High Fermentation Efficiency Research Articles

The Evaluation of Indicators Used to Assess the Suitability of Agricultural Waste for Fermentation.

To ensure high fermentation efficiency, it is necessary to assess the biodegradability of a substrate. These parameters are most often determined on the basis of the amount of loss on ignition and total organic carbon. We are more and more often using chemical indices. However, these indices do not provide information on how much an organic substance is susceptible to biodegradation. The actual assessment of the content of easily biodegradable matter in substrates that are used for fermentation should be performed on the basis of aerobic (AT4) and anaerobic tests (BMP), which require specialised equipment and are time consuming. The AT4 index is being more and more frequently adopted for the analysis of substrates that are used in the fermentation process, because AT4 takes a much shorter time than BMP and provides information on the biodegradability of substrates. The aim of the article is to answer the question of whether the AT4 parameter can be used to assess the suitability of the substrate from the agricultural sector for the fermentation process. The results show that the AT4 index could be used instead of the BMP parameter.

Integrating sugarcane molasses into sequential cellulosic biofuel production based on SSF process of high solid loading

BackgroundSugarcane bagasse (SCB) is one of the most promising lignocellulosic biomasses for use in the production of biofuels. However, bioethanol production from pure SCB fermentation is still limited by its high process cost and low fermentation efficiency. Sugarcane molasses, as a carbohydrate-rich biomass, can provide fermentable sugars for ethanol production. Herein, to reduce high processing costs, molasses was integrated into lignocellulosic ethanol production in batch modes to improve the fermentation system and to boost the final ethanol concentration and yield.ResultsThe co-fermentation of pretreated SCB and molasses at ratios of 3:1 (mixture A) and 1:1 (mixture B) were conducted at solid loadings of 12% to 32%, and the fermentation of pretreated SCB alone at the same solid loading was also compared. At a solid loading of 32%, the ethanol concentrations of 64.10 g/L, 74.69 g/L, and 75.64 g/L were obtained from pure SCB, mixture A, and mixture B, respectively. To further boost the ethanol concentration, the fermentation of mixture B (1:1), with higher solid loading from 36 to 48%, was also implemented. The highest ethanol concentration of 94.20 g/L was generated at a high solid loading of 44%, with an ethanol yield of 72.37%. In addition, after evaporation, the wastewater could be converted to biogas by anaerobic digestion. The final methane production of 312.14 mL/g volatile solids (VS) was obtained, and the final chemical oxygen demand removal and VS degradation efficiency was 85.9% and 95.9%, respectively.ConclusionsMolasses could provide a good environment for the growth of yeast and inoculum. Integrating sugarcane molasses into sequential cellulosic biofuel production could improve the utilization of biomass resources.

The potential of the newly isolated thermotolerant Kluyveromyces marxianus for high-temperature ethanol production using sweet sorghum juice

In this work, the newly isolated thermotolerant Kluyveromyces marxianus DBKKUY-103 exhibited a high ethanol fermentation efficiency at high temperatures using sweet sorghum juice (SSJ). The highest ethanol concentrations and productivities achieved under the optimum conditions using thermotolerant K. marxianus DBKKUY-103 were 85.16g/l and 1.42g/l.h at 37°C and 83.46g/l and 1.39g/l.h at 40°C, respectively. The expression levels of genes during ethanol fermentation at 40°C were evaluated and the results found that the transcriptional levels of the RAD10, RAD14, RAD33, RAD50, ATPH, ATP4, ATP16, and ATP20 genes were up-regulated compared with those at 30°C, suggesting that the high growth and high ethanol production efficiencies of K. marxianus DBKKUY-103 during high-temperature ethanol production associated with the genes involved in DNA repair and ATP production.

<i>In vitro</i> digestibility and fermentation kinetics of agricultural and agro-industrial by-products used in ruminant feeding in Benin Republic

Objective: The global energy value of the feed remains essential in the choice of raw materials used in ruminant feeds.Methodology and results: This study analyzes the variability in vitro of degradation parameters in the presence of rumen juice from agricultural and agro-industrial by-products. It reveals that the tuber byproducts very rich in starch have a high fermentation efficiency. The by-products poor in fiber and rich in sugars are quickly degraded by the rumen microbes.Conclusion and application of results: The estimation of the digestibility of agricultural and agro-industrial by-products through the in vitro feed incubation methodology in the presence of rumen juice is an effective way to categorize feeds used in small ruminant feeding in Benin republic. The digestibility of the byproducts studied is related to the volumes of gas produced during the incubation. More digestible is Byproduct, the more gas it produces. This study strengthens the rationing tables. The By-products which is more digestible must be prioritized for production targets for fattening. Beyond the feed degradation, other constraints must be considered in the choice of agricultural and agro-industrial by-products in the development of the ruminant rations.Keywords: by-products, digestibility, Benin Republic, rumen juice, ruminant.

