xylose in the treated liquor were fermented to ethanol using a glucose-xylose fermenting bacteria, Zymomonas mobilis 8b. Sugar losses up to 10% occurred during treatment with Ca(OH)2, but these losses were two to fourfold lower with NH4OH treatment. Ethanol yields for NH4OH-treated hydrolysate were 33% greater than those achieved in Ca(OH)2-treated hydrolysate and pH adjustment to either 6.0 or 8.5 with NH4OH prior to fermentation produced equivalent ethanol yields.

Particle properties of sugar maple hemicellulose hydrolysate and its influence on growth and metabolic behavior of Pichia stipitis

In this study the influence of the insoluble solids in nano-filtrated sugar maple hemicellulosic hydrolysate on the metabolic behavior of Pichia stiptis was investigated. The particle properties of hemicellulosic hydrolysate were analyzed. Phosphoric acid and ammonium (PA) were applied to remove the particles. The metabolic behavior and growth property of P. stipitis in particle--removed hydrolysate was measured. Results demonstrated that the average particle size and zeta potential of the untreated hydrolysate were 2266.9±78.2 nm and -6.09±0.49 mV. Xylose consumption and ethanol production rate were significantly decreased when particle content is greater than 1.63 g/L. Because the majority of particles (34 g/L) were removed from hydrolysates by phosphoric acid and ammonium treatment, the fermentability of the hydrolysate was significantly improved. These results indicated particles play an important role in hydrolysate inhibition effect.

An economic comparison of different fermentation configurations to convert corn stover to ethanol using Z. mobilis and Saccharomyces

Numerous routes are being explored to lower the cost of cellulosic ethanol production and enable large-scale production. One critical area is the development of robust cofermentative organisms to convert the multiple, mixed sugars found in biomass feedstocks to ethanol at high yields and titers without the need for processing to remove inhibitors. Until such microorganisms are commercialized, the challenge is to design processes that exploit the current microorganisms' strengths. This study explored various process configurations tailored to take advantage of the specific capabilities of three microorganisms, Z. mobilis 8b, S. cerevisiae, and S. pastorianus. A technoeconomic study, based on bench-scale experimental data generated by integrated process testing, was completed to understand the resulting costs of the different process configurations. The configurations included whole slurry fermentation with a coculture, and separate cellulose simultaneous saccharification and fermentation (SSF) and xylose fermentations with none, some or all of the water to the SSF replaced with the fermented liquor from the xylose fermentation. The difference between the highest and lowest ethanol cost for the different experimental process configurations studied was $0.27 per gallon ethanol. Separate fermentation of solid and liquor streams with recycle of fermented liquor to dilute the solids gave the lowest ethanol cost, primarily because this option achieved the highest concentrations of ethanol after fermentation. Further studies, using methods similar to ones employed here, can help understand and improve the performance and hence the economics of integrated processes involving enzymes and fermentative microorganisms.

Metabolic effects of furaldehydes and impacts on biotechnological processes

There is a growing awareness that lignocellulose will be a major raw material for production of both fuel and chemicals in the coming decades--most likely through various fermentation routes. Considerable attention has been given to the problem of finding efficient means of separating the major constituents in lignocellulose (i.e., lignin, hemicellulose, and cellulose) and to efficiently hydrolyze the carbohydrate parts into sugars. In these processes, by-products will inevitably form to some extent, and these will have to be dealt with in the ensuing microbial processes. One group of compounds in this category is the furaldehydes. 2-Furaldehyde (furfural) and substituted 2-furaldehydes--most importantly 5-hydroxymethyl-2-furaldehyde--are the dominant inhibitory compounds found in lignocellulosic hydrolyzates. The furaldehydes are known to have biological effects and act as inhibitors in fermentation processes. The effects of these compounds will therefore have to be considered in the design of biotechnological processes using lignocellulose. In this short review, we take a look at known metabolic effects, as well as strategies to overcome problems in biotechnological applications caused by furaldehydes.

Effect of pretreatment chemicals on xylose fermentation by Pichia stipitis

Pretreatment of biomass with dilute H2SO4 results in residual acid which is neutralized with alkalis such as Ca(OH)2, NaOH and NH4OH. The salt produced after neutralization has an effect on the fermentation of Pichia stipitis. Synthetic media of xylose (60 g total sugar/l) was fermented to ethanol in the presence and absence of the salts using P. stipitis CBS 6054. CaSO4 enhanced growth and xylitol production, but produced the lowest ethanol concentration and yield after 140 h. Na2SO4 inhibited xylitol production, slightly enhanced growth towards the end of fermentation but had no significant effect on xylose consumption and ethanol concentration. (NH4)2SO4 inhibited growth, had no effect on xylitol production, and enhanced xylose consumption and ethanol production.