Fermented Slurry Research Articles

Ethanol and hydrogen production from sunflower straw: The effect of pretreatment on the whole slurry fermentation

In the present study the whole slurry obtained after different pretreatment methods applied on sunflower straw (SS) biomass, was used for biological production of hydrogen (BHP) and ethanol (BEP). BHP from pretreated SS was studied in batch experiments using mixed anaerobic sludge, via simultaneous saccharification and fermentation (SSF), while for BEP, SSF was carried out using the xylose – fermenting yeast Pichia stipitis. The experimental results obtained showed that for BHP, alkaline pretreatment with 20g NaOH/100 gTS was optimal, leading to the highest hydrogen yield of 155.3±6.5L/kg TS. This yield was 52.5 times higher than the respective of raw SS (2.9±0.0 LH2/kgTS) without enzyme addition. Pretreatment with the same chemical loadings of HCl or H2SO4 resulted in the lowest BHP yields, due to the generation of inhibitory compounds. Although pretreatment with 20g H2SO4/100gTS also caused inhibition of P. stipitis giving the lowest BEP yield, pretreatment with 20g HCl/100 gTS led to the highest BEP yield of 104.78±0.08mg ethanol/gTS, indicating the varying sensitivity of different microbial species to the inhibitory compounds that are generated during pretreatment.

Proteolytic activity of thermophilic Bacillus licheniformis strain SF5-1 for the efficient bioconversion of pork waste to amino acid fertiliser

The protease activity of the thermophilic Bacillus licheniformis strain SF5-1 (SF5-1) was efficient at 56 °C when applied to the management of pork waste. For the tissues treated with SF5-1, within 48 h, the lipid and fat layers formed at the top with the digested protein slurry as sediment. The amino acid content of the SF5-1 fermented pork tissue was significantly higher than that of the untreated controls. The concentration of most of the amino acids increased in the SF5-1-treated pork slurry. Glutamic acid, lysine and threonine were the most abundant amino acids in the hydrolysates. The fermented slurry from the SF5-1 increased the growth of chili pepper and cucumber in germination pouch (in vitro) and under greenhouse condition. The increase in growth was affected by the concentration of the slurry, and the plants treated with higher concentrations (50× and 100×) of the fermented product produced higher plants and more biomass compared to plants grown with diluted suspensions (200× and 300×). The cucumber plants showed variable disease symptoms when challenged with the pathogens of anthracnose and downy mildew, Colletotrichum orbiculare and Pseudoperonospora cubensis, respectively. In the pork slurry-treated plants, the disease symptoms were significantly lower than those in the non-treated control. Taken together, our results demonstrated the beneficial use of B. licheniformis strain SF5-1 in the efficient conversion of pork protein waste to a nutrient fertiliser with properties of plant disease control.

Waste cleaning waste: Ammonia abatement in bio-waste anaerobic digestion by soluble substances isolated from bio-waste compost

Soluble bio-based substances are isolated from the alkaline hydrolysate of composted urban gardening wastes. These substances are capable to reduce the ammonia content in municipal biowaste anaerobic digestates. The present work reports the anaerobic fermentation of the organic humid fraction of solid municipal biowaste carried out in the presence the above soluble substances, added at 0.05–0.2% concentration in fermentation slurry. The results show that the ammonia in the digestate obtained under these conditions is 70–100% lower than the ammonia present in the digestate of control fermentation without addition of soluble biobased substances. Operational cost savings (up to 1.1 € per abated ammonia N kg) may result for the anaerobic digestion performed in the presence of the above soluble susbtances, compared to conventional technology for the digestate secondary treatment.

