Integrated bioethanol production to boost low-concentrated cellulosic ethanol without sacrificing ethanol yield

Genome-centric metagenomics revealed the effect of pH on the microbiome involved in short-chain fatty acids and ethanol production

Added-value chemicals production via food waste (FWs) valorization using open-mixed cultures is an emerging approach to replace petrochemical-based compounds. Nevertheless, the effects of operational parameters on the product spectrum remain uncertain given the wide number of co-occurring species and metabolisms. In this study, the identification of 58 metagenome-assembled genomes and their investigation assessed the effect of slight pH variations on microbial dynamics and the corresponding functions when FWs were subjected to anaerobic fermentation (AF) in 1-L continuous stirred tank reactors at 25 °C. The initial pH of 6.5 promoted a microbial community involved in acetate, butyrate and ethanol production, mediated by Bifidobacterium subtile IE007 and Eubacteriaceae IE027 as main species. A slight pH decrease to 6.1 shaped microbial functions that resulted in caproate and H₂ production, increasing the relevance of Eubacteriaceae IE037 role. This study elucidated the strong pH effect on product outputs when minimal variations take place in AF.

Overproduction of palmitoleic acid from corn stover hydrolysate by engineered Saccharomyces cerevisiae

Palmitoleic acid (POA) has been widely applied to nutrition and pharmaceutical industry. However, high cost of scale-up fermentation restricts the extensive application of POA. Hence, we investigated the availability of corn stover hydrolysate (CSH) as carbon source in POA production by engineered S. cerevisiae. Although the yeast growth was inhibited to some extent by CSH, the POA production with CSH was slightly higher than that with pure glucose. The C/N ratio of 120 and addition of 1 g/L lysine raised the POA titer up to 2.19 g/L and 2.05 g/L, respectively. Two-stage cultivation could increase the POA titer by upregulating the gene expression of key enzymes in fatty acid synthesis pathway. A high POA content of 57.5% (v/v) and a highest POA titer of 6.56 g/L were achieved under the optimized conditions. These findings provide a feasible approach for sustainable production of POA or its derivatives from CSH.

CelS-Catalyzed Processive Cellulose Degradation and Cellobiose Extraction for the Production of Bioethanol

Bacterial cellulase enzymes are potent candidates for the efficient production of bioethanol, a promising alternative to fossil fuels, from cellulosic biomass. These enzymes catalyze the breakdown of cellulose in plant biomass into simple sugars and then to bioethanol. In the absence of the enzyme, the cellulosic biomass is recalcitrant to decomposition due to fermentation-resistant lignin and pectin coatings on the cellulose surface, which make them inaccessible for hydrolysis. Cellobiohydrolase CelS is a microbial enzyme that binds to cellulose fiber and efficiently cleaves it into a simple sugar (cellobiose) by a repeated processive chopping mechanism. The two contributing factors to the catalytic reaction rate and the yield of cellobiose are the efficient product expulsion from the product binding site of CelS and the movement of the substrate or cellulose chain into the active site. Despite progress in understanding product expulsion in other cellulases, much remains to be understood about the molecular mechanism of processive action of these enzymes. Here, nonequilibrium molecular dynamics simulations using suitable reaction coordinates are carried out to investigate the energetics and mechanism of the substrate dynamics and product expulsion in CelS. The calculated free energy barrier for the product expulsion is three times lower than that for the processive action indicating that product removal is relatively easier and faster than the sliding of the substrate to the catalytic active site. The water traffic near the active site in response to the product expulsion and the processive action is also explored.

Ethanol Production from the Mixture of Waste French Fries and Municipal Wastewater via Separate Hydrolysis and Fermentation

The potential of bioethanol generation using the mixture of waste French fries (WFF) and municipal wastewater (MWW) via separate hydrolysis and fermentation (SHF) was evaluated in this study. The effect of WFF substrate loading (SL, 10%, 16%, and 20%, w/v) on the SHF was also examined. Both glucose production and hydrolysis efficiency increased with increasing of SL from 10 to 16% and the maximum glucose yield of 0.236 g glucose/g WFF and hydrolysis efficiency of 91.9% were obtained at SL of 16%. However, the glucose production and hydrolysis efficiency decreased when the SL further increased to 20% due to the inhibition on enzyme caused by higher glucose production. The mixture hydrolysate was then used as feedstock for ethanol fermentation. The maximum ethanol production of 22.69 g/L was obtained from SL of 16%. The highest rate of glucose conversion to ethanol was 84.2%. The results demonstrated that the mixture of WFF and MWW could be used for ethanol production by the SHF.

