Dilute Sulfuric Acid Pretreatment Research Articles

Isolation of Highly Crystalline Cellulose via Combined Pretreatment/Fractionation and Extraction Procedures within a Biorefinery Concept.

Sustainable production of bio-based materials and chemicals requires integrated approaches which utilize all fractions of lignocellulosic biomass. In this work, highly crystalline cellulose was isolated via combined pretreatment/fractionation and extraction processes from beechwood sawdust. The proposed approach was based on the selective recovery of hemicellulose components in the first step, followed by enhanced delignification in the second step, permitting the efficient recovery of the remaining cellulose via bleaching in the final step. Hydrothermal pretreatment under tailored conditions in neat water or dilute acid resulted in almost complete hemicellulose removal (80-96 wt %) in the liquid fraction. In the second step, the formed surface lignin was isolated via mild extraction while enhanced removal of both native/structural and surface lignin (71 wt %) was achieved by applying the organosolv treatment using dilute sulfuric acid as catalyst. Dilute sulfuric acid pretreatment followed by acid catalyzed organosolv pretreatment proved to be the most efficient combined approach, leading to 80 wt % hemicellulose removal as xylose monomer, and 71 wt % delignification. High crystallinity cellulose (<88%), with an overall cellulose recovery of 68-91 wt % based on native cellulose in parent biomass was isolated in the last step via bleaching of all pretreated biomass solids. The proposed integrated biorefinery procedures that aim to whole "waste" biomass valorization, replacing fossil resources, with the use of green solvents (water, ethanol) at relatively mild temperature/pressure conditions, are in line with the scope of several United Nations Sustainable Development Goals, such as UN SDG 8, 11, 12, and 13.

Valorization of Afzelia bella oilseeds cake in bioethanol production using 1-butyl-3-methyl-imidazolium chloride ionic liquid and dilute sulfuric acid pretreatment

The use of renewable feedstocks and the variability of lignocellulosic feedstocks generating fermentable sugars also offer the advantages of the most abundant, sustainable and competitive non-food biomass. The aim of this study is to valorize the oilcake obtained from non-conventional Congolese Afzelia bella oilseeds, after extraction of the oil used in biodiesel production. The conversion of A. bella oilseeds cake into ethanol was carried out using the two methods of pretreatment (with the synthesized ionic liquid 1-butyl-3-methyl imidazolium chloride and dilute sulfuric acid), followed by hydrolysis with dilute sulfuric acid and fermentation by Saccharomyces cerevisiae. The lignocellulosic composition of the sample was 47.98% cellulose, 28.15% hemicellulose and 22.3% lignin. Pretreatment with 1-butyl-3-methyl-imidazolium chloride (ionic liquid) showed an increase in cellulose content of around 4.25% and hemicellulose content of around 7.7%, with simultaneous delignification of 12.25%. In contrast, with dilute sulfuric acid, the lignin and cellulose content of the sample fell to 3.88% and 1.88% respectively. Hemicellulose content, on the other hand, increased by only 3.58%. After dilute acid hydrolysis (5% H2SO4), the values for total reducing sugar, glucose and xylose were 66.06±1.34 mg.g-1; 36.25 ± 1.2 mg.g-1 and 29.71 ± 0.6 mg.g-1 respectively. The percentage conversion of sugars was 55.16 ± 0.5% for cellulose to glucose and 53.76 ± 0.75% for hemicellulose to xylose. The hydrolyzed product of the complex polysaccharide was then converted to ethanol using commercial yeast. Results showed an ethanol yield of around 10.3%, and Fourier Transform Infrared (FTIR) spectroscopy results confirmed bioethanol production. A. bella oilseeds cakes are therefore a potential source for ethanol production.