Antimicrobial Efficiency in the Fermented Slurry of Unpolished Rice

현미는 도정한 백미보다 이로운 영양분을 더 많이 함유하고 있다. 본 연구에서는 현미를 이용하여 만든 현미 발효 슬러리의 이화학적 특성과 항균활성에 대해 시험하였으며, 현미발효슬러리의 항균활성은 paper disc-agar diffusion 방법을 이용하여 6가지 병원성 균주(Staphylococcus aureus, Escherichia coli, Listeria monocytogenes, Pseudomonas aeruginosa, Salmonella typhimurium and Yersinia enterocolitica)와 2가지 발효균주(Gluconacetobacter intermedius and Lodderomyces elongisporus)에 대해 항균성을 조사하였다. 특히, Staphylococcus aureus, E. coli, Listeria monocytogenes, P. aeruginosa, Salmonella typhimurium, Y. enterocolitica 그리고 Lodderomyces elongisporus에 대해서는 시판 항생제인 카베니실린과 테트라사이클린보다 더 높은 항균활성을 보였다. 항산화활성은 2,2-diphenyl-1-picrylhydrazyl (DPPH) 라디칼 소거능을 이용하여 측정하였을 때, 대표되는 항산화제인 아스코르빅 산과 비슷한 활성을 나타내었다. 현미발효슬러리의 발효중에 나타나는 균주를 동정하기 위해 TSB 고체배지와 YPD 고체배지에 현미발효슬러리를 도포하였을 때, 분리된 콜로니를 16S rDNA sequence 분석을 통하여, 네가지 균주를 분리하였으며, phylogenic tree 분석법을 이용하여 조사하였을 때, 각각 G. intermedius, Lactobacillus casei, Lactobacillus plantarum 그리고 Acetobacter peroxydans와 유사하였다. 【Unpolished rice (UR) is considered to be a healthy alternative to white rice when coping with chronic diseases. In the present study, the fermented slurry of unpolished rice (FSUR) was evaluated with respect to its antimicrobial activities and biochemical characteristics, including the quantities of sugar, total soluble sugar, organic acids, free amino acids, pH, and physiological activity. The antimicrobial efficiency of FSUR was assessed using the paper disc-agar diffusion method. FSUR exhibited strong antimicrobial activity against six pathogenic bacterial strains (Staphylococcus aureus, Escherichia coli, Listeria monocytogenes, Pseudomonas aeruginosa, Salmonella typhimurium, and Yersinia enterocolitica) and two fermentation strains (Gluconacetobacter intermedius and Lodderomyces elongisporus). The antimicrobial activity of FSUR was higher than the commercial antibiotics, carbenicillin ( $50{\mu}g/ml$ ) and tetracycline ( $50{\mu}g/ml$ ) against S. aureus, E. coli, L. monocytogenes, P. aeruginosa, S. typhimurium, Y. enterocolitica, and L. elongisporus. Also FSUR had a high antioxidant activity. The microorganisms were isolated from FSUR using tryptic soy broth and yeast extract-peptone-dextrose agar media. The isolated microorganisms were characterized using physiological and biochemical analyses as well as by 16S rRNA gene sequencing and phylogenic analysis. 16S rRNA gene sequence analysis showed that the isolated microorganisms had a high similarity to G. intermedius, Lactobacillus casei, Lactobacillus plantarum, and Acetobacter peroxydans.】

Combination of high solids loading pretreatment and ethanol fermentation of whole slurry of pretreated rice straw to obtain high ethanol titers and yields

In cellulosic ethanol production using lignocellulose, an increase in biomass solids loading during the pretreatment process significantly affects the final ethanol titer and the production cost. In this study, pretreatment using rice straw at high solids loading (20% (w/v)) was evaluated, using maleic acid as a catalyst. After pretreatment at optimal conditions of 190°C, 20min, and 0.2% or 5% (w/v) maleic acid, the highest enzymatic digestibility obtained was over 80%. Simultaneous saccharification and fermentation (SSF) of the whole slurry of pretreated rice straw in the presence of activated carbon to separate inhibitory compounds generated a high ethanol yield of 62.8%, based on the initial glucan in unpretreated rice straw. These findings suggest that high solids loading pretreatment using maleic acid and SSF of the whole slurry of pretreated rice straw can be combined to improve the process economics of ethanol production.

