Autocatalytic hydrothermal pretreatment by recycling byproduct organic acids to directionally depolymerize cassava straw

A two-stage autocatalytic hydrothermal pretreatment was proposed to improve the cassava straw utilization. The two-stage hydrothermal pretreatment was a process of which the first stage adopted lower-severity conditions (temperature and time) to improve the C-5 sugar yields and the second stage employed more severities to enhance C-6 sugar yield during enzyme hydrolysis. After employing this process, the maximum yields of C-5 and C-6 sugars were 68.49% and 81.02% when treating at 180 °C for 60 min for the first stage and 200 °C for 20 min for the second stage. Based on this, the autocatalytic pretreatment was investigated, which was a method to further enhance the pretreatment intensity by recycling pretreated liquid rich in byproduct organic acids (acetic acid, lactic acid and formic acid) during two-stage hydrothermal pretreatment. The results showed that the C-5 sugar yields of the first stage increased to 81.12% when recycled pretreated liquid twice, which led to 0.93 wt% byproduct organic acid. After the second stage, the C-6 sugar yield increased to 88.60% during enzymatic hydrolysis. Besides, mass balance and development potentials were analyzed. The results revealed that two-stage autocatalytic hydrothermal pretreatment could effectively enhance pretreatment intensity and provide promising methods of directionally depolymerizing cassava straws.     

Isolation of xylose isomerases by sequence- and function-based screening from a soil metagenomic library

Background

The recent development of improved enzymes and pentose-using yeast for cellulosic ethanol processes calls for new attention to the lignocellulose pretreatment step. This study assessed the influence of pretreatment pH, temperature, and time, and their interactions on the enzymatic glucose and xylose yields from mildly pretreated wheat straw in multivariate experimental designs of acid and alkaline pretreatments.

Results

The pretreatment pH was the most significant factor affecting both the enzymatic glucose and xylose yields after mild thermal pretreatments at maximum 140°C for 10 min. The maximal enzymatic glucose and xylose yields from the solid, pretreated wheat straw fraction were obtained after pretreatments at the most extreme pH values (pH 1 or pH 13) at the maximum pretreatment temperature of 140°C. Surface response models revealed significantly correlating interactions of the pretreatment pH and temperature on the enzymatic liberation of both glucose and xylose from pretreated, solid wheat straw. The influence of temperature was most pronounced with the acidic pretreatments, but the highest enzymatic monosaccharide yields were obtained after alkaline pretreatments. Alkaline pretreatments also solubilized most of the lignin.

Conclusions

Pretreatment pH exerted significant effects and factor interactions on the enzymatic glucose and xylose releases. Quite extreme pH values were necessary with mild thermal pretreatment strategies (T ≤ 140°C, time ≤ 10 min). Alkaline pretreatments generally induced higher enzymatic glucose and xylose release and did so at lower pretreatment temperatures than required with acidic pretreatments.     

Bioethanol production from ammonia percolated wheat straw

This study examined the effectiveness of ammonia percolation pretreatment of wheat straw for ethanol production. Ground wheat straw at a 10% (w/v) loading was pretreated with a 15% (v/v) ammonia solution. The experiments were performed at treatment temperature of 50∼170°C and residence time of 10∼150 min. The solids treated with the ammonia solution showed high lignin degradation and sugar availability. The pretreated wheat straw was hydrolyzed by a cellulase complex (NS50013) and β-glucosidase (NS50010) at 45°C. After saccharification, Saccharomyces cerevisiae was added for fermentation. The incubator was rotated at 120 rpm at 35°C. As a result of the pretreatment, the delignification efficiency was > 70% (170°C, 30 min) and temperature was found to be a significant factor in the removal of lignin than the reaction time. In addition, the saccharification results showed an enzymatic digestibility of > 90% when 40 FPU/g cellulose was used. The ethanol concentration reached 24.15 g/L in 24 h. This paper reports a total process for bioethanol production from agricultural biomass and an efficient pretreatment of lignocellulosic material.     