Insight into key obstacles and technological strategy for enzymatic digestion of full cellulose fraction from poplar sawdust

Lignocellulosic biomass mainly consists of hemicellulose, lignin, and cellulose, which differently affect the enzymatic digestibility of cellulose. As for the typical representative for inert woody biomass, three components of cellulose were proposed conceptually for poplar sawdust, i.e., active cellulose, inert cellulose, and resistant cellulose. Dilute sulfuric acid pretreatment, hydrogen peroxide-sulfuric acid delignification, and sulfuric acid-assisted glycerol swelling were, respectively, proven to break the three obstacle mechanisms that affect the cellulase of poplar. The removal of key obstacles improved the cellulase digestibility of poplar enzyme-hydrolyzed residues by 188.7 %, and glucose yield increased from 34.6 % to 99.9 %. Therefore, a total of 39.5 g glucose was obtained from 100 g poplar sawdust by integrating the above three technologies. This work presented insight into and removed the key obstacles to enzymatic digestibility of poplar cellulose and developed an integrated technology to effectively convert full cellulose fraction to glucose from woody biomass.

Bio-based succinic acid production from durian husk: A rising Southeast Asia agricultural waste

The spiking availability of durian husk in Southeast Asia reveals a promising yet underutilized lignocellulosic biomass. Compositional characterization via National Renewable Energy Laboratories protocols (NREL) showed that durian husk contained 40.96% glucan, 9.51% xylan, and 26.48% of lignin. Enzymatic hydrolysis of durian husk was improved upon dilute sulfuric acid pretreatment. The optimal conditions for particle size and pretreatment duration were determined to be − 80/+ 100 mesh size durian husk exposing to an hour of acid pretreatment, resulting in a maximum reducing sugar recovery of 37.29 and 24.32 g/L for acid and enzyme hydrolysate, respectively. Fermentability tests using Actinobacillus succinogenes 130Z for both acid and enzyme hydrolysates demonstrated succinic acid yields of 49% and 63% (g/g), respectively. A stoichiometry equation describing the formation of substrate and all products was established based on the results obtained from batch fermentation: C6H12O6 + 0.377 C5H10O5 + 0.130NH3 + 0.238CO2 → 0.864 C4H6O4 + 0.224 C2H4O2 + 0.134CH2O2 + 0.292CH2.096O0.111N0.658 + 0.231 H2O. This equation serves as a useful reference for future investigations into succinic acid production reactions. The cost-effectiveness of succinic acid production and its environmental performance were evaluated to highlight the potential of durian husk as feedstock. This study presents the first report on the utilization of durian husk as an alternative and sustainable feedstock for the production of bio-based succinic acid.

Pilot-scale bioethanol production from the starch of avocado seeds using a combination of dilute acid-based hydrolysis and alcoholic fermentation by Saccharomyces cerevisiae

BackgroundA processing methodology of raw starch extraction from avocado seeds (ASs) and a sequential hydrolysis and fermentation bioprocess in just a few steps was successfully obtained for the bioethanol production by a single yeast Saccharomyces cerevisiae strain and this research was also to investigate the optimum conditions for the pretreatment of biomass and technical procedures for the production of bioethanol. It successfully resulted in high yields and productivity of all the experiments from the laboratory scale and the pilot plant. Ethanol yields from pretreated starch are comparable with those in commercial industries that use molasses and hydrolyzed starch as raw materials.ResultsBefore the pilot-scale bioethanol production, studies of starch extraction and dilute sulfuric acid-based pretreatment was carefully conducted. The amount of starch extracted from dry and fresh avocado seed was 16.85 g ± 0.34 g and 29.79 ± 3.18 g of dry starch, representing a yield of ∼17% and 30%, respectively. After a dilute sulfuric acid pretreatment of starch, the released reducing sugars (RRS) were obtained and the hydrolysate slurries containing glucose (109.79 ± 1.14 g/L), xylose (0.99 ± 0.06 g/L), and arabinose (0.38 ± 0.01 g/L). The efficiency of total sugar conversion was 73.40%, with a productivity of 9.26 g/L/h. The ethanol fermentation in a 125 mL flask fermenter showed that Saccharomyces cerevisiae (Fali, active dry yeast) produced the maximum ethanol concentration, pmax at 49.05 g/L (6.22% v/v) with a yield coefficient, Yp/s of 0.44 gEthanol/gGlucose, a productivity or production rate, rp at 2.01 g/L/h and an efficiency, Ef of 85.37%. The pilot scale experiments of the ethanol fermentation using the 40-L fermenter were also successfully achieved with essentially good results. The values of pmax,Yp/s, rp, and Ef of the 40-L scale were at 50.94 g/L (6.46% v/v), 0.45 gEthanol/gGlucose, 2.11 g/L/h, and 88.74%, respectively. Because of using raw starch, major by-products, i.e., acetic acid in the two scales were very low, in ranges of 0.88–2.45 g/L, and lactic acid was not produced, which are less than those values in the industries.ConclusionsThe sequential hydrolysis and fermentation process of two scales for ethanol production using the combination of hydrolysis by utilizing dilute sulfuric acid-based pretreatment and fermentation by a single yeast Saccharomyces cerevisiae strain is practicable and feasible for realistic and effective scale-up strategies of bioethanol production from the starch of avocado seeds.