PENGARUH WAKTU FERMENTASI DAN KOMPOSISI LIMBAH KULIT BUAH AREN (Arenga pinnata) DENGAN STARTER KOTORAN SAPI TERHADAP BIOGAS YANG DIHASILKAN

Sugar palm or commonly called as Aren (Arenga pinnata) is a multipurpose crop. The widely use of sugar palm for food industries, generates sugar palm leather waste which can be used as a biomass for biogas production. This study aims to determine the quality and the quantity of biogas made from a mixed of sugar palm leather waste and water using batch anaerobic digester system and to evaluate the economic potential of biogas production from the mixture. Research carried out by mixing sugar palm leather with water in a ratio of 1:13; 1:11,5; 1:10; 1:9; and 1:8 (w/w) with cow dung as a starter mixed with water in an anaerobic digester batch system. Observed variables are biogas, biogas composition, Total Suspended Solid (TSS) and Chemical Oxygen Demand (COD) and analyzed fermentation slurry for every 3 days. pH for this study is set in the range of 6,4-8. The largest volume of biogas production is found at a ratio 1:11,5 (w/w) mixture of sugar palm leather and water that is 637 mL for total waste 1,5 L with 60,983 % methane contain. TSS removal percentage obtained was 88,10 %and percentage of COD removal by 82,43%. Slurry produces unqualified TSS and COD with the standard quality of liquid waste. The economic potential of sugar palm leather waste and water for producing biogas is beneficial.

Inhibitory Activity of Lactobacillus plantarum Strains from Akamu - A Nigerian Fermented Maize Food against Escherichia coli

Lactobacillus plantarum strains: NGL5 and NGL7 previously identified from akamu-a lactic acid bacteria fermented maize food using PCR sequencing analysis were evaluated for inhibitory activity against Eschericia coli NCTC 11560 in fermenting ground maize slurries at 22 and 30°C, and in porridges prepared from 24 h fermented slurries. The L. plantarum strains fermented ground maize slurries and produced significant levels of titratable acidity ≥5 g L-1 and low pH ≤3.63, displaying inhibitory activity against E. coli NCTC 11560. Inhibition was significantly (p≤0.05) greater at 30°C than at 22°C. At 30°C, E. coli was inhibited after 24 h in the co-inoculation assay (µ 5 g L-1. For safety of the product however fermentation has to be for at least 24 h at 30°C or more at 22°C.

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

A parametric study, with an initial load of 15%w/w of dry stover from white corn, was conducted to evaluate the sequential thermochemical hydrolysis (TH), enzymatic saccharification (ES) and fermentation of the whole slurry with ethanologenic Escherichia coli. The TH was designed to release the maximum amount of xylose with a concomitant formation of minimal amounts of furans. It was found that 29.0% or 93.2% of the xylan was recovered as free xylose at 130°C after 8min in the presence of 1% or 2%w/w H2SO4 and produced only 0.06 or 0.44g/L of total furans, respectively. After 24h of ES, 76.14–77.18g/L of monosaccharides (pentoses and hexoses) were obtained. These slurries, which contained 0.03–0.26g/L of total furans and 5.14–5.91g/L of acetate, were fermented with 3.7g/L of ethanologenic E. coli to produce 24.5–23.5g/L of ethanol.

Influence of Co-substrates for the Methane Production Rate Kinetics in Anaerobic Fermentation of Pig Slurry

The influence of co-substrates on kinetics of methane production in anaerobic fermentation of pig slurry was studied by performing a series of laboratory experiments using food waste and biowaste as co-substrates. Experiments were performed in the laboratory reactors of «Hohenheim» biogas testing system in batch mode at the temperature of 37ºC. Given 40 g of fresh pig slurry was fed to each digester and mixed with co-substrates in the ration of 55:45% by organic dry matter content. Methane production rate was used to evaluate co-digestion of pig slurry. Research results showed that co-substrates gave significant effect to methane production kinetics. The kinetic parameters of methane production i.e. potential yield of methane (P), maximum methane yield rate (Rm) and duration of lag-phase (λ) were analyzed using modified Gompertz equation. Technical time to produce 95% of potential methane yield was investigated and efficient period of anaerobic digestion was calculated. The potential production of methane was reached in lag-phase duration (λ) of 10-13 days. Maximal methane production rate was reduced 2-3 times in co-digestion than in mono-digestion of pig slurry. The potential yield of methane (P) was 0.420 Nm3(kg оDM)-1, 0.246 Nm3(kg оDM)-1 and 0.198 Nm3(kg оDM)-1 in mono-digestion and co-digestion of pig slurry with food waste and biowaste, respectively.