Autohydrolysis extraction process as a pretreatment of lignocelluloses for their enzymatic hydrolysis

Autohydrolysis was studied as a pretreatment to enhance sugar yields from enzymatic hydrolysis of wheat and rape straw, beech, birch and poplar sawdust. Reaction temperatures were 185°C to 212°C and the reaction time 20 min. The pretreated slurries were hydrolyzed with “Novo” cellulase and Fusarium sp. 27 cellulase at 45°C and pH 4.8 for 24 h with addition of Fusarium sp. 27 cellbound cellobiase. From 85% to 90% sugar content of substrates were converted to reducing sugars after 24 h enzymatic hydrolysis, with exception of poplar wood. 10.8 g biomass was obtained after cultivation of Fusarium sp. 27 with water solution hemicellulose fraction from 100 g beech sawdust autohydrolyzed at 200°C during 20 min.     

Optimization of Ethanol Production From Microfluidized Wheat Straw by Response Surface Methodology

In this study, wheat straw was pretreated with a microfluidizer to improve its enzymatic hydrolysis and ethanol yields. The pretreatment was performed at various pressures (500, 1000, and 1500 bar) and solid loadings (1, 2, and 3%). The microfluidized biomass was then subjected to hydrolysis and simultaneous saccharification and co-fermentation (SSCF) experiments at different enzyme loadings (5, 10, and 15 FPU/g dry wheat straw) using a mutant yeast. The results indicated that the microfluidization method alters the structure of biomass and leads to a reduction in lignin content. The samples pretreated at 1% solid loading contained the minimum lignin concentration and provided the maximum sugar and ethanol yields. These results signified that the microfluidization method is more effective on biomass at low solid loadings. The process conditions were optimized for higher ethanol and sugar yields using response surface methodology (RSM). The optimum pressure and solid and enzyme loadings were found as 1500 bar, 1%, and 15 FPU/g dry wheat straw, respectively. The yields obtained at this condition were 82%, 94%, and 65% for glucose, xylose, and ethanol, respectively. High sugar yields implied that microfluidization is an effective pretreatment method for cellulosic ethanol production. On the other hand, low ethanol yield may indicate that the microorganism was sensitive to inhibitory compounds present in the fermentation medium.     

Whole slurry fermentation of maleic acid-pretreated oil palm empty fruit bunches for ethanol production not necessitating a detoxification process

The yield of ethanol from oil palm empty fruit bunches (EFB) was increased on exploiting maleic acid pretreatment combined with fermentation of the pretreated whole slurry. The optimized conditions for pretreatment were to expose EFB to a high temperature (190 °C) with 1 % (w/v) maleic acid for a short time duration (3 min ramping to the set temperature with no holding) in a microwave digester. An enzymatic digestibility of 60.9 % (based on theoretical glucose yield) was exhibited using pretreated and washed EFB after 48 h of hydrolysis. Simultaneous saccharification and fermentation (SSF) of the whole slurry of pretreated EFB for 48 h resulted in 61.3 % theoretical yield of ethanol based on the initial amount of glucan in untreated EFB. These results indicate that maleic acid is a suitable catalyst not requiring detoxification steps for whole slurry fermentation of EFB for ethanol production, thus improving the process economics. Also, the whole slurry fermentation can significantly increase the biomass utilization by converting sugar from both solid and liquid phases of the pretreated slurry.     

Monosaccharide yields and lignin removal from wheat straw in response to catalyst type and pH during mild thermal pretreatment

The influence of various low temperature (140 °C) pretreatments, using different acid and alkaline catalysts and different pH values, was studied for enzymatic hydrolysis of wheat straw. The pretreated wheat straw was treated by a standard blend of Celluclast 1.5L and Novozym 188. While pretreatment at pH 1 gave the highest yield of saccharides in the liquid fraction, the solid fraction was more susceptible to enzymatic attack when pretreated at pH 13. The highest yields were obtained after pretreatment with hydrochloric acid at pH 1, and with sodium hydroxide at pH 13 when enzymatic hydrolysis was employed. A two-step pretreatment strategy at pH 1 (hydrochloric acid) and subsequently at pH 13 (sodium hydroxide) released 69% and 95% of the theoretical maximal amounts of glucose and xylose, respectively. Furthermore, this two-step pretreatment removed 68% of the lignin from the straw with only minor losses of monosaccharides and production of only low amounts of inhibitors. Type of catalyst and pH indeed influenced the monosaccharide yields and lignin removal from wheat straw, and need more attention in the choice of pretreatment strategy.     