Continuous Bioethanol Production by Fungi and Yeast Working in Tandem

Biofuel is considered one of the most viable alternatives to fossil fuels derived from the dwindling petroleum resources that damage the environment. Bioethanol could be manufactured from agricultural wastes, thus providing inexpensive natural resources. Several strategies have been utilized to convert lignocellulosic hydrolysate to bioethanol with various suspended microorganisms. In this study, we alternatively propose to encapsulate these microorganisms in bioreactor setups. An immobilized cell system can provide resistance to the inhibitors present in hydrolysates, enhance productivity, facilitate the separation process, and improve microorganism recycling. Herein, we developed a continuous bioethanol production process by encapsulating three types of micro-organisms: T. reesei, S. cerevisiae, and P. stipitis. These microorganisms were encapsulated in SBP (“Small Bioreactor Platform”) capsules and tested for their viability post encapsulation, biological activity, and bioethanol production. Encapsulating microorganisms in SBP capsules provided a confined protective environment for the microorganisms, facilitated their acclimation, and ensured their long-term prosperity and activity. An additional significant benefit of utilizing SBP capsules was the simultaneous availability of saccharification and fermentation over a very long time—about 2.5–3 months—with no need to renew the cells or encapsulating matrices. Two different configurations were tested. The first one consisted of columns packed with fungal cells and specific yeast cells together. In the second configuration, the fungal cells were separated from the yeast cells into two columns in series. The presented systems achieved an efficiency of 60–70%, suggesting the long-term prosperity and uninterrupted metabolic activity of the microorganisms.

Bioconversion of food and lignocellulosic wastes employing sugar platform: A review of enzymatic hydrolysis and kinetics

Bioenergy and biochemicals can be sustainably produced through fermentation and anaerobic digestion (AD). However, this bioconversion processes could be more economical if the hydrolysis rates of substrates in bioreactors can be accelerated. In this review, the feasibilities of including enzymatic hydrolysis (EH) in various bioconversion systems were studied to facilitate the biological synergy. The reaction kinetics of EH in bioconversion systems comparing pretreated lignocellulosic biomass (LCB) and food waste (FW) substrates were reviewed. Possible strategies to improve the hydrolysis efficiency were explored, including co-cultivation during enzyme production and replacement of pure enzyme with on-site produced fungal mash during EH. Key insights into improvement of current AD and fermentation technologies were summarized and further formed into suggestions of future directions in techno-economic feasibility of biorefinery using mixture of the first-generation food crop feedstock with FW; and/or co-digestion of FW with LCB.

Recent advances in commercial biorefineries for lignocellulosic ethanol production: Current status, challenges and future perspectives

Cellulosic ethanol production has received global attention to use as transportation fuels with gasoline blending virtue of carbon benefits and decarbonization. However, due to changing feedstock composition, natural resistance, and a lack of cost-effective pretreatment and downstream processing, contemporary cellulosic ethanol biorefineries are facing major sustainability issues. As a result, we've outlined the global status of present cellulosic ethanol facilities, as well as main roadblocks and technical challenges for sustainable and commercial cellulosic ethanol production. Additionally, the article highlights the technical and non-technical barriers, various R&D advancements in biomass pretreatment, enzymatic hydrolysis, fermentation strategies that have been deliberated for low-cost sustainable fuel ethanol. Moreover, selection of a low-cost efficient pretreatment method, process simulation, unit integration, state-of-the-art in one pot saccharification and fermentation, system microbiology/ genetic engineering for robust strain development, and comprehensive techno-economic analysis are all major bottlenecks that must be considered for long-term ethanol production in the transportation sector.

Co-production of functional xylo-oligosaccharides and fermentable sugars from corn stover through fast and facile ball mill-assisted alkaline peroxide pretreatment

The aim of this work was to develop a feasible ball mill-assisted alkaline peroxide pretreatment followed by stepwise hydrolysis to improve the yield of xylo-oligosaccharides (XOS) and fermentable sugars. The hydrogen peroxide charge, ball-milling time, and solid-to-liquid ratio affected the compositions, particle sizes, morphology, and crystallinity of the corn stover, directly improving the following hydrolytic efficiency. The optimal pretreatment was with 0.45 g/g (H₂O₂: substrate) and 1:3 solid-to-liquid ratio (w/v) for 1.0 h ball-milling, resulting in 84.29% delignification. Physicochemical properties of the pretreated samples were characterized and their correlations to the enzymatic hydrolysis were revealed. Compared with one-step cellulase hydrolysis, the two-step xylanase-cellulase hydrolysis of the pretreated corn stover showed significant advance in preparing XOS, producing 69.65% (on the base of xylan content in pretreated sample) of XOS, along with 20.55% xylose, 68.94% glucose, and 21.15% gluco-oligosaccharides. The yield of XOS was 2-7 times higher than those in previous studies.

