Lignocellulose-derived thin stillage composition and efficient biological treatment with a high-rate hybrid anaerobic bioreactor system

BACKGROUND

This study aims to chemically characterize thin stillage derived from lignocellulosic biomass distillation residues in terms of organic strength, nutrient, and mineral content. The feasibility of performing anaerobic digestion on these stillages at mesophilic (40 °C) and thermophilic (55 °C) temperatures to produce methane was demonstrated. The microbial communities involved were further characterized.

RESULTS

Energy and sugar cane stillage have a high chemical oxygen demand (COD of 43 and 30 g/L, respectively) and low pH (pH 4.3). Furthermore, the acetate concentration in sugar cane stillage was high (45 mM) but was not detected in energy cane stillage. There was also a high amount of lactate in both types of stillage (35-37 mM). The amount of sugars was 200 times higher in energy cane stillage compared to sugar cane stillage. Although there was a high concentration of sulfate (18 and 23 mM in sugar and energy cane stillage, respectively), both thin stillages were efficiently digested anaerobically with high COD removal under mesophilic and thermophilic temperature conditions and with an organic loading rate of 15-21 g COD/L/d. The methane production rate was 0.2 L/g COD, with a methane percentage of 60 and 64, and 92 and 94 % soluble COD removed, respectively, by the mesophilic and thermophilic reactors. Although both treatment processes were equally efficient, there were different microbial communities involved possibly arising from the differences in the composition of energy cane and sugar cane stillage. There was more acetic acid in sugar cane stillage which may have promoted the occurrence of aceticlastic methanogens to perform a direct conversion of acetate to methane in reactors treating sugar cane stillage.

CONCLUSIONS

Results showed that thin stillage contains easily degradable compounds suitable for anaerobic digestion and that hybrid reactors can efficiently convert thin stillage to methane under mesophilic and thermophilic conditions. Furthermore, we found that optimal conditions for biological treatment of thin stillage were similar for both mesophilic and thermophilic reactors. Bar-coded pyrosequencing of the 16S rRNA gene identified different microbial communities in mesophilic and thermophilic reactors and these differences in the microbial communities could be linked to the composition of the thin stillage.

Does size matter? Separations on guard columns for fast sample analysis applied to bioenergy research

BACKGROUND

Increasing sample throughput is needed when large numbers of samples have to be processed. In chromatography, one strategy is to reduce column length for decreased analysis time. Therefore, the feasibility of analyzing samples simply on a guard column was explored using refractive index and ultraviolet detection. Results from the guard columns were compared to the analyses using the standard 300 mm Aminex HPX-87H column which is widely applied to the analysis of samples from many biotechnology- and bioenergy-related experiments such as biomass conversions or fermentations.

RESULTS

The 50 mm Rezex RFQ Fast Acid H(+) guard column was able to separate the most common fermentation products (ethanol, acetone, iso- and n-butanol) and promising precursors (furfural and 5-hydroxymethylfurfural) of biofuels and value-added chemicals. Compound profiles in fermentation samples were analyzed with similar accuracy compared to results using the 300 mm column. However, separation of glucose and xylose was not achieved. Nevertheless, it was possible to monitor the consumption of one of the two sugars during fermentation if the other one was absent or remained constant over the course of the experiment. If correct peak integration and interference subtraction was applied, concentration profiles from enzymatic digestibility experiments and even more complex samples (e.g. acetone-butanol-ethanol (ABE) fermentation) were reliably obtained. With the 50 mm guard column, samples were analyzed up to ten-times faster compared to the 300 mm column. A further decrease in analysis time was achieved by using the 30 mm Micro Guard Cation H guard column. This column is especially suitable for the rapid analysis of compounds with long elution times on the standard 300 mm column, such as biofuel-related alcohols (e.g., n-butanol, n-hexanol) and furan- and tetrahydrofuran-type molecules.

CONCLUSION

Applied to a suitable set of samples, separations on a guard column can give rapid and sufficiently accurate information on compound changes over the course of an experiment. Therefore, it is an inexpensive and ideal tool for processing a large amount of samples, such as in screening or discovery experiments, where detecting relative changes is often sufficient to identify promising candidates for further analysis.

