Ethanol production from high dry matter corncob using fed-batch simultaneous saccharification and fermentation after combined pretreatment

Utilization of plant-derived wastes as the potential biohydrogen source: a sustainable strategy for waste management

The sustainable economy has shown a renewed interest in acquiring access to the resources required to promote innovative practices that favor recycling and the reuse of existing, unconsidered things over newly produced ones. The production of biohydrogen through dark anaerobic fermentation of organic wastes is one of the intriguing possibilities for replacing fossil-based fuels through the circular economy. At present, plant-derived waste from the agro-based industry is the main global concern. When these wastes are improperly disposed of in landfills, they become the habitat for several pathogens. Additionally, it contaminates surface water as a result of runoff, and the leachate that is created from the waste enters groundwater and degrades its quality. However, cellulose and hemicellulose-rich plant wastes from agriculture fields and agro-based industries have been employed as the most efficient feedstock since carbohydrates are the primary substrate for the synthesis of biohydrogen. To produce biohydrogen from plant-derived wastes on a large scale, it is necessary to explore comprehensive knowledge of lab-scale parameters and pretreatment strategies. This paper summarizes the problems associated with the improper management of plant-derived wastes and discusses the recent developments in dark fermentation and substrate pretreatment techniques with the goal of gaining significant insight into the biohydrogen production process. It also highlights the utilization of anaerobic digestate, which is left over after biohydrogen gas as feedstock for the development of value-added products such as volatile fatty acids (VFA), biochar, and biofertilizer.

Emerging trends and advances in valorization of lignocellulosic biomass to biofuels

Sustainable technologies pave the way to address future energy demand by converting lignocellulosic biomass into fuels, carbon-neutral materials, and chemicals which might replace fossil fuels. Thermochemical and biochemical technologies are conventional methods that convert biomass into value-added products. To enhance biofuel production, the existing technologies should be upgraded using advanced processes. In this regard, the present review explores the advanced technologies of thermochemical processes such as plasma technology, hydrothermal treatment, microwave-based processing, microbial-catalyzed electrochemical systems, etc. Advanced biochemical technologies such as synthetic metabolic engineering and genomic engineering have led to the development of an effective strategy to produce biofuels. The microwave-plasma-based technique increases the biofuel conversion efficiency by 97% and the genetic engineering strains increase the sugar production by 40%, inferring that the advanced technologies enhances the efficiency. So understanding these processes leads to low-carbon technologies which can solve the global issues on energy security, the greenhouse gases emission, and global warming.

Pretreatment Strategies to Enhance Enzymatic Hydrolysis and Cellulosic Ethanol Production for Biorefinery of Corn Stover

There is a rising interest in bioethanol production from lignocellulose such as corn stover to decrease the need for fossil fuels, but most research mainly focuses on how to improve ethanol yield and pays less attention to the biorefinery of corn stover. To realize the utilization of different components of corn stover in this study, different pretreatment strategies were used to fractionate corn stover while enhancing enzymatic digestibility and cellulosic ethanol production. It was found that the pretreatment process combining dilute acid (DA) and alkaline sodium sulfite (ASS) could effectively fractionate the three main components of corn stover, i.e., cellulose, hemicellulose, and lignin, that xylose recovery reached 93.0%, and that removal rate of lignin was 85.0%. After the joint pretreatment of DA and ASS, the conversion of cellulose at 72 h of enzymatic hydrolysis reached 85.4%, and ethanol concentration reached 48.5 g/L through fed-batch semi-simultaneous saccharification and fermentation (S-SSF) process when the final concentration of substrate was 18% (w/v). Pretreatment with ammonium sulfite resulted in 83.8% of lignin removal, and the conversion of cellulose and ethanol concentration reached 86.6% and 50 g/L after enzymatic hydrolysis of 72 h and fed-batch S-SSF, respectively. The results provided a reference for effectively separating hemicellulose and lignin from corn stover and producing cellulosic ethanol for the biorefinery of corn stover.

Mechanistic insights into morphological and chemical changes during benzenesulfonic acid pretreatment and simultaneous saccharification and fermentation process for ethanol production

The anatomical and histochemical characterization of pretreated substrates is essential for the further valorization of biomass during the biorefinery process. In this work, the benzenesulfonic acid (BA)-treated substrates were employed for simultaneous saccharification and fermentation (SSF) of ethanol for the first time. An ethanol yield of 50.36% was attained at 10% solids loading and 47.45 g/L of ethanol accumulated at 30 % solids loading. The dramatic improvements could result from the deconstruction of cell walls, which were evidenced by fluorescence microscope and confocal Raman microscopy spectra. Additionally, for a thorough comprehension of the inherent chemistry of lignin during the BA pretreatment, the changes in lignin structure features were identified for the first time by gel permeation chromatography (GPC) and nuclear magnetic resonance (NMR). In summary, this study tried to probe the possibility of BA-treated Miscanthus for the SSF process and unveiled the mechanism of the efficient BA pretreatment.