Effect of co-products of enzyme-assisted aqueous extraction of soybeans on ethanol production in dry-grind corn fermentation

Enzyme-assisted aqueous extraction processing (EAEP) is an environmentally-friendly alternative to solvent and mechanical oil extraction methods, and can achieve ∼97% oil recovery from soybeans. The present study utilized soy skim (protein rich) and insoluble fiber (IF; carbohydrate rich), both co-products of EAEP, in dry-grind corn fermentation. The effects of adding soy skim and untreated IF (UIF), either separately or together, and adding pretreated IF (TIF), on ethanol production were investigated. Maximum ethanol production was achieved when UIF and skim were slurried together (corn-to-UIF ratio 1:0.16; skim-to-UIF ratio 6.5:1) and when fiber-hydrolyzing enzymes were added to corn fermentation. This modification to corn fermentation increased ethanol yield by 20%, ethanol production rate by 3%, and decreased fermentation time by 38h compared to corn-only fermentation. An attempt was also made to utilize pentoses (from soy skim and IF) in integrated corn-soy fermentation slurry by an additional Escherichia coli KO11 fermentation step.

Whole slurry saccharification and fermentation of maleic acid-pretreated rice straw for ethanol production.

We evaluated the feasibility of whole slurry (pretreated lignocellulose) saccharification and fermentation for producing ethanol from maleic acid-pretreated rice straw. The optimized conditions for pretreatment were to treat rice straw at a high temperature (190°C) with 1% (w/v) maleic acid for a short duration (3min ramping to 190°C and 3min holding at 190°C). Enzymatic digestibility (based on theoretical glucose yield) of cellulose in the pretreated rice straw was 91.5%. Whole slurry saccharification and fermentation of pretreated rice straw resulted in 83.2% final yield of ethanol based on the initial quantity of glucan in untreated rice straw. These findings indicate that maleic acid pretreatment results in a high yield of ethanol from fermentation of whole slurry even without conditioning or detoxification of the slurry. Additionally, the separation of solids and liquid is not required; therefore, the economics of cellulosic ethanol fuel production are significantly improved. We also demonstrated whole slurry saccharification and fermentation of pretreated lignocellulose, which has rarely been reported.

Fermentative l-lactic acid production from pretreated whole slurry of oil palm trunk treated by hydrothermolysis and subsequent enzymatic hydrolysis

A simple and cost-effective biochemical conversion process consisting of hydrothermal treatment, enzymatic hydrolysis and fermentation of pretreated whole slurry (PWS) was developed for producing l-lactic acid (L-LA) from oil palm trunk (OPT). When OPT was hydrothermally treated at optimal condition capable of achieving maximum yield of hemicellulosic sugars after enzymatic hydrolysis, the enzymatic digestibility of the PWS afforded a yield of 81.4% of the theoretical glucose yield (TGY). However, glucose yield from washed pretreated solid (WPS) was only 43.5% of TGY. The use of two hydrolysates from PWS and WPS for fermentation by Lactobacillus paracasei engineered to selectively produce L-LA afforded yields of 89.5% and 45.8% of the theoretical LA yield (TLY), respectively. This study confirmed the inevitable extensive sugar loss during washing of pretreated slurry due to loss of soluble starch. Alternatively, the proposed design process is considered suitable for converting OPT to L-LA without such starch loss.

High titer and yield ethanol production from undetoxified whole slurry of Douglas-fir forest residue using pH profiling in SPORL.

