Non-severe thermochemical hydrolysis of stover from white corn and sequential enzymatic saccharification and fermentation to ethanol

Simultaneous saccharification and fermentation for D-lactic acid production using a metabolically engineered Escherichia coli adapted to high temperature

BACKGROUND

Escherichia coli JU15 is a metabolically engineered strain capable to metabolize C5 and C6 sugars with a high yield of D-lactic acid production at its optimal growth temperature (37 °C). The simultaneous saccharification and fermentation process allow to use lignocellulosic biomass as a cost-effective and high-yield strategy. However, this process requires microorganisms capable of growth at a temperature close to 50 °C, at which the activity of cellulolytic enzymes works efficiently.

RESULTS

The thermotolerant strain GT48 was generated by adaptive laboratory evolution in batch and chemostat cultures under temperature increments until 48 °C. The strain GT48 was able to grow and ferment glucose to D-lactate at 47 °C. It was found that a pH of 6.3 conciliated with GT48 growth and cellulase activity of a commercial cocktail. Hence, this pH was used for the SSF of a diluted acid-pretreated corn stover (DAPCS) at a solid load of 15% (w/w), 15 FPU/g-DAPCS, and 47 °C. Under such conditions, the strain GT48 exhibited remarkable performance, producing D-lactate at a level of 1.41, 1.42, and 1.48-fold higher in titer, productivity, and yield, respectively, compared to parental strain at 45 °C.

CONCLUSIONS

In general, our results show for the first time that a thermal-adapted strain of E. coli is capable of being used in the simultaneous saccharification and fermentation process without pre-saccharification stage at high temperatures.

Co-Fermentation of Glucose-Xylose Mixtures from Agroindustrial Residues by Ethanologenic Escherichia coli: A Study on the Lack of Carbon Catabolite Repression in Strain MS04

The production of biofuels, such as bioethanol from lignocellulosic biomass, is an important task within the sustainable energy concept. Understanding the metabolism of ethanologenic microorganisms for the consumption of sugar mixtures contained in lignocellulosic hydrolysates could allow the improvement of the fermentation process. In this study, the ethanologenic strain Escherichia coli MS04 was used to ferment hydrolysates from five different lignocellulosic agroindustrial wastes, which contained different glucose and xylose concentrations. The volumetric rates of glucose and xylose consumption and ethanol production depend on the initial concentration of glucose and xylose, concentrations of inhibitors, and the positive effect of acetate in the fermentation to ethanol. Ethanol yields above 80% and productivities up to 1.85 g_EtOH/Lh were obtained. Furthermore, in all evaluations, a simultaneous co-consumption of glucose and xylose was observed. The effect of deleting the xyIR regulator was studied, concluding that it plays an important role in the metabolism of monosaccharides and in xylose consumption. Moreover, the importance of acetate was confirmed for the ethanologenic strain, showing the positive effect of acetate on the co-consumption rates of glucose and xylose in cultivation media and hydrolysates containing sugar mixtures.

Cardoon Hydrolysate Detoxification by Activated Carbon or Membranes System for Bioethanol Production

Advanced biofuels incorporation into the transportation sector, particularly cellulosic bioethanol, is crucial for attaining carbon neutrality by 2050, contributing to climate changes mitigation and wastes minimization. The world needs biofuel to be commercially available to tackle the socioeconomic challenges coming from the continued use of fossil fuels. Cynara cardunculus (cardoon) is a cheap lignocellulosic raw biomass that easily grows in Mediterraneous soils and is a potential renewable resource for a biorefinery. This work aimed to study the bioethanol production from cardoon hemicellulosic hydrolysates, which originated from dilute sulfuric acid hydrolysis pretreatment. A detoxification step to remove released microbial fermentative inhibitors was evaluated by using both activated carbon adsorption and a nanofiltration membrane system. The Scheffersomyces stipitis CBS5773 yeast and the modified Escherichia coli MS04 fermentation performances at different experimental conditions were compared. The promising results with E. coli, using detoxified cardoon by membrane nanofiltration, led to a bioethanol volumetric productivity of 0.30 g·L−1·h−1, with a conversion efficiency of 94.5%. Regarding the S. stipitis, in similar fermentation conditions, volumetric productivity of 0.091 g·L−1·h−1 with a conversion efficiency of 64.9% was obtained. Concluding, the production of bioethanol through detoxification of hemicellulosic cardoon hydrolysate presents a suitable alternative for the production of second-generation bioethanol, especially using the modified E. coli.