Enhanced treatment of low-temperature and low carbon/nitrogen ratio wastewater by corncob-based fixed bed bioreactor coupled sequencing batch reactor

In this study, a combined corncob-based fixed bed bioreactor and sequencing batch reactor system (CCF-SBR) was developed to treat low-temperature (3-12 °C) and low carbon/nitrogen ratio (C/N = 2) wastewater with a single SBR as the control. Results showed similarly low COD concentration of CCF-SBR (20.4 ± 3.7 mg·L^-1) and control SBR (24.9 ± 6.7 mg·L^-1) effluent. However, the total nitrogen (TN) removal rate of CCF-SBR was significantly higher than that of control SBR (29.6 ± 2.7% vs 8.6 ± 2.3%). According to the nitrification and denitrification activities and the analysis of microbial community, CCF mainly played the role of denitrification based on fermentation genera and denitrifying genera, and SBR mainly implemented nitrification with Nitrospira and Acinetobacter. This study explores a promising way for agricultural waste resource utilization and wastewater treatment under low-temperature and low C/N ratio.

Agricultural waste management strategies for environmental sustainability

Globally, abundant agricultural wastes (AWs) are being generated each day to fulfil the increasing demands of the fast-growing population. The limited and/or improper management of the same has created an urgent need to devise strategies for their timely utilization and valorisation, for agricultural sustainability and human-food and health security. The AWs are generated from different sources including crop residue, agro-industries, livestock, and aquaculture. The main component of the crop residue and agro-industrial waste is cellulose, (the most abundant biopolymer), followed by lignin and hemicellulose (lignocellulosic biomass). The AWs and their processing are a global issue since its vast majority is currently burned or buried in soil, causing pollution of air, water and global warming. Traditionally, some crop residues have been used in combustion, animal fodder, roof thatching, composting, soil mulching, matchsticks and paper production. But, lignocellulosic biomass can also serve as a sustainable source of biofuel (biodiesel, bioethanol, biogas, biohydrogen) and bioenergy in order to mitigate the fossil fuel shortage and climate change issues. Thus, valorisation of lignocellulosic residues has the potential to influence the bioeconomy by producing value-added products including biofertilizers, bio-bricks, bio-coal, bio-plastics, paper, biofuels, industrial enzymes, organic acids etc. This review encompasses circular bioeconomy based various AW management strategies, which involve 'reduction', 'reusing' and 'recycling' of AWs to boost sustainable agriculture and minimise environmental pollution.

Lignocellulosic Biomass Valorization for Bioethanol Production: a Circular Bioeconomy Approach

Lignocellulosic biomass generated from different sectors (agriculture, forestry, industrial) act as biorefinery precursor for production of second-generation (2G) bioethanol and other biochemicals. The integration of various conversion techniques on a single platform under biorefinery approach for production of biofuel and industrially important chemicals from LCB is gaining interest worldwide. The waste generated on utilization of bio-resources is almost negligible or zero in a biorefinery along with reduced greenhouse gas emissions, which supports the circular bioeconomy concept. The economic viability of a lignocellulosic biorefinery depends upon the efficient utilization of three major components of LCB-cellulose, hemicellulose and lignin. The heterogeneous structure and recalcitrant nature of LCB is main obstacle in its valorization into bioethanol and other value-added products. The success of bioconversion process depends upon methods used during pre-treatment, hydrolysis and fermentation processes. The cost involved in each step of the bioconversion process affects the viability of cellulosic ethanol. The lignocellulose biorefinery has ample scope, but much-focused research is required to fully utilize major parts of lignocellulosic biomass with zero wastage. The present review entails lignocellulosic biomass valorization for ethanol production, along with different steps involved in its production. Various value-added products produced from LCB components were also discussed. Recent technological advances and significant challenges in bioethanol production are also highlighted in addition to future perspectives.

Directional Structure Modification of Poplar Biomass-Inspired High Efficacy of Enzymatic Hydrolysis by Sequential Dilute Acid-Alkali Treatment

A major challenge in converting lignocellulose to biofuel is overcoming the resistance of the biomass structure. Herein, sequential dilute acid-alkali/aqueous ammonia treatment was evaluated to enhance enzymatic hydrolysis of poplar biomass by removing hemicellulose first and then removing lignin with acid and base, respectively. The results show that glucose release in sequential dilute acid-alkali treatments (61.4-71.4 mg/g) was 7.3-24.8% higher than sequential dilute acid-aqueous ammonia treatments (57.2-61.8 mg/g) and 283.8-346.3% higher than control (16.0 mg/g), respectively. Dilute acid treatment removed most hemicellulose (84.9%) of the biomass, followed by alkaline treatment with 27.5% removal of lignin. Roughness, surface area, and micropore volume of the biomass were crucial for the enzymatic hydrolysis. Furthermore, the ultrastructure changes observed using crystallinity, Fourier transform infrared spectroscopy, thermogravimetric analysis, and pyrolysis gas chromatography/mass spectrometry support the effects of sequential dilute acid-alkali treatment. The results provide an efficient approach to facilitate a better enzymatic hydrolysis of the poplar samples.