BackgroundForest residue is one of the most cost-effective feedstock for biofuel production. It has relatively high bulk density and can be harvested year round, advantageous for reducing transportation cost and eliminating onsite storage. However, forest residues, especially those from softwood species, are highly recalcitrant to biochemical conversion. A severe pretreatment for removing this recalcitrance can result in increased sugar degradation to inhibitors and hence cause difficulties in fermentation at high solid loadings. Here, we presented high titer ethanol production from Douglas-fir forest residue without detoxification. The strong recalcitrance of the Douglas-fir residue was removed by sulfite pretreatment to overcome the recalcitrance of lignocelluloses (SPORL). Sugar degradation to inhibitors was substantially reduced using a novel approach of “pH profiling” by delaying acid application in pretreatment, which facilitated the simultaneous enzymatic saccharification and fermentation of undetoxified whole slurry at a solid loading of 21%.Results“pH profiling” reduced furan production by approximately 70% in using SPORL pretreating Douglas-fir forest residue (FS-10) comparing with the control run while without sacrificing enzymatic saccharification of the resultant substrate. pH profiling also reduced carbohydrate degradation. The improved carbohydrate yield in pretreated solids and reduced fermentation inhibitors with pH profiling resulted in a terminal ethanol titer of 48.9 ± 1.4 g/L and yield of 297 ± 9 L/tonne FS-10, which are substantially higher, i.e., by 27% in titer and by 38% in yield, than those of a control SPORL run without pH profiling.ConclusionsEconomical and large-volume production of commodity biofuels requires the utilization of feedstocks with low value (therefore low cost) and sustainably producible in large quantities, such as forest residues. However, most existing pretreatment technologies cannot remove the strong recalcitrance of forest residues to produce practically fermentable high titer sugars. Here, we demonstrated a commercially scalable and efficient technology capable of removing the strong recalcitrant nature of forest residues using “pH profiling” together with “low temperature SPORL”. The resultant pretreated whole slurry of a Douglas-fir forest residue using this technology can be easily processed at high solids of 21% without detoxification to achieve a high ethanol yield of 297 L/tonne at 48.9 g/L.Graphical Graphic table of content.Electronic supplementary materialThe online version of this article (doi:10.1186/s13068-015-0205-3) contains supplementary material, which is available to authorized users.

Using sulfite chemistry for robust bioconversion of Douglas-fir forest residue to bioethanol at high titer and lignosulfonate: A pilot-scale evaluation

This study demonstrated at the pilot-scale (50kg) use of Douglas-fir forest harvest residue, an underutilized forest biomass, for the production of high titer and high yield bioethanol using sulfite chemistry without solid–liquor separation and detoxification. Sulfite Pretreatment to Overcome the Recalcitrance of Lignocelluloses (SPORL) was directly applied to the ground forest harvest residue with no further mechanical size reduction, at a low temperature of 145°C and calcium bisulfite or total SO2 loadings of only 6.5 or 6.6wt% on oven dry forest residue, respectively. The low temperature pretreatment facilitated high solids fermentation of the un-detoxified pretreated whole slurry. An ethanol yield of 282L/tonne, equivalent to 70% theoretical, with a titer of 42g/L was achieved. SPORL solubilized approximately 45% of the wood lignin as directly marketable lignosulfonate with properties equivalent to or better than a commercial lignosulfonate, important to improve the economics of biofuel production.

Dual application of agricultural residues for xylanase production and dye removal through solid state fermentation

The present study is a novel attempt to utilize agricultural residues for addressing the dual challenge faced by pulp and paper industries to control pollution caused due to pulp bleaching and release of colored waste water. Fungal isolate, Aspergillus lentulus, was utilized for the production of xylanase through solid state fermentation utilizing various low-cost agro-residues as substrate. Maximum xylanase production was obtained on the 4th day of incubation using wheat bran as the substrate (158.4 U/g) followed by corn cob (153.0 U/g), sugarcane bagasse (129.9 U/g) and wheat straw (49.4 U/g). These activities were accompanied by very low cellulase activities. The enzyme exhibited good stability at high pH and temperature (>75% activity retained at pH 9 and 70 °C). Later, the left over spent fermented slurry was utilized to remove anionic (>85.0% removal) and cationic (>96.0% removal) dyes. Results indicate cellulase-free; pH and thermo stable nature of the xylanase enzyme which is required during bio-bleaching process. Moreover, successful utilization of spent residues from fermentation in dye removal process signify that the proposed technology can be utilized to meet the requirements of pulp and bleaching industries through an effective and sustainable approach.