Ethanol production by Escherichia coli from detoxified lignocellulosic teak wood hydrolysates with high concentration of phenolic compounds

Teak wood residues were subjected to thermochemical pretreatment, enzymatic saccharification, and detoxification to obtain syrups with a high concentration of fermentable sugars for ethanol production with the ethanologenic Escherichia coli strain MS04. Teak is a hardwood, and thus a robust deconstructive pretreatment was applied followed by enzymatic saccharification. The resulting syrup contained 60 g L-1 glucose, 18 g L-1 xylose, 6 g L-1 acetate, less than 0.1 g L-1 of total furans, and 12 g L-1 of soluble phenolic compounds (SPC). This concentration of SPC is toxic to E. coli, and thus two detoxification strategies were assayed: 1) treatment with Coriolopsis gallica laccase followed by addition of activated carbon and 2) overliming with Ca(OH)2. These reduced the phenolic compounds by 40 and 76%, respectively. The detoxified syrups were centrifuged and fermented with E. coli MS04. Cultivation with the over-limed hydrolysate showed a 60% higher volumetric productivity (0.45 gETOH L-1 h-1). The bioethanol/sugars yield was over 90% in both strategies.

Limited oxygen conditions as an approach to scale-up and improve D and L-lactic acid production in mineral media and avocado seed hydrolysates with metabolically engineered Escherichia coli

The effectiveness of micro-aeration on lactate (LA) production by metabolically engineered Escherichia coli was evaluated in 1 L bioreactors containing mineral media and glucose (70 g/L). Volumetric oxygen transfer coefficients (k_La) between 12.6 and 28.7 h^-1 increased the specific growth rate (µ) and volumetric productivity (Q_LA) by 300 and 400%, respectively, without a significant decrease in lactate yield (Y_LA), when compared with non-aerated fermentations. A k_La of 12.6 h^-1 was successfully used as a criterion to scale-up the production of L and D-lactate from 1 to 11 and 130 L. Approximately constant Q_LA and Y_LA values were obtained throughout the fermentation scale-up process. Furthermore, a D-lactogenic fermentation was carried out in 1 L bioreactors using avocado seed hydrolysate as a culture medium under the same k_La value, displaying high Q_LA and Y_LA.

Bioethanol from hydrolyzed Spirulina (Arthrospira platensis) biomass using ethanologenic bacteria

Photosynthetic microorganisms are considered excellent feedstock for biofuel production in developing biomass production technologies. A study was conducted to evaluate ethanol production with the sequential enzymatic saccharification and fermentation of Arthrospira platensis (Spirulina) biomass with the metabolically engineered Escherichia coli strain MS04. A. platensis was cultivated semicontinuously in an open raceway pond, and the carbohydrate content was determined to be as high as 40%. The enzymatic saccharification was designed to release the maximum amount of glucose. After 40 h of enzymatic saccharification, 27 g L−1 of monosaccharides was obtained. These slurries were fermented with ethanologenic bacteria, achieving 12.7 g L−1 ethanol after 9 h of fermentation, which corresponds to 92% conversion yield of the glucose content in the hydrolysate, 0.13 g of ethanol per 1 g of Spirulina biomass and a volumetric productivity of 1.4 g of ethanol L−1 h−1. Therefore, we conclude that it is possible, in a short time, to obtain a high ethanol yield corresponding to 160 L per ton of dry biomass with a high productivity.

Production of d-Lactate from Avocado Seed Hydrolysates by Metabolically Engineered Escherichia coli JU15

Agroindustry residues can be used to produce valuable chemicals such as lactic acid, which is a primary chemical platform with many industrial applications. Biotechnological processes are the main approach of lactic acid production; however, culture media has an important impact on their costs. As a result, researchers are exploring various methods of production that use residual or waste biomass as raw materials, most of which are rich in lignocellulose. Nevertheless, starch and micronutrients such as those contained in avocado seeds stand out as promising feedstock for the bioprocess as well. In this study, the lactogenic Escherichia coli strain JU15 was evaluated for producing d-lactate using an avocado seed hydrolysate medium in a controlled stirred-tank bioreactor. The highest lactic acid concentration achieved was 37.8 g L−1 using 120 g L−1 as the content of initial reducing sugars. The results showed that d-lactate can be produced from avocado seed, which hydrolysates to 0.52 g L−1 h−1 using the engineered E. coli JU15. This study may serve as a starting point to further develop bioprocesses for producing metabolites using avocado seed hydrolysates.