Ethanol Production from High Solid Loading of Rice Straw by Simultaneous Saccharification and Fermentation in a Non-Conventional Reactor

Simultaneous saccharification and fermentation (SSF) at high solid loading is a potential approach to improve the economic feasibility of cellulosic ethanol. In this study, SSF using high loading of rice straw was assessed using a vertical ball mill reactor. First, the conditions of temperature and number of glass spheres were optimized at 8% (w/v) initial solids (41.5 °C, 18 spheres). Then, assays were carried out at higher solid loadings (16% and 24% w/v). At 8% or 16% solids, the fermentation efficiency was similar (ηF~75%), but the ethanol volumetric productivity (QP) reduced from 1.50 to 1.14 g/L.h. By increasing the solids to 24%, the process was strongly affected (ηF = 40% and QP = 0.7 g/L.h). To overcome this drawback, three different feeding profiles of 24% pre-treated rice straw were investigated. Gradual feeding of the substrate (initial load of 16% with additions of 4% at 10 and 24 h) and an inoculum level of 3 g/L resulted in a high ethanol titer (52.3 g/L) with QP of 1.1 g/L.h and ηF of 67%. These findings demonstrated that using a suitable fed-batch feeding strategy helps to overcome the limitations of SSF in batch mode caused by the use of high solid content.

Study on inhibitors from acid pretreatment of corn stalk on ethanol fermentation by alcohol yeast

The inhibitory effects of the main inhibitors formed during acid pretreatment of corn stalk were studied through ethanol fermentations of model substrates and hydrolysates from corn stalk by alcohol yeast. Experimental results showed that the tested inhibitors had no significant effect on ethanol fermentations when they were added separately at a concentration according to analysis results from hydrolysate of corn stalk. However, when they were added as a mixture, the inhibitory effects became obvious. With the increase of concentration, there was a delay in ethanol productivity. But complete inhibition was observed at 5.0 g L⁻¹ furfural, 10.0 g L⁻¹ acetic acid, 7.0 g L⁻¹ ferulic acid, and 7.0 g L⁻¹p-coumaric acid, respectively. The inhibitory effect decreased in the order: furfural > acetic acid > ferulic acid > p-coumaric acid > HMF. These results suggest that a high concentration of inhibitor has a strong negative influence on ethanol fermentation, but the inhibiting abilities of various inhibitors are different.

The inhibitory effects of the main inhibitors formed during acid pretreatment of corn stalk were studied through ethanol fermentations of model substrates and hydrolysates from corn stalk by alcohol yeast.

Recycling Strategy and Repression Elimination for Lignocellulosic-Based Farnesene Production with an Engineered Escherichia coli

Farnesene is an important chemical platform for many industrial products, such as biofuels and polymers. We performed high-efficiency utilization of corncobs for β-farnesene production by separate hydrolysis and fermentation with an optimized Escherichia coli strain. First, we developed a recycling strategy for both corncob pretreatment and cellulose hydrolysis, which saved great amounts of pretreatment reagents and presented a 96.83% cellulose conversion rate into glucose. However, the corncob hydrolysate strongly repressed cell growth and β-farnesene production, being caused by high-concentrated citrate. Through expressing a heterologous ATP citrate lyase and screening for a suitable expression host, an optimized strain was constructed that produced β-farnesene at 4.06 g/L after 48 h in a 5 L fermenter, representing an approximately 2.3-fold increase over the initial strain. Therefore, the proposed strategy about the recycling process and repression elimination was successful and suitable for the production of lignocellulosic-based β-farnesene, which can be further studied to scale up for industrialization.

Fabrication and Properties of a Bio-Based Biodegradable Thermoplastic Polyurethane Elastomer

Using the melt polycondensation of five bio-based aliphatic monomers (succinic acid, sebacic acid, fumaric acid, 1,3-propanediol, and 1,4-butanediol), we first synthesized the more flexible and biodegradable polyester diols (BPD) with an average molecular weight of 3825. Then, the BPD was polymerized with excessive 4,4'-diphenylmethane diisocyanate (MDI). Finally, the molecular chain extender of 1,4-butanediol (BDO) was used to fabricate the biodegradable thermoplastic polyurethane elastomer (BTPU), comprising the soft segment of BPD and the hard segment polymerized by MDI and BDO. Atomic force microscope (AFM) images showed the two-phase structure of the BTPU. The tensile strength of the BTPU containing 60% BPD was about 30 MPa and elongation at break of the BTPU was over 800%. Notably, the BTPU had superior biodegradability in lipase solution and the biodegradation weight loss ratio of the BTPU containing 80% BPD reached 36.7% within 14 days in the lipase solution.