Fungi isolated from Miscanthus and sugarcane: biomass conversion, fungal enzymes, and hydrolysis of plant cell wall polymers

BACKGROUND

Biofuel use is one of many means of addressing global change caused by anthropogenic release of fossil fuel carbon dioxide into Earth's atmosphere. To make a meaningful reduction in fossil fuel use, bioethanol must be produced from the entire plant rather than only its starch or sugars. Enzymes produced by fungi constitute a significant percentage of the cost of bioethanol production from non-starch (i.e., lignocellulosic) components of energy crops and agricultural residues. We, and others, have reasoned that fungi that naturally deconstruct plant walls may provide the best enzymes for bioconversion of energy crops.

RESULTS

Previously, we have reported on the isolation of 106 fungi from decaying leaves of Miscanthus and sugarcane (Appl Environ Microbiol 77:5490-504, 2011). Here, we thoroughly analyze 30 of these fungi including those most often found on decaying leaves and stems of these plants, as well as four fungi chosen because they are well-studied for their plant cell wall deconstructing enzymes, for wood decay, or for genetic regulation of plant cell wall deconstruction. We extend our analysis to assess not only their ability over an 8-week period to bioconvert Miscanthus cell walls but also their ability to secrete total protein, to secrete enzymes with the activities of xylanases, exocellulases, endocellulases, and beta-glucosidases, and to remove specific parts of Miscanthus cell walls, that is, glucan, xylan, arabinan, and lignin.

CONCLUSION

This study of fungi that bioconvert energy crops is significant because 30 fungi were studied, because the fungi were isolated from decaying energy grasses, because enzyme activity and removal of plant cell wall components were recorded in addition to biomass conversion, and because the study period was 2 months. Each of these factors make our study the most thorough to date, and we discovered fungi that are significantly superior on all counts to the most widely used, industrial bioconversion fungus, Trichoderma reesei. Many of the best fungi that we found are in taxonomic groups that have not been exploited for industrial bioconversion and the cultures are available from the Centraalbureau voor Schimmelcultures in Utrecht, Netherlands, for all to use.

Analytical method for the determination of organic acids in dilute acid pretreated biomass hydrolysate by liquid chromatography-time-of-flight mass spectrometry

BACKGROUND

For the development of lignocellulosic biofuels a common strategy to release hemicellulosic sugars and enhance the enzymatic digestibility of cellulose is the heat pretreatment of biomass with dilute acid. During this process, fermentation inhibitors such as 5-hydroxymethylfurfural, furfural, phenolics, and organic acids are formed and released into the so-called hydrolysate. The phenolic inhibitors have been studied fairly extensively, but fewer studies have focused on the analysis of the organic acids profile. For this purpose, a simple and fast liquid chromatography/mass spectrometry (LC/MS) method for the analysis of organic acids in the hydrolysate has been developed using an ion exchange column based on a polystyrene-divinylbenzene polymer frequently used in biofuel research. The application of the LC/MS method to a hydrolysate from Miscanthus has been evaluated.

RESULTS

The presented LC/MS method involving only simple sample preparation (filtration and dilution) and external calibration for the analysis of 24 organic acids present in dilute acid pretreated biomass hydrolysate is fast (12 min) and reasonably sensitive despite the small injection volume of 2 μL used. The lower limit of quantification ranged from 0.2 μg/mL to 2.9 μg/mL and the limit of detection from 0.03 μg/mL to 0.7 μg/mL. Analyte recoveries obtained from a spiked hydrolysate were in the range of 70 to 130% of the theoretical yield, except for glyoxylic acid, malic acid, and malonic acid, which showed a higher response due to signal enhancement. Relative standard deviations for the organic acids ranged from 0.4 to 9.2% (average 3.6%) for the intra-day experiment and from 2.1 to 22.8% (average 8.9%) for the inter-day (three-day) experiment.

CONCLUSION

We have shown that the analysis of the profile of 24 organic acids present in biomass hydrolysate can be achieved by a simple LC/MS method applying external calibration and minimal sample preparation. The organic acids eluted within only 12 min by isocratic elution, enabling high sample throughput. Repeatability (precision and accuracy) and recovery were sufficiently accurate for most of the organic acids tested, making the method suitable for their fast determination in hydrolysate. We envision that this method can be further expanded to a larger number of organic acids, including phenolic acids such as p-coumaric acid and ferulic acid and other molecules depending on the researchers' needs.

Dissecting a complex chemical stress: chemogenomic profiling of plant hydrolysates

Complex chemical stress arises during the production of biofuels. Large-scale mutant fitness profiling was used to identify bacterial and yeast tolerance genes and to model fitness in a complex hydrolysate mixture. The resulting model can be used to engineer more tolerant strains.

Genome-wide fitness profiling was used to identify plant hydrolysate tolerance genes in Zymomonas mobilis and Saccharomyces cerevisiae.