Implementing dilute acid pretreatment coupled with solid acid catalysis and enzymatic hydrolysis to improve bioconversion of bamboo shoot shells

Exploiting bamboo shoot shells (BSS) as feedstocks for biorefining is a crucial scheme to advance the bioavailability of bamboo shoots. This work applied traditional dilute sulfuric acid pretreatment (DAP) to treat BSS and simultaneously prepared the solid-acid-catalyst by using BSS as carbon-based carriers. The biocatalysis of the prehydrolysate from DAP and enzymatic hydrolysis of pretreated BSS was subsequently performed to achieve efficient bioconversion of its carbohydrates. The results displayed that 0.1 g/L H2SO4 employed in DAP was the optimal condition for furfural conversion of BSS during biocatalysis, reaching the maximum of 41%. Meanwhile, the enzymatic hydrolysis efficiency of the pretreated BSS also reached the maximum of 97%. This increment of efficiency was ascribed to the enhancement of accessibility and cellulosic crystal size, and also the reduction of surface area of lignin in BSS. Ultimately, the efficient bioutilization of BSS and bioconversion of its carbohydrates were realized by DAP technology.

Glucose Conversion for Biobutanol Production from Fresh Chlorella sorokiniana via Direct Enzymatic Hydrolysis

Microalgae, which accumulate considerable carbohydrates, are a potential source of glucose for biofuel fermentation. In this study, we investigated the enzymatic hydrolysis efficiency of wet microalgal biomass compared with freeze-dried and oven-dried biomasses, both with and without an acidic pretreatment. With the dilute sulfuric acid pretreatment followed by amy (α-amylase and amyloglucosidase) and cellulase hydrolysis, approximately 95.4% of the glucose was recovered; however, 88.5% was released by the pretreatment with 2% (w/v) sulfuric acid, which indicates the potential of the acids for direct saccharification process. There were no considerable differences in the glucose yields among the three kinds of materials. In the direct amy hydrolysis without any pretreatment, a 78.7% glucose yield was obtained, and the addition of cellulase had no significant effect on the hydrolysis to glucose. Compared with the oven-dried biomass, the wet biomass produced a substantially higher glucose yield, which is possibly because the cross-linked cells of the oven-dried biomass prevented the accessibility of the enzymes. According to the results, the fresh microalgal biomass without cell disruption can be directly used for enzymatic hydrolysis to produce glucose. The enzymatic hydrolysate of the wet microalgal biomass was successfully used for acetone–butanol–ethanol (ABE) fermentation, which produced 7.2 g/L of ABE, indicating the application potential of wet microalgae in the bioalcohol fuel fermentation process.

Enhancing enzymatic hydrolysis of Chinese fir sawdust by using the synergistic effect of dilute sulfuric acid and sodium chlorite pretreatment

Enhancing enzymatic hydrolysis of Chinese fir sawdust by using the synergistic effect of dilute sulfuric acid and sodium chlorite pretreatment

Effect of Dilute Acid Pretreatment and Lignin Extraction Conditions on Lignin Properties and Suitability as a Phenol Replacement in Phenol-Formaldehyde Wood Adhesives.

Corn stover was subjected to dilute sulfuric acid pretreatment to assess the impact of pretreatment conditions on lignin extractability, properties, and utility as a phenol replacement in wood phenol-formaldehyde (PF) adhesives. It was identified that both formic acid and NaOH could extract and recover 60-70% of the lignin remaining after pretreatment and enzymatic hydrolysis under the mildest pretreatment conditions while simultaneously achieving reasonable enzymatic hydrolysis yields (>60%). The availability of reaction sites for the incorporation of lignins into the PF polymer matrix (i.e., unsubstituted phenolic hydroxyl groups) was shown to be strongly impacted by the pretreatment time and the recovery. Finally, a lignin-based wood adhesive was formulated by replacing 100% of the phenol with formic-acid-extracted lignin, which exhibited a dry shear strength exceeding a conventional PF adhesive. These findings suggest that both pretreatment and lignin extraction conditions can be tailored to yield lignins with properties targeted for this co-product application.