Pretreatment and enzymatic saccharification of oak at high solids loadings to obtain high titers and high yields of sugars

Production of high-titer sugar from lignocellulose is important in terms of process economics of bio-based product industry. In this study, to obtain high titers and yields of sugars, we combined pretreatment and saccharification steps, both at high solids loadings. First, pretreatment of oak was optimized at a 30% (w/w) solids loading. The whole slurry of the pretreated oak was subjected to a fed-batch saccharification step at the final solids loading of 30%, to minimize loss of fermentable sugars and simplify the processes. As a result, high-titer sugars (157.5 g/L) consisting of 120.2 g/L of glucose and 37.3 g/L of xylose were obtained at 75.9% and 58.6%, respectively, of theoretical maximum yields, based on the initial glucan and xylan contents. Thus, through proper optimization processes of oak, the combination of pretreatment and saccharification at high solids loadings was effective in obtaining both high titers and high yields of sugars from lignocellulose.

Bioethanol production from wheat straw by phosphoric acid plus hydrogen peroxide (PHP) pretreatment via simultaneous saccharification and fermentation (SSF) at high solid loadings

Phosphoric acid plus hydrogen peroxide (PHP) pretreatment was employed on wheat straw for ethanol conversion by simultaneous saccharification and fermentation (SSF) at high loadings. Results showed solid loading of PHP-pretreated wheat straw can be greatly promoted to 20%. Although more enzyme input improved ethanol conversion significantly, it still can be potentially reduced to 10-20 mg protein/g cellulose. Increasing yeast input also promoted ethanol conversion, however, the responses were not significant. Response surface method was employed to optimize SSF conditions with the strategy of maximizing ethanol conversion and concentration and minimizing enzyme and yeast input. Results indicated that ethanol conversion of 88.2% and concentration of 69.9 g/L were obtained after 120 h SSF at solid loading of 15.3%, and CTec2 enzyme and yeast were in lower input of 13.2 mg protein/g cellulose and 1.0 g/L, respectively. Consequently, 15.5 g ethanol was harvested from 100 g wheat straw in the optimal conditions.

Feasibility of high-concentration cellulosic bioethanol production from undetoxified whole Monterey pine slurry

The economic feasibility of high-concentration cellulosic bioethanol production remains challenging because it requires easily available feedstock and low energy consumption process. Simultaneous saccharification and fermentation (SSF) of sulfite pretreated Momentary pine slurry at 20% (w/w) loadings increased ethanol concentration from 59.3 g/L to 68.5 g/L by washing strategy. Effects of inhibitors in pretreatment liquor were further investigated. Besides HMF, furfural and acetic acid, other inhibitors and/or their synergistic effects proved to be responsible for a lower fermentability. To bypass the inhibition and achieve high-efficient bioethanol concentration, a fermentation temperature of 28 °C was optimized for both cell growth and ethanol production. Under the optimal conditions with prehydrolyzed 25% (w/w) whole undetoxified slurry, a high ethanol concentration (up to 82.1 g/L) were produced with a yield of 205 kg/ton Monterey pine in the SSF. Thus, this high cellulosic bioethanol production from Monterey pine makes it a potential strategy for biofuel production.

Torrefaction of corncob to produce charcoal under nitrogen and carbon dioxide atmospheres

Corncob was torrefied under nitrogen and carbon dioxide atmospheres at 220-300 °C, obtaining solid products with mass yields of 69.38-95.03% and 67.20-94.99% and higher heating values of 16.58-24.77 MJ/kg and 16.68-24.10 MJ/kg, respectively. The changes of physicochemical properties of the charcoal was evaluated by many spectroscopies, contact angle determination, and combustion test. Hemicelluloses were not detected for the torrefaction under the hard conditions. As the severity increased, C concentration raised while H and O concentrations reduced. Combustion test showed that the burnout temperature of charcoal declined with the elevation of reaction temperature, and torrefaction at a high temperature shortened the time for the whole combustion process. Base on the data, torrefaction at 260 °C under carbon dioxide was recommended for the torrefaction of corncob.