Ethanol production from cotton gin trash using optimised dilute acid pretreatment and whole slurry fermentation processes

Cotton ginning trash (CGT) collected from Australian cotton gins was evaluated for bioethanol production. CGT composition varied between ginning operations and contained high levels of extractives (26–28%), acid-insoluble material (17–22%) and holocellulose (42–50%). Pretreatment conditions of time (4–20min), temperature (160–220°C) and sulfuric acid concentration (0–2%) were optimised using a central composite design. Response surface modelling revealed that CGT fibre pretreated at 180°C in 0.8% H2SO4 for 12min was optimal for maximising enzymatic glucose recoveries and achieved yields of 89% theoretical, whilst the total accumulated levels of furans and acetic acid remained relatively low at <1 and 2g/L respectively. Response surface modelling also estimated maximum xylose recovery in pretreated liquors (87% theoretical) under the set conditions of 150°C in 1.9% H2SO4 for 23.8min. Yeast fermentations yielded high ethanol titres of 85%, 88% and 70% theoretical from glucose generated from: (a) enzymatic hydrolysis of washed pretreated fibres, (b) enzymatic hydrolysis of whole pretreated slurries and (c) simultaneous saccharification fermentations, respectively.

Production of bioethanol from rice straw by simultaneous saccharification and fermentation of whole pretreated slurry usingSaccharomyces cerevisiaeKF-7

Rice straw is one of the most abundant lignocellulosic waste materials worldwide, and it can serve as feedstock for bioethanol production. In this study, ethanol was produced from rice straw by simultaneous saccharification and fermentation (SSF) of the whole pretreated slurry with high solid load. Pretreatment of rice straw using 0.5% (w/w) H2SO4 was performed to increase the efficiency of enzymatic saccharification. Enzymatic saccharification and SSF of the whole pretreated slurry were subsequently performed to evaluate the inhibitory effect on enzymatic saccharification and the final ethanol yield. The result showed that pretreatment was the key for achieving high ethanol yield of the whole pretreated slurry. Rice straw pretreated at 170°C with a solid load of 15% exhibited efficient enzymatic saccharification. By the result of ethanol yields, no inhibitory effect was found when the whole pretreated slurry was subjected to SSF. The ethanol yield of SSF of the whole pretreated slurry was enhanced to 77.3%, compared with the glucose recovery efficiency of enzymatic saccharification of the washed pretreated rice straw, which was 74.4%. © 2014 American Institute of Chemical Engineers Environ Prog, 34: 582–588, 2015

Anaerobic Co-digestion of Water Hyacinth and Sheep Waste

Water hyacinth (WH) and sheep waste (SW) can be used to generate energy which could save on the fossil fuels conventionally used as source of energy. In this study, the possibility was explored to mix water hyacinth with sheep waste in different combinations for anaerobic co-digestion, so that energy can be generated as biogas and at the same time digested sludge can be used as fertilizer for agricultural applications. Pretreatment of water hyacinth was done by alkali method. Anaerobic co-digestion was carried out in mesophilic temperature range of 30oC to 37oC with different fermentation slurries of 8% total solids. Co-digestion was carried for a retention period of 60 days. The gas produced was collected by the downward displacement of water, and was subsequently measured and analyzed. Fermentation slurry SW3 (mixing ratio of 4: 12.01:83.90 for Water Hyacinth: Sheep Waste: Water) was found to be optimum, which gave the highest biogas yield of 0.36 l/gVS. with composition 60.84% % CH4, 21.53% CO2 and 17.63% others (H2, N2, H20 and H2S). The overall results showed that blending water hyacinth with Sheep waste had significant improvement on the biogas yield.