Optimization of dilute sulfuric acid pretreatment of kitchen garbage for increased lactic acid production

Optimization of dilute sulfuric acid pretreatment of kitchen garbage for increased lactic acid production

Enhanced pyrolysis of lignocellulosic biomass by room-temperature dilute sulfuric acid pretreatment

In the present work, the effects of dilute sulfuric acid (0.2 wt%) pretreatment at room temperature (30 °C) on the pyrolytic behaviors of pubescens, rice straw, willow, and cypress were studied. For herbaceous biomass, the degradation temperatures (Ta) of cellulose increased by around 20 °C, and the yields of bio-char in slow pyrolysis decreased by around 4 % after the pretreatment. For the pretreated softwood biomass, a decreased Ta and an increased yield of bio-char were found for willow, while little variation for cypress was detected. Regarding the typical small molecular products in the bio-oil after the pretreatment, levoglucosan (LGA) was only found in those of the pretreated pubescens (14.3 %, 400 °C) and cypress (4.5 %, 400 °C; 9.3 %, 500 °C), whereas levoglucosenone (LGO) was only found in that of the pretreated willow (9.7 %, 400 °C; 10.5 %, 500 °C). Increased yield of the small lignin-derived compounds was found for rice straw (from 7.9 % to 14.5 %), while decreased yields were found for willow and cypress. For the oligomeric part in bio-oil, significantly promoted production of holocellulose-derived oligomers was found with slightly increased proportions of heavy oligomers (>1300 Da). Meanwhile, the production of lignin-derived oligomers was apparently promoted as well, especially for rice straw. In the 500 °C pyrolysis of the two woody biomass samples, in addition to more complete degradation (higher yields of bio-oil and small-molecular compounds, and lower yield of bio-char), it was also found that the proportion of the oligomers in 256–500 Da increased significantly, and more holocellulose-derived oligomers were found. It was, therefore, speculated that a higher pyrolysis temperature would mainly promote the pyrolysis of cellulose and hemicellulose.

A novel approach for 3D in situ visualization of morphological changes in dilute sulfuric acid-pretreated straw by micro-computed tomography

Lignocellulosic biomass is a crucial agricultural and industrial product. Dilute acid pretreatment is a common step to break down the dense and complex lignocellulose-based structure of straw to enhance its biochemical conversion efficiency. The straw derived from diverse crops, such as corn, cotton, and rape, displays diverse internal morphologies and varying responses to pretreatment methods. It is, therefore, necessary to understand the underlying mechanisms of these microstructural changes, to maximize their industrial value. This is the first study to demonstrate X-ray micro-computed tomography (micro-CT) based 3D visualization to observe the changes in the histology of straw caused by pretreatment using dilute acids, understand the mechanism of dilute sulfuric acid pretreatment, and visualize the reason behind the resulting morphological changes in the microstructure of straw. Three types of straw from corn, cotton, and rape were treated with 1.5% (w/w) dilute sulfuric acid at 121 °C for 30 and 60 min. Subsequently, the straw samples were imaged using Skyscan1275 under optimized micro-CT scan and image reconstruction conditions. In addition, the algorithms for image segmentation were determined. Volume- and slice-rendered images were analyzed via pseudo-color processing for different straw samples. Furthermore, the results of the micro-CT 3D in situ visualization aligned with the chemical analysis of lignocellulose composition. Overall, the presented non-invasive 3D micro-CT technique can provide crucial insights into the morphology and thus microstructural changes of lignocellulose-based materials not only qualitatively, but also semi-quantitatively. The non-invasive imaging methodology described in the present study will prove to be extremely helpful in prompt in situ analysis and comparison of diverse treatment approaches using lignocellulosic biomass.