Fed-batch production of green coconut hydrolysates for high-gravity second-generation bioethanol fermentation with cellulosic yeast

The residual biomass obtained from the production of Cocos nucifera L. (coconut) is a potential source of feedstock for bioethanol production. Even though coconut hydrolysates for ethanol production have previously been obtained, high-solid loads to obtain high sugar and ethanol levels remain a challenge. We investigated the use of a fed-batch regime in the production of sugar-rich hydrolysates from the green coconut fruit and its mesocarp. Fermentation of the hydrolysates obtained from green coconut or its mesocarp, containing 8.4 and 9.7% (w/v) sugar, resulted in 3.8 and 4.3% (v/v) ethanol, respectively. However, green coconut hydrolysate showed a prolonged fermentation lag phase. The inhibitor profile suggested that fatty acids and acetic acid were the main fermentation inhibitors. Therefore, a fed-batch regime with mild alkaline pretreatment followed by saccharification, is presented as a strategy for fermentation of such challenging biomass hydrolysates, even though further improvement of yeast inhibitor tolerance is also needed.

Optimization of simultaneous saccharification and fermentation conditions with amphipathic lignin derivatives for concentrated bioethanol production

Amphipathic lignin derivatives (A-LDs) prepared from the black liquor of soda pulping of Japanese cedar are strong accelerators for bioethanol production under a fed-batch simultaneous enzymatic saccharification and fermentation (SSF) process. To improve the bioethanol production concentration, conditions such as reaction temperature, stirring program, and A-LDs loadings were optimized in both small scale and large scale fed-batch SSF. The fed-batch SSF in the presence of 3.0g/L A-LDs at 38°C gave the maximum ethanol production and a high enzyme recovery rate. Furthermore, a jar-fermenter equipped with a powerful mechanical stirrer was designed for 1.5L-scale fed-batch SSF to achieve rigorous mixing during high substrate loading. Finally, the 1.5L fed-batch SSF with a substrate loading of 30% (w/v) produced a high ethanol concentration of 87.9g/L in the presence of A-LDs under optimized conditions.

Chemical input reduction in the arabinoxylan and lignocellulose alkaline extraction and xylooligosaccharides production

Lignocellulosic material breakdown by hydrolysis is an important step to open new perspectives for bioenergy and special foods production like prebiotic xylooligosaccharides. Improvement of lignocellulose and arabinoxylan alkaline extraction from sugarcane bagasse and enzymatic hydrolysis were performed. Treatments 1 (10% KOH at 70°C), 3 (5% KOH at 121°C) and ZD method (24% KOH at 35°C) showed solid lignocellulose recovery of respectively 75.2%, 74.2% and 73%. A range of 24.8-27% extracted material with high arabinoxylan content (72.1-76.3%) was obtained with these treatments. Treatment 1 and 3 exhibited great KOH reduction in the method reaction, 54.1% and 76.2%, respectively. Likewise, in treatment 3 there was a decrease in ethanol consumption (40.9%) when compared to ZD method. The extracted arabinoxylan showed susceptibility to enzymatic hydrolysis with high solid loading (7%) since Trichoderma reesei xylanases were advantageous for xylose production (54.9%), while Aspergillus fumigatus xylanases achieved better XOS production (27.1%).

Cost-effective simultaneous saccharification and fermentation of l-lactic acid from bagasse sulfite pulp by Bacillus coagulans CC17

The main barriers to cost-effective lactic acid production from lignocellulose are the high cost of enzymes and the ineffective utilization of the xylose within the hydrolysate. In the present study, the thermophilic Bacillus coagulans strain CC17 was used for the simultaneous saccharification and fermentation (SSF) of bagasse sulfite pulp (BSP) to produce l-lactic acid. Unexpectedly, SSF by CC17 required approximately 33.33% less fungal cellulase than did separate hydrolysis and fermentation (SHF). More interestingly, CC17 can co-ferment cellobiose and xylose without any exogenous β-glucosidase in SSF. Moreover, adding xylanase could increase the concentration of lactic acid produced via SSF. Up to 110g/L of l-lactic acid was obtained using fed-batch SSF, resulting in a lactic acid yield of 0.72g/g cellulose. These results suggest that SSF using CC17 has a remarkable advantage over SHF and that a potentially low-cost and highly-efficient fermentation process can be established using this protocol.

Synergetic effect of dilute acid and alkali treatments on fractional application of rice straw

BACKGROUND

The biorefinery based on an effective and economical process is to fractionate the three primary constituents (cellulose, hemicelluloses, and lignin) from lignocellulosic biomass, in which the constituents can be respectively converted into high-value-added products. In this study, a successive treatment with dilute acid (0.25-1.0 % aqueous H2SO4, 100-150 °C, 0.5-3.0 h) and alkali (1.5 % aqueous NaOH, 80 °C, 3 h) was performed to produce xylooligosaccharides (XOS), high-purity lignin, and cellulose-rich substrates to produce glucose for ethanol production from rice straw (RS).