We modeled fitness in hydrolysate as a mixture of fitness in its components.

Outliers in our model led to the identification of a previously unknown component of hydrolysate.

Overexpression of a Z. mobilis tolerance gene of unknown function improved ethanol productivity in plant hydrolysate.

The efficient production of biofuels from cellulosic feedstocks will require the efficient fermentation of the sugars in hydrolyzed plant material. Unfortunately, plant hydrolysates also contain many compounds that inhibit microbial growth and fermentation. We used DNA-barcoded mutant libraries to identify genes that are important for hydrolysate tolerance in both Zymomonas mobilis (44 genes) and Saccharomyces cerevisiae (99 genes). Overexpression of a Z. mobilis tolerance gene of unknown function (ZMO1875) improved its specific ethanol productivity 2.4-fold in the presence of miscanthus hydrolysate. However, a mixture of 37 hydrolysate-derived inhibitors was not sufficient to explain the fitness profile of plant hydrolysate. To deconstruct the fitness profile of hydrolysate, we profiled the 37 inhibitors against a library of Z. mobilis mutants and we modeled fitness in hydrolysate as a mixture of fitness in its components. By examining outliers in this model, we identified methylglyoxal as a previously unknown component of hydrolysate. Our work provides a general strategy to dissect how microbes respond to a complex chemical stress and should enable further engineering of hydrolysate tolerance.

Characterization of Miscanthus giganteus lignin isolated by ethanol organosolv process under reflux condition

Miscanthus giganteus lignin was extracted by an organosolv process under reflux conditions (4 h) with varying concentrations of ethanol (65%, 75%, 85%, 95%) and 0.2 M hydrochloric acid as catalyst. The resulting lignin was extensively characterized by size exclusion chromatography (SEC), Fourier-transform infrared spectroscopy (FTIR), gas chromatography-mass spectrometry (GC/MS), two-dimensional nuclear magnetic resonance spectroscopy (2D-NMR), and chemical analysis (residual sugars, Klason lignin, ash). The predominant linkage units present were β-O-4' (82-84%), resinol (6-7%), and phenylcoumaran (10-11%). The 65% ethanol solvent system gave the lowest lignin yield (14% of starting biomass) compared to 29-32% of the other systems. Increasing ethanol concentration resulted in decreasing carbohydrate content of the lignins (3.6-1.1%), a higher solubility in tetrahydrofuran (THF), a slight reduction of the molecular weight (M(w) 2.72-2.25 KDa), an increasing α-ethoxylation, and an increase in ethoxylated phenylpropenoic compounds (p-coumaric and ferulic acid), but the S/G ratio of the monolignols (0.63, GC/MS) and Klason lignin content (86-88%) were unaffected. An extraction method for these ethyl-esterified phenylpropenoids and smaller molecular weight lignin compounds was developed. The effect of reaction time (2, 4, and 8 h) was investigated for the 95% ethanol solvent system. Besides increased lignin yield (13-43%), a slight increase in M(w) (2.21-2.38 kDa) and S/G ratio (0.53-0.68, GC-MS) was observed. Consecutive extractions suggested that these changes were not from lignin modifications (e.g., condensations) but rather from extraction of lignin of different composition. The results were compared to similar solvent systems with 95% acetone and 95% dioxane.

Simultaneous determination of phenolic compounds by means of an automated voltammetric “electronic tongue”

This contribution describes the simultaneous determination of three phenolic compounds, o-cresol, p-chlorophenol and 4-chloro-3-methylphenol, using direct oxidation and amperometric detection coupled by signal deconvolution, accomplished via chemometric methods. Direct oxidation of phenolic compounds is performed at the surface of an epoxy-graphite transducer, by linear scan voltammetry. Due to strong signal overlapping, artificial neural networks (ANNs) were used during data treatment, in a combination of chemometrics and electrochemical sensors known as an "electronic tongue". To calibrate this system properly, a total of 80 mixed samples were prepared automatically by employing a sequential injection analysis (SIA) system designed to automatically generate the information needed to train the network. The phenolic compound concentration varied from 1 to 70 microM for o-cresol, from 0.5 microM to 140 microM for p-chlorophenol and from 1 microM to 100 microM for 4-chloro-3-methylphenol. A good prediction capability was obtained, with correlation coefficients >0.964 when the obtained values were compared with those expected for a set of 24 external test samples not used for training. The results presented here indicate that this technique is a simple and robust analytical method of environmental interest.