Comparative evaluations of cellulosic raw materials for second generation bioethanol production

Aims: To evaluate sugar recoveries and fermentabilities of eight lignocellulosic raw materials following mild acid pretreatment and enzyme hydrolysis using a recombinant strain of Zymomonas mobilis. Methods and Results: Dilute acid pretreatment (2% H₂SO₄) with 10% (w/v) substrate loading was performed at 134°C for 60 min followed by enzyme hydrolysis at 60°C. The results demonstrated that hydrolysis of herbaceous raw materials resulted in higher sugar recoveries (up to 60–75%) than the woody sources (<50%). Fermentation studies with recombinant Z. mobilis ZM4 (pZB5) demonstrated that final ethanol concentrations and yields were also higher for the herbaceous hydrolysates. Significant reduction in growth rates and specific rates of sugar uptake and ethanol production occurred for all hydrolysates, with the greatest reductions evident for woody hydrolysates. Further studies on optimization of enzyme hydrolysis established that higher sugar recoveries were achieved at 50°C compared to 60°C following acid pretreatment. Conclusions: Of the various raw materials evaluated, the highest ethanol yields and productivities were achieved with wheat straw and sugarcane bagasse hydrolysates. Sorghum straw, sugarcane tops and Arundo donax hydrolysates were similar in their characteristics, while fermentation of woody hydrolysates (oil mallee, pine and eucalyptus) resulted in relatively low ethanol concentrations and productivities. The concentrations of a range of inhibitory compounds likely to have influence the fermentation kinetics were determined in the various hydrolysates. Significance and Impact of the Study: The study focuses on lignocellulosic materials available for second generation ethanol fermentations designed to use renewable agricultural/forestry biomass rather than food‐based resources. From the results, it is evident that relatively good sugar and ethanol yields can be achieved from some herbaceous raw materials (e.g. sugarcane bagasse and sorghum straw), while much lower yields were obtained from woody biomass.     

Effects of preheating on briquetting and subsequent hydrothermal pretreatment for enzymatic saccharification of wheat straw

Briquetting of plant biomass with low bulk density is an advantage for handling, transport, and storage of the material, and heating of the biomass prior to the briquetting facilitates the densification process and improves the physical properties of the briquettes. This study investigates the effects of preheating prior to briquetting of wheat straw (WS) on subsequent hydrothermal pretreatment and enzymatic conversion to fermentable sugars. WS (11% moisture content) was densified to briquettes under different conditions; without preheating or with preheating at 75 or 125°C for either 5 or 10 min. Subsequent hydrothermal pretreatment was done for both un-briquetted WS and for briquettes. Enzymatic saccharification was afterwards performed for all samples. The results showed that as expected, nonpretreated WS briquettes gave very low sugar yields (22–29% of the cellulose content), even though preheating at 125°C prior to briquetting (without pretreatment) improved sugar yields somewhat. When combined with pretreatment, briquetting with preheating showed neutral or negative effects on sugar yield. This result suggests that moderate preheating (75°C for 5 min) before briquetting improved bulk density and compressive resistance of briquettes without impeding subsequent enzymatic conversion. However, excessive preheating (75 or 125°C for 10 min) before briquetting may result in irreversible structural modifications that hinder the interaction between biomass and water during pretreatment, thereby decreasing the accessibility of cellulose to enzymatic saccharification.     