RESULTS

During the dilute acid pretreatment, the maximum production of XOS (12.8 g XOS/100 g RS) with a relatively low level of byproducts was achieved at a relatively low temperature (130 °C) and a low H2SO4 concentration (0.5 %) for a reaction time of 2.0 h. During the alkali post-treatment, 14.2 g lignin with a higher purity of 99.2 % and 30.3 g glucose with a higher conversion rate by enzymatic hydrolysis were obtained from the successively treated substrates with 100 g RS as starting material. As the pretreatment temperature, H2SO4 concentration, or time increased, more β-O-4 linkages in lignins were cleaved, which resulted in an increase of phenolic OH groups in lignin macromolecules. The signal intensities of G2 and G6 in HSQC spectra gradually reduced and vanished, indicating that a condensation reaction probably occurred at C-2 and C-6 of guaiacyl with the side chains of other lignin.

CONCLUSIONS

The present study demonstrated that the successive treatments with dilute acid and alkali had a synergetic effect on the fractionation of the three main constituents in RS. It is believed that the results obtained will enhance the availability of the combined techniques in the lignocellulosic biorefinery for the application of the main components, cellulose, hemicelluloses, and lignin as biochemical and biofuels.

Potential of Using Maize Cobs in Pig Diets - A Review

The quest to broaden the narrow range of feed ingredients available to pig producers has prompted research on the use of low cost, unconventional feedstuffs, which are typically fibrous and abundant. Maize cobs, a by-product of a major cereal grown worldwide, have potential to be used as a pig feed ingredient. Presently, maize cobs are either dumped or burnt for fuel. The major challenge in using maize cobs in pig diets is their lignocellulosic nature (45% to 55% cellulose, 25% to 35% hemicellulose, and 20% to 30% lignin) which is resistant to pigs' digestive enzymes. The high fiber in maize cobs (930 g neutral detergent fiber/kg dry matter [DM]; 573 g acid detergent fiber/kg DM) increases rate of passage and sequestration of nutrients in the fiber reducing their digestion. However, grinding, heating and fermentation can modify the structure of the fibrous components in the maize cobs and improve their utilization. Pigs can also extract up to 25% of energy maintenance requirements from fermentation products. In addition, dietary fiber improves pig intestinal health by promoting the growth of lactic acid bacteria, which suppress proliferation of pathogenic bacteria in the intestines. This paper reviews maize cob composition and the effect on digestibility of nutrients, intestinal microflora and growth performance and proposes the use of ensiling using exogenous enzymes to enhance utilization in diets of pigs.

Enhanced anti-oxidative activity and lignocellulosic ethanol production by biotin addition to medium in Pichia guilliermondii fermentation

Commercialization of lignocellulosic ethanol fermentation requires its high titer, but the reactive oxygen species (ROS) accumulation during the bioprocess damaged the cells and compromised this goal. To improve the cellular anti-oxidative activity during non-detoxified corncob residue hydrolysate fermentation, seed cells were prepared to possess a higher level of intracellular biotin pool (IBP), which facilitated the biosyntheses of catalase and porphyrin. As a result, the catalase activity increased by 1.3-folds compared to control while the ROS level reduced by 50%. Cell viability in high-IBP cells was 1.7-folds of control and the final ethanol titer increased from 31.2 to 41.8 g L(-1) in batch fermentation. The high-IBP cells were further used for repeated-batch fermentation in the non-detoxified lignocellulosic hydrolysate, and the highest titer and average productivity of ethanol reached 63.7 g L(-1) and 1.2 g L(-1)h(-1). The results were favorable to future industrial application of this lignocellulosic bioethanol process.

Second generation bioethanol production at high gravity of pilot-scale pretreated wheat straw employing newly isolated thermotolerant yeast Kluyveromyces marxianus DBTIOC-35

Application of thermotolerant yeast Kluyveromyces marxianus DBTIOC-35 in SSF decreases overall process time, and increases productivity and yield by allowing elimination of presaccharification step and use of high biomass concentration, respectively.

Augmented digestion of lignocellulose by steam explosion, acid and alkaline pretreatment methods: a review

Lignocellulosic materials can be explored as one of the sustainable substrates for bioethanol production through microbial intervention as they are abundant, cheap and renewable. But at the same time, their recalcitrant structure makes the conversion process more cumbersome owing to their chemical composition which adversely affects the efficiency of bioethanol production. Therefore, the technical approaches to overcome recalcitrance of biomass feedstock has been developed to remove the barriers with the help of pretreatment methods which make cellulose more accessible to the hydrolytic enzymes, secreted by the microorganisms, for its conversion to glucose. Pretreatment of lignocellulosic biomass in cost effective manner is a major challenge to bioethanol technology research and development. Hence, in this review, we have discussed various aspects of three commonly used pretreatment methods, viz., steam explosion, acid and alkaline, applied on various lignocellulosic biomasses to augment their digestibility alongwith the challenges associated with their processing.