Lactic acid production from glucose and xylose using the lactogenic Escherichia coli strain JU15: Experiments and techno-economic results

In this work, d-lactic acid production was evaluated from a simulated hydrolysate of corn stover (32 g/L xylose, 42 g/L glucose) with the metabolically engineered Escherichia coli strain JU15. Based on the experimental results, a technical and economic analysis of the entire process was performed using the Aspen Plus software. As a result, it was possible to show that the strain can efficiently produce lactic acid from both sugars, reaching a final concentration of 40 g/L and a yield of 0.6 g lactic acid/g sugars. The process is economically viable at higher scales of 1000 tons/day. The cost distribution is influenced by the scale of the process; on a larger scale, the cost of raw materials represents a higher percentage of total cost than it does on smaller scales. The use of a metabolically engineered strain allows a better use of the sugars obtained from agroindustrial residues.

Autohydrolysis pretreatment assessment in ethanol production from agave bagasse

The aim of the present work was to assess the autohydrolysis pretreatment of Agave tequilana bagasse for ethanol production. The pretreatment was conducted using a one-liter high pressure Parr reactor under different severity factors (SF) at a 1:6w/v ratio (solid:liquid) and 200rpm. The solids obtained under the selected autohydrolysis conditions were subjected to enzymatic hydrolysis with a commercial cellulase cocktail, and the enzymatic hydrolysate was fermented using Saccharomyces cerevisiae. The results obtained from the pretreatment process showed that the glucan content in the pretreated solid was mostly preserved, and an increase in the digestibility was observed for the case with a SF of 4.13 (190°C, 30min). Enzymatic hydrolysis of the pretreated solids showed a yield of 74.3%, with a glucose concentration of 126g/L, resulting in 65.26g/L of ethanol after 10h of fermentation, which represent a 98.4% conversion according to the theoretical ethanol yield value.

Sequential enzymatic saccharification and fermentation of ionic liquid and organosolv pretreated agave bagasse for ethanol production

Agave bagasse (AGB) has gained recognition as a drought-tolerant biofuel feedstock with high productivity in semiarid regions. A comparative analysis of ionic liquid (IL) and organosolv (OV) pretreatment technologies in AGB was performed using a sequential enzymatic saccharification and fermentation (SESF) strategy with cellulolytic enzymes and the ethanologenic Escherichia coli strain MS04. After pretreatment, 86% of xylan and 45% of lignin were removed from OV-AGB, whereas IL-AGB reduced lignin content by 28% and xylan by 50% when compared to the untreated biomass. High glucan (>90%) and xylan (>83%) conversion was obtained with both pretreated samples. During the fermentation stage (48h), 12.1 and 12.7kg of ethanol were produced per 100kg of untreated AGB for IL and OV, respectively. These comparative analyses showed the advantages of SESF using IL and OV in a biorefinery configuration where a better understanding of AGB recalcitrance is key for future applications.

Ethanol production by Escherichia coli from Arundo donax biomass under SSF, SHF or CBP process configurations and in situ production of a multifunctional glucanase and xylanase

Diluted acid or liquid hot water (LHW) pretreated Arundo donax biomass was converted into ethanol under separated hydrolysis and fermentation (SHF) or simultaneous saccharification and fermentation (SSF) using Escherichia coli as the fermentative organism. Up to 0.26gL^-1h^-1 and 25.0gL^-1 of ethanol were obtained with diluted acid pretreated biomass under SSF compared to 0.17gL^-1h^-1 and 24gL^-1 under SHF. LHW pretreated biomass elicited 25% lower yields on average. Saccharification was carried out with Cellic CTec2 cocktail. Alternatively, under a consolidated bioprocess (CBP) where the ethanologenic bacteria was complemented with a novel multifunctional glucanase and xylanase, ethanol concentration was 7.6gL^-1 and 7.2gL^-1 after 96h for dilute acid or LHW pretreated biomass, respectively, without any prior saccharification step. According to these results, a bacterial fermentative host combined with in situ enzyme expression can improve ethanol production from A. donax biomass.

Production of d-lactate from sugarcane bagasse and corn stover hydrolysates using metabolic engineered Escherichia coli strains

In this study, the lactogenic Escherichia coli strain JU15 was used and modified to produce d-lactate (d-LA) from plant hydrolysates with a minimal nutrient addition in pH controlled fermenters. Results showed that strain JU15 produces d-LA with high yield and productivity in laboratory simulated hydrolysate media and actual sugar cane bagasse hemicellulosic hydrolysate. Strain JU15 showed sequential carbon source utilization and acetic acid production. The l-lactic and acetic acid production pathways were deleted in JU15, resulting strain AV03 (JU15 ΔpoxB, ΔackA-pta, ΔmgsA), which showed simultaneous consumption of glucose and xylose and no acetic acid production in the simulated hydrolysate. The d-LA yield from hydrolysate sugars was close to 0.95gD-LA/gsugars in all cases. Our results show that d-LA can be produced from plant hydrolysates in simple batch fermentation processes with a high productivity using engineered E. coli strains at fermenter scales from 0.2 up to 10L.