Comparing the Recalcitrance of Eucalyptus, Pine, and Switchgrass Using Ionic Liquid and Dilute Acid Pretreatments

Pine, eucalyptus, and switchgrass were evaluated for the production of fermentable sugars via ionic liquid and dilute acid pretreatments and subsequent enzymatic hydrolysis. The results show that among the three feedstocks, switchgrass has the highest sugar yields and faster hydrolysis rates for both pretreatment technologies by achieving 48 % (dilute acid) and 96 % (ionic liquid) sugar yields after 24 h. Of the two wood species, eucalyptus has a higher and faster sugar recovery after ionic liquid pretreatment than pine (93 vs. 62 % in 24 h) under 160 °C for 3 h with [C₂mim][OAc]. Pretreatment of pine and eucalyptus is observed to be ineffective under 1.2 % dilute acid condition and 160 °C for 15 min, indicating that further enhancement of reaction temperature or acid concentration is necessary to increase the digestibility of pretreated materials. Raman spectroscopy data show that the extent of lignin depolymerization that occurs during pretreatment also varies for the three different feedstocks. Under similar hemicellulose removal conditions, lignin removal in ionic liquid pretreatment can help improve cellulose conversion. This finding may help explain the observed variation in the saccharification yields and kinetics. These results indicate that ionic liquid pretreatment not only improved saccharification over dilute acid for all three feedstocks but also better dealt with the differences among them, suggesting better tolerance to feedstock variability.     

Ultrasonic pretreatment and acid hydrolysis of sugarcane bagasse for succinic acid production using Actinobacillus succinogenes

Immense interest has been devoted to the production of bulk chemicals from lignocellulose biomass. Diluted sulfuric acid treatment is currently one of the main pretreatment methods. However, the low total sugar concentration obtained via such pretreatment limits industrial fermentation systems that use lignocellulosic hydrolysate. Sugarcane bagasse hemicellulose hydrolysate is used as the carbon and nitrogen sources to achieve a green and economical production of succinic acid in this study. Sugarcane bagasse was ultrasonically pretreated for 40 min, with 43.9 g/L total sugar obtained after dilute acid hydrolysis. The total sugar concentration increased by 29.5 %. In a 3-L fermentor, using 30 g/L non-detoxified total sugar as the carbon source, succinic acid production increased to 23.7 g/L with a succinic acid yield of 79.0 % and a productivity of 0.99 g/L/h, and 60 % yeast extract in the medium could be reduced. Compared with the detoxified sugar preparation method, succinic acid production and yield were improved by 20.9 and 20.2 %, respectively.     

Steam pretreatment of H(2)SO(4)-impregnated Salix for the production of bioethanol

In the bioconversion of lignocellulosic materials to ethanol, pretreatment of the material prior to enzymatic hydrolysis is essential to obtain high overall yields of sugar and ethanol. In this study, steam pretreatment of fast-growing Salix impregnated with sulfuric acid has been investigated by varying the temperature (180-210 degrees C), the residence time (4, 8 or 12 min), and the acid concentration (0.25% or 0.5% (w/w) H(2)SO(4)). High sugar recoveries were obtained after pretreatment, and the highest yields of glucose and xylose after the subsequent enzymatic hydrolysis step were 92% and 86% of the theoretical, respectively, based on the glucan and xylan contents of the raw material. The most favorable pretreatment conditions regarding the overall sugar yield were 200 degrees C for either 4 or 8 min using 0.5% sulfuric acid, both resulting in a total of 55.6g glucose and xylose per 100g dry raw material. Simultaneous saccharification and fermentation experiments were performed on the pretreated slurries at an initial water-insoluble content of 5%, using ordinary baker's yeast. An overall theoretical ethanol yield of 79%, based on the glucan and mannan content in the raw material, was obtained.     

Comparison of Feedstock Pretreatment Performance and Its Effect on Soluble Sugar Availability

Cellulosic feedstocks for bioenergy differ in composition and processing requirements for efficient conversion to chemicals and fuels. This study discusses and compares the processing requirements for three lignocellulosic feedstocks??soybean hulls, wheat straw, and de-starched wheat bran. They were ground with a hammer mill to investigate how differences in composition and particle size affect the hydrolysis process. Enzyme hydrolysis was conducted using cellulase from Trichoderma reesei at 50°C and pH 5. Ground fractions were also subjected to dilute sulfuric acid treatment at 125°C, 15 psi for 30 min prior to cellulase treatment. Reducing particle size of biomass resulted in segregated components of feedstock. Grinding wheat straw to particle size <132 ??m resulted in measured lignin content from 20% to ??5% and reduced hemicellulose content. Reducing lignin content increased the effectiveness of enzyme hydrolysis of wheat straw. Particles sized <132 ??m exhibited the highest soluble sugar release upon hydrolysis for all three feedstocks studied. Hemicellulose digestion improved with dilute sulfuric acid treatment with residual hemicellulose content <5% in all three feedstocks after acid treatment. This enhanced the cellulase action and resulted in approximately 1.6-fold increase in sugar availability in de-starched wheat bran and ??1.5-fold for wheat straw and soybean hulls. Higher sugar availability in wheat bran after acid-mediated enzyme treatment correlated to higher ethanol yields during yeast fermentation compared with soybean hulls and wheat straw.     