An alternative feedstock of corn meal for industrial fuel ethanol production: delignified corncob residue

Delignified corncob residue is an industrial solid waste from xylose production using corncob as feedstock. In this study, delignified corncob residue was used as the feedstock of ethanol production by simultaneous saccharification and fermentation (SSF) and the optimal fermentation performance was investigated under various operation conditions. The ethanol titer and yield reached 75.07 g/L and 89.38%, respectively, using a regular industrial yeast strain at moderate cellulase dosage and high solids loading. A uniform SSF temperature of 37°C at both prehydrolysis and SSF stages was tested. The fermentation performance and cost of delignified corncob residue and corn meal was compared as feedstock of ethanol fermentation. The result shows that the delignified corncob residue is competitive to corn meal as ethanol production feedstock. The study gives a typical case to demonstrate the potential of intensively processed lignocellulose as the alternative feedstock of corn meal for industrial fuel ethanol production.

Bioethanol production from hemicellulose rich Populus nigra involving recombinant hemicellulases from Clostridium thermocellum

Bioethanol was produced from poplar leafy biomass rich in hemicelluloses content involving recombinant Clostridium thermocellum hemicellulases and pentose sugar utilizing Candida shehatae. FT-IR analysis revealed effective AFEX pretreatment of poplar leaves. Repetitive batch strategy yielded ∼1.5-fold rise in cell biomass and specific activity of both, acetylxylanesterase (Axe) and GH43 hemicellulase. TLC and HPAEC exhibited xylose and arabinose release from hydrolyzed biomass. SSF trial with 1% (wv(-1)) pretreated poplar and mixed enzymes showed ∼1.5-fold higher ethanol titre as compared with SHF. The shake flask SSF with 5% (wv(-1)) pretreated poplar furnished 4.56 and 5.43gL(-1) ethanol with Axe and mixed enzymes, respectively. Whereas, bioreactor scale-up exhibited ∼1.25-fold increase in ethanol titres (5.68, 6.75gL(-1)) as compared with shake flask with an yield of 0.295 (gg(-1)) and 0.351 (gg(-1)), respectively with Axe and mixed enzymes.

Combined process for ethanol fermentation at high-solids loading and biogas digestion from unwashed steam-exploded corn stover

A combined process was designed for the co-production of ethanol and methane from unwashed steam-exploded corn stover. A terminal ethanol titer of 69.8 g/kg mass weight (72.5%) was achieved when the fed-batch mode was performed at a final solids loading of 35.5% (w/w) dry matter (DM) content. The whole stillage from high-solids ethanol fermentation was directly transferred in a 3-L anaerobic digester. During 52-day single-stage digester operation, the methane productivity was 320 mL CH₄/g volatile solids (VS) with a maximum VS reduction efficiency of 55.3%. The calculated overall product yield was 197 g ethanol + 96 g methane/kg corn stover. This indicated that the combined process was able to improve overall content utilization and extract a greater yield of lignocellulosic biomass compared to ethanol fermentation alone.

Enhanced ethanol production from pomelo peel waste by integrated hydrothermal treatment, multienzyme formulation, and fed-batch operation

Pomelo peel is an abundant pectin-rich biomass waste in China and has the potential to serve as a source of fuels and chemicals. This study reports a promising way to deal with pomelo peel waste and to utilize it as raw material for ethanol production via simultaneous saccharification and fermentation (SSF). An integrated strategy, incorporating hydrothermal treatment, multienzyme formulation, and fed-batch operation, was further developed to enhance the ethanol production. The results show that hydrothermal treatment (120 °C, 15 min) could significantly reduce the use of cellulase (from 7 to 3.8 FPU g(-1)) and pectinase (from 20 to 10 U g(-1)). A multienzyme complex, which consists of cellulase, pectinase, β-glucosidase, and xylanase, was also proven to be effective to improve the hydrolysis of pretreated pomelo peel, leading to higher concentrations of fermentative sugars (36 vs 14 g L(-1)) and galacturonic acid (23 vs 9 g L(-1)) than those with the use of a single enzyme. Furthermore, to increase the final ethanol concentration, fed-batch operation by adding fresh substrate was employed in the SSF process. A final solid loading of 25% (w/v), which is achieved by adding 15% fresh substrate to the SSF system at an initial solid loading of 10%, produced 36 g L(-1) ethanol product in good yield (73.5%). The ethanol concentration is about 1.73-fold that at the maximum solid loading of 14% for batch operation, whereas both of them have a closed ethanol yield. The results indicate that the use of the fed-batch mode could alleviate the decrease in ethanol yield at high solid loading, which is caused by significant mass transfer limitation and increased inhibition of toxic compounds in the SSF process. The integrated strategy demonstrated in this work could open a new avenue for dealing with pectin-rich biomass wastes and utilization of the wastes to produce ethanol.