Optimization of Dilute Acid Pretreatment for Enhanced Release of Fermentable Sugars from Sugarcane Bagasse and Validation by Biophysical Characterization

Pretreatment of biomass is one of the most challenging steps in the process of second-generation (2G) ethanol and biochemical production. Dilute acid pretreatment is a widely adapted and convenient method to recover pentose (C5) as well as hexose (C6) sugars due to its featured solubilization of hemicellulose and cellulose before and after enzymatic saccharification, respectively. In the present study, dilute sulfuric acid (H2SO4) pretreatment of sugarcane bagasse (SCB) was statistically optimized using the face-centered composite design (FCCD) of response surface methodology (RSM) in terms of acid concentration (0.1–3% v/v), solid loading (5–20% w/v) and residence time (15–60 min) at constant temperature of 121 °C followed by enzymatic hydrolysis using commercial cellulase (Novozymes Cellic CTec2) for enhanced combined sugar yield (CSY) comprising of C5 and C6 sugars in pretreated as well as saccharified hydrolysates. Optimized process parameters found in the study were 2.18% (v/v) acid; 14.35% (w/v) solid loading; and 29.49 min residence time. CSY under optimized conditions was found to be 521.42 ± 7.2 g/kg raw SCB with 72.06 ± 1.0% sugars recovered out of the maximum theoretical sugars present in the raw biomass. Total reducing sugar yields in pretreated and saccharified hydrolysates were found to be 215.28 ± 2.4 and 306.14 ± 5.3 g/kg raw SCB, respectively. Morphological and structural changes in optimized pretreated and saccharified biomass further validated the efficiency of optimized pretreatment applied in the present study. The maximum ethanol concentration, volumetric productivity and yield from released sugars were calculated as 10.82 ± 2.2 g/L, 0.45 ± 0.9 g/L/h and 0.42 g/g-glucose consumed or 71.45 ± 2.5 g/kg raw SCB, respectively. Ethanol yield obtained from the fermentation of dilute H2SO4-pretreated SCB was corresponding to 82.4% of the theoretical ethanol yield.

Coupling of pretreatment and pyrolysis improving the production of levoglucosan from corncob

Influence of organic biphasic pretreatment on levoglucosan (LG) yield from biomass pyrolysis was investigated with dilute sulfuric acid and dilute sodium hydroxide pretreatment as comparison, simultaneously, the mechanism was explored in depth. It was found that with monophasic (water) conditions, dilute sulfuric acid and dilute sodium hydroxide pretreatment, the yield of LG was all increase largely (from untreated 28% to treated 60–85%) due to the removal of hemicellulose or lignin. Phenoxyethanol was introduced to construct a biphasic system, and it showed that acid biphasic pretreatment of corncob followed by pyrolysis at 450 °C increased the LG yield to 91% and limited the formation of by-products, especially acids, ketones, and esters. In addition, it can easily realize the efficient recovery of the removed hemicellulose (75%) and lignin (49%). The different LG yields implied that removing lignin is more effective in increasing LG than removing hemicellulose. These findings provide instruction for the highly selective production of anhydrous sugars from biomass waste.

Comprehensive investigation of multiples factors in sulfuric acid pretreatment on the enzymatic hydrolysis of waste straw cellulose

The prerequisite for cellulosic biochemical production from lignocellulosic materials is efficient enzymatic hydrolysis that is a complicated heterogeneous catalytic process and affected by the complex lignin-cellulose-hemicellulose network. Understanding the main influencing factors for enzymatic hydrolysis is of substantial significance to guide the design of a biorefinery process. An experimental study of the pretreatment indicated that acid pretreatment is preferable for herbaceous feedstocks. Therefore, the classic dilute sulfuric acid pretreatment was utilized to hydrolyze and remove hemicellulose from three representative types of agricultural straws at various intensities. From the enzymatic hydrolysis of residual cellulose perspective, the crystallinity index and enzyme accessibility of the pretreated materials were also mathematically correlated to hemicellulose removals, respectively. For the better insight and understanding of the mathematical logics, the linear and nonlinear kinetic models were therefore compared, and the relationship was established by the five-parameter logistic equations and Allosteric sigmoidal models with well fittings.