Increasing Ethanol Titer and Reducing Enzyme Dosage via Fed-Batch, Simultaneous Saccharification and Fermentation in a High Solids Bioreactor

Abstract- Fed-batch, solid state, simultaneous saccharification and fermentation (SSF) was evaluated as an approach to reduce enzyme use in converting cellulose to ethanol, and to maximize ethanol titer. Kraft pulp, an intermediate in paper production, was used to represent a fractionated cellulose feedstock. Following a literature survey, average dosages were determined as 34 FPU of cellulase (Celluclast 1.5L) and 135 CBU o f β -glucosidase (Novozyme 188) per gram glucan, and were set as 100% dosages. Initially, submerged fed-batch SSF trials were conducted in a traditional bioreactor using enzyme dosages of 17, 33, 67, and 133%, with a final solids loading rate (SLR) of 14%. Ethanol production was similar (77.3-83.4% of theoretical yield) for trials with 33-133% enzyme dosages, but fell to 36% of theoretical at 17% enzyme dosage. Fed-batch saccharification and fed-batch SSF were then performed in a solid state bioreactor, achieving a 34.8% SLR. This reduced the initial 133% enzyme dosage to 19%. In saccharification trials the glucose yield was only 35% of theoretical (103.6 g/L), due to feedback inhibition of enzymes. Companion SSF trials achieved an ethanol yield of only 20% of theoretical (30.1 g/L ethanol). While some ethanol was lost due to evaporation, yeast inhibition by low water activity was presumed to be the primary limitation. Performance at lower solid loading rates were evaluated, and even diluted the fermented slurry and conducted a secondary fermentation. These trials suggest that enzyme inactivation via irreversible binding may be the primary limitation, instead of low water activity or nutrient limitation.

Bioethanol production from steam-pretreated corn stover through an isomerase mediated process

Agricultural by-products such as corn stover are considered strategic raw materials for the production of second-generation bioethanol from renewable and non-food sources. This paper describes the conversion of steam-pretreated corn stover to ethanol utilising a multi-step process including enzymatic hydrolysis, isomerisation, and fermentation of mixed hydrolysates with native Saccharomyces cerevisiae. An immobilised isomerase enzyme was used for the xylose isomerisation along with high concentrations of S. cerevisiae. The objective was to assess the extent of simultaneity of the various conversion steps, through a detailed analysis of process time courses, and to test this process scheme for the conversion of lignocellulosic hydrolysates containing several inhibitors of the isomerase enzyme (e.g. metal ions, xylitol and glycerol). The process was tested on two types of hydrolysate after acid-catalysed steam pretreatment: (a) the water soluble fraction (WSF) in which xylose was the largest carbon source and (b) the entire slurry, containing both cellulose and hemicellulose carbohydrates, in which glucose predominated. The results indicated that the ethanol concentration rose when the inoculum concentration was increased in the range 10-75 g/L. However, when xylose was the largest carbon source, the metabolic yields were higher than 0.51g(ethanol)/g(consumed) sugars probably due to the use of yeast internal cellular resources. This phenomenon was not observed in the fermentation of mixed hydrolysates obtained from the entire pretreated product and in which glucose was the largest carbon source. The ethanol yield from biomass suspensions with dry matter (DM) concentrations of 11-12% (w/v) was 70% based on total sugars (glucose, xylose, galactose). The results suggest that xylulose uptake was more effective in mixed hydrolysates containing glucose levels similar to, or higher than, xylose. Analysis of the factors that limit isomerase activity in lignocellulosic hydrolysates excluded any inhibition due to residual calcium ions after the detoxification of the hemicellulose hydrolysates with Ca(OH)2. By contrast, most of the enzyme activity ceased during the fermentation of the entire slurry after steam explosion, probably due to synergistic inhibition effects of various fermentation co-products.