Combined substrate, enzyme and yeast feed in simultaneous saccharification and fermentation allow bioethanol production from pretreated spruce biomass at high solids loadings

BACKGROUND

Economically feasible cellulosic ethanol production requires that the process can be operated at high solid loadings, which currently imparts technical challenges including inefficient mixing leading to heat and mass transfer limitations and high concentrations of inhibitory compounds hindering microbial activity during simultaneous saccharification and fermentation (SSF) process. Consequently, there is a need to develop cost effective processes overcoming the challenges when working at high solid loadings.

RESULTS

In this study we have modified the yeast cultivation procedure and designed a SSF process to address some of the challenges at high water insoluble solids (WIS) content. The slurry of non-detoxified pretreated spruce when used in a batch SSF at 19% (w/w) WIS was found to be inhibitory to Saccharomyces cerevisiae Thermosacc that produced 2 g l-1 of ethanol. In order to reduce the inhibitory effect, the non-washed solid fraction containing reduced amount of inhibitors compared to the slurry was used in the SSF. Further, the cells were cultivated in the liquid fraction of pretreated spruce in a continuous culture wherein the outflow of cell suspension was used as cell feed to the SSF reactor in order to maintain the metabolic state of the cell. Enhanced cell viability was observed with cell, enzyme and substrate feed in a SSF producing 40 g l-1 ethanol after 96 h corresponding to 53% of theoretical yield based on available hexose sugars compared to 28 g l-1 ethanol in SSF with enzyme and substrate feed but no cell feed resulting in 37% of theoretical yield at a high solids loading of 20% (w/w) WIS content. The fed-batch SSF also significantly eased the mixing, which is usually challenging in batch SSF at high solids loading.

CONCLUSIONS

A simple modification of the cell cultivation procedure together with a combination of yeast, enzyme and substrate feed in a fed-batch SSF process, made it possible to operate at high solids loadings in a conventional bioreactor. The proposed process strategy significantly increased the yeast cell viability and overall ethanol yield. It was also possible to obtain 4% (w/v) ethanol concentration, which is a minimum requirement for an economical distillation process.

Inhibitor analysis and adaptive evolution of Saccharomyces cerevisiae for simultaneous saccharification and ethanol fermentation from industrial waste corncob residues

Industrial waste corncob residues (CCR) are rich in cellulose and can be hydrolyzed directly without pretreatment. However, a poor fermentation performance was frequently observed in the simultaneous saccharification and ethanol fermentation (SSF) of CCR, although the furans and organic acid inhibitors were very low. In this study, the high level of water-insoluble phenolic compounds such as 2-furoic acid, ferulic acid, p-coumaric acid, guaiacol, and p-hydroxybenzoic acid were detected in CCR and inhibited the growth and metabolism of Saccharomyces cerevisiae DQ1. An evolutionary adaptation strategy was developed by culturing the S. cerevisiae DQ1 strain in a series of media with the gradual increase of CCR hydrolysate. The high ethanol concentration (62.68g/L) and the yield (55.7%) were achieved in the SSF of CCR using the adapted S. cerevisiae DQ1. The results provided a practical method for improving performance of simultaneous saccharification and ethanol production from CCR.

Utilization of household food waste for the production of ethanol at high dry material content

BACKGROUND

Environmental issues and shortage of fossil fuels have turned the public interest to the utilization of renewable, environmentally friendly fuels, such as ethanol. In order to minimize the competition between fuels and food production, researchers are focusing their efforts to the utilization of wastes and by-products as raw materials for the production of ethanol. household food wastes are being produced in great quantities in European Union and their handling can be a challenge. Moreover, their disposal can cause severe environmental issues (for example emission of greenhouse gasses). On the other hand, they contain significant amounts of sugars (both soluble and insoluble) and they can be used as raw material for the production of ethanol.

RESULTS

Household food wastes were utilized as raw material for the production of ethanol at high dry material consistencies. A distinct liquefaction/saccharification step has been included to the process, which rapidly reduced the viscosity of the high solid content substrate, resulting in better mixing of the fermenting microorganism. This step had a positive effect in both ethanol production and productivity, leading to a significant increase in both values, which was up to 40.81% and 4.46 fold, respectively. Remaining solids (residue) after fermentation at 45% w/v dry material (which contained also the unhydrolyzed fraction of cellulose), were subjected to a hydrothermal pretreatment in order to be utilized as raw material for a subsequent ethanol fermentation. This led to an increase of 13.16% in the ethanol production levels achieving a final ethanol yield of 107.58 g/kg dry material.

CONCLUSIONS

In conclusion, the ability of utilizing household food waste for the production of ethanol at elevated dry material content has been demonstrated. A separate liquefaction/saccharification process can increase both ethanol production and productivity. Finally, subsequent fermentation of the remaining solids could lead to an increase of the overall ethanol production yield.