Structure of the Bacterial Cellulose Ribbon and Its Assembly-Guiding Cytoskeleton by Electron Cryotomography

This work’s relevance for the microbiology community is twofold. It delivers for the first time high-resolution near-native snapshots of Gluconacetobacter spp. (previously Komagataeibacter spp.) in the process of cellulose ribbon synthesis, in their native biofilm environment.

Cellulose is a widespread component of bacterial biofilms, where its properties of exceptional water retention, high tensile strength, and stiffness prevent dehydration and mechanical disruption of the biofilm. Bacteria in the genus Gluconacetobacter secrete crystalline cellulose, with a structure very similar to that found in plant cell walls. How this higher-order structure is produced is poorly understood. We used cryo-electron tomography and focused-ion-beam milling of native bacterial biofilms to image cellulose-synthesizing Gluconacetobacter hansenii and Gluconacetobacter xylinus bacteria in a frozen-hydrated, near-native state. We confirm previous results suggesting that cellulose crystallization occurs serially following its secretion along one side of the cell, leading to a cellulose ribbon that can reach several micrometers in length and combine with ribbons from other cells to form a robust biofilm matrix. We were able to take direct measurements in a near-native state of the cellulose sheets. Our results also reveal a novel cytoskeletal structure, which we have named the cortical belt, adjacent to the inner membrane and underlying the sites where cellulose is seen emerging from the cell. We found that this structure is not present in other cellulose-synthesizing bacterial species, Agrobacterium tumefaciens and Escherichia coli 1094, which do not produce organized cellulose ribbons. We therefore propose that the cortical belt holds the cellulose synthase complexes in a line to form higher-order cellulose structures, such as sheets and ribbons.

IMPORTANCE This work’s relevance for the microbiology community is twofold. It delivers for the first time high-resolution near-native snapshots of Gluconacetobacter spp. (previously Komagataeibacter spp.) in the process of cellulose ribbon synthesis, in their native biofilm environment. It puts forward a noncharacterized cytoskeleton element associated with the side of the cell where the cellulose synthesis occurs. This represents a step forward in the understanding of the cell-guided process of crystalline cellulose synthesis, studied specifically in the Gluconacetobacter genus and still not fully understood. Additionally, our successful attempt to use cryo-focused-ion-beam milling through biofilms to image the cells in their native environment will drive the community to use this tool for the morphological characterization of other studied biofilms.

Consolidated bioprocessing of raw starch to ethanol by Saccharomyces cerevisiae: Achievements and challenges

Recent advances in amylolytic strain engineering for starch-to-ethanol conversion have provided a platform for the development of raw starch consolidated bioprocessing (CBP) technologies. Several proof-of-concept studies identified improved enzyme combinations, alternative feedstocks and novel host strains for evaluation and application under fermentation conditions. However, further research efforts are required before this technology can be scaled up to an industrial level. In this review, different CBP approaches are defined and discussed, also highlighting the role of auxiliary enzymes for a supplemented CBP process. Various achievements in the development of amylolytic Saccharomyces cerevisiae strains for CBP of raw starch and the remaining challenges that need to be tackled/pursued to bring yeast raw starch CBP to industrial realization, are described. Looking towards the future, it provides potential solutions to develop more cost-effective processes that include cheaper substrates, integration of the 1G and 2G economies and implementing a biorefinery concept where high-value products are also derived from starchy substrates.

Integrating bran starch hydrolysates with alkaline pretreated soft wheat bran to boost sugar concentration

Soft wheat bran (SWB), one of the most abundant byproducts from the wheat milling industry, is a potential candidate for biofuel production. In this study, bran starch hydrolysates were separately integrated with dilute acid pretreated SWB and alkaline pretreated SWB to boost fermentable sugar concentration. Alkaline pretreatment showed higher sugar recoveries than acid pretreatment. Significant sugar degradation for acid pretreatment was observed when pretreatment temperature higher than 170 ℃. The optimal pretreatment condition was 15% solid loading with 0.08 mol/L NaOH at 150 ℃ for 20 min. The neutralization reaction between dilute alkaline and released acids reduced sugar decomposition and inhibitors formation. Integrating bran starch hydrolysates with alkaline pretreated SWB yielded the highest glucose concentration of 50.91 g/L and a total sugar concentration of 101.29 g/L.