Liquid hot water and alkaline pretreatment of soybean straw for improving cellulose digestibility

Soybean straw was pretreated with either liquid hot water (LHW) (170-210 °C for 3-10 min) or alkaline soaking (4-40 g NaOH/100 g dry straw) at room temperature to evaluate the effects on cellulose digestibility. Nearly 100% cellulose was recovered in pretreated solids for both pretreatment methods. For LHW pretreatment, xylan dissolution from the raw material increased with pretreatment temperature and time. Cellulose digestibility was correlated with xylan dissolution. A maximal glucose yield of 70.76%, corresponding to 80% xylan removal, was obtained with soybean straw pretreated at 210 °C for 10 min. NaOH soaking at ambient conditions removed xylan up to 46.37% and the subsequent glucose yield of pretreated solids reached up to 64.55%. Our results indicated LHW pretreatment was more effective than NaOH soaking for improving cellulose digestibility of soybean straw.     

Optimization of the dilute maleic acid pretreatment of wheat straw

Background  

In this study, the dilute maleic acid pretreatment of wheat straw is optimized, using pretreatment time, temperature and maleic acid concentration as design variables. A central composite design was applied to the experimental set up. The response factors used in this study are: (1) glucose benefits from improved enzymatic digestibility of wheat straw solids; (2) xylose benefits from the solubilization of xylan to the liquid phase during the pretreatment; (3) maleic acid replenishment costs; (4) neutralization costs of pretreated material; (5) costs due to furfural production; and (6) heating costs of the input materials. For each response factor, experimental data were fitted mathematically. After data translation to €/Mg dry straw, determining the relative contribution of each response factor, an economic optimization was calculated within the limits of the design variables.     

Dilute sulfuric acid cycle spray flow-through pretreatment of corn stover for enhancement of sugar recovery

A cycle spray flow-through reactor was designed and used to pretreat corn stover in dilute sulfuric acid medium. The dilute sulfuric acid cycle spray flow-through (DCF) process enhanced xylose sugar yields and cellulose digestibility while increasing the removal of lignin. Within the DCF system, the xylose sugar yields of 90–93% could be achieved for corn stover pretreated with 2% (w/v) dilute sulfuric acid at 95 °C during the optimal reaction time (90 min). The remaining solid residue exhibited enzymatic digestibility of 90–95% with cellulase loading of 60 FPU/g glucan that was due to the effective lignin removal (70–75%) in this process. Compared with flow-through and compress-hot water pretreatment process, the DCF method produces a higher sugar concentration and higher xylose monomer yield. The novel DCF process provides a feasible approach for lignocellulosic material pretreatment.     

Ethanol production from maize silage as lignocellulosic biomass in anaerobically digested and wet-oxidized manure

In this communication, pretreatment of the anaerobically digested (AD) manure and the application of the pretreated AD manure as liquid medium for the simultaneous saccharification and fermentation (SSF) were described. Furthermore, fermentation of pretreated maize silage and wheat straw was investigated using 2 l bioreactors. Wet oxidation performed for 20 min at 121 °C was found as the most suitable pretreatment conditions for AD manure. High ammonia concentration and significant amount of macro- and micro-nutrients in the AD manure had a positive influence on the ethanol fermentation. No extra nitrogen source was needed in the fermentation broth. It was shown that the AD manure could successfully substitute process water in SSF of pretreated lignocellulosic fibres. Theoretical ethanol yields of 82% were achieved, giving 30.8 kg ethanol per 100 kg dry mass of maize silage.     