Bioethanol production from pretreated whole slurry rice straw by thermophilic co-culture

In this study, an innovative approach is proposed for the valorization of all sugars (cellulosic and hemicellulosic) contained in a lignocellulosic feedstock, as is rice straw biomass (RSB), for the production of second-generation bioethanol. For this purpose, the whole slurry obtained after dilute sulfuric acid pretreatment at a biomass loading of 5% (w/v) was subjected to consolidated bioprocessing without previous solid-liquid separation and/or detoxification and the concentrated solution of carbohydrates recovered was fermented to bioethanol by a co-culture of thermophilic anaerobic bacteria. To bypass the inhibitory effect of undetoxified whole slurry on the lower fermentability of the co-culture, the concentration of different additives was optimized for improved buffering capacity, enhanced substrate-microbe interaction, and by-product elimination. The individual supplementation of 3 selected additives at optimum concentration; 20 mM calcium carbonate, 0.4% (v/v) polyethylene glycol, and 1% (w/v) sodium acetate resulted in enhanced ethanol yield of 130.83 mM, 106.37 mM, and 99.10 mM, respectively, compared to 75.03 mM ethanol yield in control. The combined effect of optimal dosage of these additives was able to improve the ethanol concentration and yield to 142 mM and 48% of the theoretical maximum respectively, suggesting about 88.58% increase compared to control medium without additives supplementation. The combined supplementation of additives proved to be an advantageous strategy for bioethanol production using undetoxified whole slurry biomass.

Curtailing citrate buffer inhibition effect on S. cerevisiae to enhance the fermentability of cellulosic hydrolysate

The current investigation emphasizes the effect of different citrate buffer strengths on cellulose conversion efficiency during the enzymatic hydrolysis of pretreated sorghum stalks, and the subsequent fermentation of enzymatic hydrolysates to achieve the industrial titer of bioethanol. A set of enzymatic hydrolysis experiments were conducted in 50 mM, 5 mM and 0.5 mM citrate buffer mediums which contain dilute sulfuric acid pretreatment derived biomass as substrate. As a result, similar cellulose conversion efficiencies such as 68.7%, 68.8% and 69% were observed during enzymatic hydrolysis of pretreated biomass at 50 mM, 5 mM and 0.5 mM citrate buffer strengths, respectively. Enzymatic hydrolysates derived from 50 mM, 5 mM and 0.5 mM citrate buffer mediums has been fermented by S. cerevisiae, which resulted in ethanol yield of 0.38 gp/gs, 0.47 gp/gs, and 0.47 gp/gs, respectively. As per the fermentation results, it has been observed that, metabolic growth of S. cerevisiae has been hindered due to the presence of high concentration of citric acid (50 mM) in the enzymatic hydrolysate.

THF co-solvent pretreatment prevents lignin redeposition from interfering with enzymes yielding prolonged cellulase activity

BackgroundConventional aqueous dilute sulfuric acid (DSA) pretreatment of lignocellulosic biomass facilitates hemicellulose solubilization and can improve subsequent enzymatic digestibility of cellulose to fermentable glucose. However, much of the lignin after DSA pretreatment either remains intact within the cell wall or readily redeposits back onto the biomass surface. This redeposited lignin has been shown to reduce enzyme activity and contribute to rapid enzyme deactivation, thus, necessitating significantly higher enzyme loadings than deemed economical for biofuel production from biomass.ResultsIn this study, we demonstrate how detrimental lignin redeposition on biomass surface after pretreatment can be prevented by employing Co-solvent Enhanced Lignocellulosic Fractionation (CELF) pretreatment that uses THF–water co-solvents with dilute sulfuric acid to solubilize lignin and overcome limitations of DSA pretreatment. We first find that enzymatic hydrolysis of CELF-pretreated switchgrass can sustain a high enzyme activity over incubation periods as long as 5 weeks with enzyme doses as low as 2 mg protein/g glucan to achieve 90% yield to glucose. A modified Ninhydrin-based protein assay revealed that the free-enzyme concentration in the hydrolysate liquor, related to enzyme activity, remained unchanged over long hydrolysis times. DSA-pretreated switchgrass, by contrast, had a 40% drop in free enzymes in solution during incubation, providing evidence of enzyme deactivation. Furthermore, measurements of enzyme adsorption per gram of lignin suggested that CELF prevented lignin redeposition onto the biomass surface, and the little lignin left in the solids was mostly integral to the original lignin–carbohydrate complex (LCC). Scanning electron micrographs and NMR characterization of lignin supported this observation.ConclusionsEnzymatic hydrolysis of solids from CELF pretreatment of switchgrass at low enzyme loadings was sustained for considerably longer times and reached higher conversions than for DSA solids. Analysis of solids following pretreatment and enzymatic hydrolysis showed that prolonged cellulase activity could be attributed to the limited lignin redeposition on the biomass surface making more enzymes available for hydrolysis of more accessible glucan.