On the evaluation of different saccharification schemes for enhanced bioethanol production from potato peels waste via a newly isolated yeast strain of Wickerhamomyces anomalus

The present study focuses on the exploration of the potential use of potato peels waste (PPW) as feedstock for bioethanol production, using a newly isolated yeast strain, Wickerhamomyces anomalus, via different saccharification and fermentation schemes. The saccharification of PPW was performed via thermal and chemical (acid, alkali) pretreatment, as well as via enzymatic hydrolysis through the use of commercial enzymes (cellulase and amylase) or enzymes produced at lab scale (alpha-amylase from Bacillus sp. Gb67), either separately or in mixtures. The results indicated that the enzymatic treatment by commercial enzymes led to a higher saccharification efficiency (72.38%) and ethanol yield (0.49 g/g_{consumed sugars}) corresponding to 96% of the maximum theoretical. In addition, acid pretreatment was found to be beneficial for the process, leading also to high hydrolysis and ethanol yields, indicating that PPW is a very promising feedstock for bio-ethanol production by W. anomalus under different process schemes.

Utilization of waste cake for fermentative ethanol production

In this study, utilization of waste cake for bioethanol production via a two-step of enzymatic hydrolysis and ethanol fermentation was developed. At the first step, namely waste cake hydrolysis, effects of α-amylase volumes (C1: 0.1 mL/L, C2: 0.4 mL/L, C3, 0.8 mL/L) on the performance of hydrolysis were investigated. Both chemical oxygen demand (COD) and reducing sugar (RS) could reach the highest values after 80 min for all the conditions. The maximum COD of 86.3 g/L, RS of 44 g/L and waste reduction of 85.2% were achieved at C3. At the second step, namely ethanol fermentation, the produced waste cake hydrolysate was used as the sole feedstock for fermentative ethanol production, and the highest ethanol production of 46.6 g/L and ethanol yield of 1.13 g/g RS were obtained (C3), respectively. This corresponds to an overall ethanol yield of 1.12 g ethanol/g initial dry cake which is the highest ethanol yield compared to using other food wastes reported to date. These values are higher than using the glucose as feedstock since the waste cake hydrolysate could provide the carbon and nitrogen sources for ethanol fermentation.

Process integration for ethanol production from corn and corn stover as mixed substrates

This work investigated all possible process integration strategies for ethanol production from corn and dilute acid pretreated corn stover (CS) as mixed substrates. Three corn to pretreated CS ratios (20%:10%, 10%:20% and 5%:25%) were examined. When the ratio of corn to pretreated CS was 20%:10%, the process integration strategy that mixed corn with CS hydrolysate for liquefaction followed by SSF resulted in the highest ethanol titer of 99.3 g/L. Mixing liquefied corn with pretreated CS for hydrolysis/saccharification followed by fermentation was the best strategy for the other two ratios. The strategy of mixing liquefied corn with pretreated CS for 6 h hydrolysis followed by fermentation showed the highest productivity for all the tested ratios.

Exploring the potential of lactic acid production from lignocellulosic hydrolysates with various ratios of hexose versus pentose by Bacillus coagulans IPE22

The aim of this study was to investigate the feasibility of utilizing different lignocellulosic hydrolysates with various hexose versus pentose (H:P) ratios to produce lactic acid (LA) from Bacillus coagulans IPE22 by fermentations with single and mixed sugar. In single sugar utilization, glucose tended to promote LA production, and xylose preferred to enhance cell growth. In mixed sugar utilization, glucose and pentose were consumed simultaneously when glucose concentration was lower than 20 g/L, and almost the same concentration of LA (50 g/L) was obtained regardless of the differences of H:P values. Finally, LA production from corn cob hydrolysates (CCH) contained 60 g/L mixed sugar verified the mechanisms found in the fermentations with simulated sugar mixture. Comparing with single glucose utilization, CCH utilization was faster and the yield of LA was not significantly affected. Therefore, the great potential of producing LA with lignocellulosic materials by B. coagulans was proved.