Dilute acid pretreatment of barley straw and its saccharification and fermentation

In this study, the optimization of the major factors for efficient dilute acid pretreatment (DAP) of Korean barley straw was conducted by response surface method (RSM). In addition, saccharification of the optimized pretreated barley straw as well as fermentation of solubilized hemicellulose and enzymatic hydrolysates was performed for bioethanol production. The factors optimized by RSM were concentration of sulfuric acid, reaction time and temperature. Optimization experiments were carried out within the scope of 0.16 ∼ 1.84% sulfuric acid, 10 ∼ 20 min of reaction time, and 116 ∼ 183°C of temperature using a statistical program, and optimal conditions (1.16% of sulfuric acid, 16.9 min of reaction time, and 150°C) were determined based on reliable statistical indicators. The predicted value at stationary point and the experimental value were 81.38 and 80.66%, respectively. Saccharification was performed at 50°C using Celluclast (cellulase) and Novozyme 188 (β-glucosidase) as biocatalysts in an enzyme loading test. Conversion of the saccharification process was approximately 65%. In addition, fermentation of glucose after saccharification and solubilization of xylose solution by DAP were performed using Saccharomyces cerevisiae and Pichia stipitis at 30°C and 200 rpm for 12 h.     

Conversion of olive tree biomass into fermentable sugars by dilute acid pretreatment and enzymatic saccharification

The production of fermentable sugars from olive tree biomass was studied by dilute acid pretreatment and further saccharification of the pretreated solid residues. Pretreatment was performed at 0.2%, 0.6%, 1.0% and 1.4% (w/w) sulphuric acid concentrations while temperature was in the range 170-210 degrees C. Attention is paid to sugar recovery both in the liquid fraction issued from pretreatment (prehydrolysate) and that in the water-insoluble solid (WIS). As a maximum, 83% of hemicellulosic sugars in the raw material were recovered in the prehydrolysate obtained at 170 degrees C, 1% sulphuric acid concentration, but the enzyme accessibility of the corresponding pretreated solid was not very high. In turn, the maximum enzymatic hydrolysis yield (76.5%) was attained from a pretreated solid (at 210 degrees C, 1.4% acid concentration) in which cellulose solubilization was detected; moreover, sugar recovery in the prehydrolysate was the poorest one among all the experiments performed. To take account of fermentable sugars generated by pretreatment and the glucose released by enzymatic hydrolysis, an overall sugar yield was calculated. The maximum value (36.3 g sugar/100 g raw material) was obtained when pretreating olive tree biomass at 180 degrees C and 1% sulphuric acid concentration, representing 75% of all sugars in the raw material. Dilute acid pretreatment improves results compared to water pretreatment.     

Comparison of saccharification process by acid and microwave-assisted acid pretreated swine manure

The saccharification process of swine manure by conventional and microwave-assisted acid pretreated were investigated using cellulose enzymes, respectively. The optima for microwave-assisted acid pretreated swine manure is achieved when swine manure of 50 g l⁻¹ of substrate concentration and water amount 40 ml was pretreated by 4% H₂SO₄ concentration with 445 W microwave powers for 30 min at pretreatment period, and temperature 50 °C, enzyme loading 2 mg g⁻¹ substrate, substrate concentration 5 g l⁻¹ and initial medium pH 4.8 at enzymes hydrolysis period by microwave-assisted acid pretreated, respectively. The optimal conditions by conventional acid pretreated is obtained when 50 g l⁻¹ swine manure was submerged in 40 ml, 4% H₂SO₄ maintained at 130 °C for 3 h at pretreatment period, and temperature 45 °C, enzyme loading 2 mg g⁻¹ substrate, substrate concentration 15 g l⁻¹ and initial medium pH 5.2 at enzymes hydrolysis period, respectively. Under the optimum conditions microwave-assisted acid pretreatment could achieve higher yield of reducing sugar, short reaction time, and lower energy consumption than from the conventional acid pretreatment, which indicates that microwave-assisted acid pretreatment is more suitable for swine manure pretreatment than by acid alone.