Integration of First- and Second-generation Bioethanol Production from Beet molasses and Distillery Stillage After Dilute Sulfuric Acid Pretreatment

The possibility of using waste distillery stillage (first-generation technology) after dilute acid pretreatment, as a medium for the preparation of beet molasses mash, for ethanol production according to the simultaneous saccharification and fermentation (SSF) technology, was assessed. The combination of lignocellulosic hydrolysates made from acid-pretreated stillage with sugar-rich beet molasses is an effective way of utilizing the first-generation ethanol production by-products in the second-generation ethanol production technology. It was demonstrated that the final ethanol concentration could be as high as 90 g/L. The process yield was over 94% of the theoretical yield when the molasses was diluted using acid-pretreated maize distillery stillage. An attempt to increase the pool of fermentable sugars by using cellulases to hydrolyze cellulose failed due to product inhibition in the fermentation medium with a high glucose concentration. A more than threefold increase in the concentration of ethyl acetate (even up to 924.4±11.8 mg/L) was observed in the distillates obtained from the media incubated with cellulases. The use of beet molasses combined with the hydrolysate of pretreated distillery stillage also changed the concentration of other volatile by-products. An increase in the concentration of aldehydes (mainly acetaldehyde to a concentration of above 1500 mg/L), methanol, 1-propanol, and 1-butanol was observed, while the concentration of higher alcohols (isobutanol, 2-methyl-1-butanol, 3-methyl-1-butanol) decreased. Interestingly, the use of cellulases in fermentation media from molasses and stillage hydrolysates resulted in an average fourfold increase in the concentration of this ester to a maximum level of 924.4±11.8 mg/L. Hydrolysates made from acid-pretreated distillery stillage, combined with sugar-rich beet molasses to boost the efficiency of the conversion process, can be successfully used in the production of second-generation fuel ethanol. However, further optimization of the cellulose enzymatic hydrolysis process is required for efficient use of the raw material.

Simultaneous saccharification and bioethanol production from underutilized biomass, cowpea haulm using co-cultures of Saccharomyces cerevisiae (BY4743) and Scheffersomyces stipitis (PsY633)

Cowpea cultivation generates large quantities of biomass after pod harvest which are underutilized and could be exploited for energy generation. In this study, evaluation of sugar production by dilute sulfuric acid pretreatment of cowpea haulm for potential ethanolic fermentation was carried out. A central composite design (CCD) was used to investigate the effects of temperature (100–120 °C), time (30–90 min), and acid concentration (1.0–4.0%) on sugar yield and inhibitor formation. The model F-values of 25.42, 78.81, and 6.96 for xylose yield, glucose yield, and total inhibitor concentration, respectively, with low probability value (p < 0.05) suggest a high significance of the models. While the R2-values of 0.9581, 0.9861, and 0.8424 indicated a satisfactory agreement of the quadratic models in predicting the responses. The quadratic model predicted optimum conditions for maximum xylose (76.5%) and glucose yield (28.6%), at minimum inhibitor concentration (2.34 g/L) and temperature (110 °C), with an acid concentration of 3.1% and reaction time of 55 min. These conditions were validated experimentally suggesting the effectiveness of experimental design towards process optimization. The resulting sugar-rich prehydrolysate was detoxified and fermented to ethanol using co-cultures of Saccharomyces cerevisiae BY4743 and Scheffersomyces stipitis (PsY633), while the recovered pretreated solid was subjected to simultaneous saccharification and fermentation with prehydrolysis (PSSF). A total ethanol titer of 15.67 g/L was obtained which corresponds to an overall conversion efficiency of 75%. This suggests that cowpea haulm could also be potentially exploited for bioethanol production either singly or in combination with other lignocellulosic biomass.