Influence of different sugar sources, nitrogen sources and inocula on the quality characteristics of apple tea wine

An infusion of tea leaves was prepared using 4 g tea/100 mL of the apple juice, ameliorated with different sugar sources, supplemented with different nitrogen sources and inoculated with 5% incola (Saccharomyces cerevisiae var. ellipsoideus, consortia 1 and consortia 2). The highest rate of fermentation and ethanol were found in the apple tea wine having sucrose as a sugar source, while apple juice concentrate as a sugar source gave the highest total phenols, caffeine content and overall acceptability. Higher alcohols, volatile acidity and antioxidant activity were the highest in honey as a sugar source. Apple tea wine having DAHP as a nitrogen source documented the highest ethanol, caffeine content and overall acceptability, lowest higher alcohols, whereas peptone as a nitrogen source had the highest total phenols. Among the different types of inocula, for most of the parameters there were non-significant differences. However, must inoculated with S. cerevisiae var. ellipsoideus for the preparation of apple tea wine resulted in the highest fermentation efficiency, ethanol and overall acceptability but the lowest volatile acidity. Apple tea wine prepared by ameliorating the apple tea must with apple juice concentrate, DAHP and inoculated with S. cerevisiae var. ellipsoideus was rated as the best. Copyright © 2017 The Institute of Brewing & Distilling

Evaluation of the multi-seeded (msd) mutant of sorghum for ethanol production

Grain sorghum [Sorghum bicolor (L.) Moench], a cost effective crop in semiarid regions, is an underestimated supplement to corn in starch based ethanol production. Twenty-three multi-seeded (msd) mutant sorghums and one wild type sorghum BTx623 were evaluated for ethanol production and chemical composition, pasting properties, and grain structure on ethanol yield and fermentation efficiency. Starch contents of the msd mutant sorghums ranged from 66.31 to 75.65%, while the starch content of BTx623 was 74.25%. The protein, fat and free amino nitrogen (FAN) contents of the msd mutant sorghums ranged from 9.44–13.33%, 0.81–2.03% and 873.2–925.4μg/g, respectively. The ethanol fermentation efficiency of msd mutant sorghums ranged from 87.29 to 92.54%. Taking into account that msd mutant sorghum can produce more than twice the amount of seed weight per panicle as wild type BTx623, msd mutant M2, the highest ethanol yield sample, could produce more than twice the amount of ethanol as wild type BTx623. All sorghum samples can be classified into three groups based fermentation rate: fast, medium and slow. For msd mutant sorghums with starch content higher than 71%, fermentation efficiencies were higher than 90%, and ethanol yields were higher than 470L/ton sorghum. In addition, the fermentation curves showed most of msd mutant sorghums took less time to complete fermentation (40–50h), indicating msd mutant sorghums have a great potential for biofuel application due to their high starch content, high fermentation efficiency, and final ethanol yield. Taking into account the best msd mutant sorghum that can produce more than twice the amount of seed weight as wild type sorghum, the best msd mutant could produce more than twice the amount of ethanol as wild type.

Efficient hydrolysis of raw starch and ethanol fermentation: a novel raw starch-digesting glucoamylase from Penicillium oxalicum.

BackgroundStarch is a very abundant and renewable carbohydrate and is an important feedstock for industrial applications. The conventional starch liquefaction and saccharification processes are energy-intensive, complicated, and not environmentally friendly. Raw starch-digesting glucoamylases are capable of directly hydrolyzing raw starch to glucose at low temperatures, which significantly simplifies processing and reduces the cost of producing starch-based products.ResultsA novel raw starch-digesting glucoamylase PoGA15A with high enzymatic activity was purified from Penicillium oxalicum GXU20 and biochemically characterized. The PoGA15A enzyme had a molecular weight of 75.4 kDa, and was most active at pH 4.5 and 65 °C. The enzyme showed remarkably broad pH stability (pH 2.0–10.5) and substrate specificity, and was able to degrade various types of raw starches at 40 °C. Its adsorption ability for different raw starches was consistent with its degrading capacities for the corresponding substrate. The cDNA encoding the enzyme was cloned and heterologously expressed in Pichia pastoris. The recombinant enzyme could quickly and efficiently hydrolyze different concentrations of raw corn and cassava flours (50, 100, and 150 g/L) with the addition of α-amylase at 40 °C. Furthermore, when used in the simultaneous saccharification and fermentation of 150 g/L raw flours to ethanol with the addition of α-amylase, the ethanol yield reached 61.0 g/L with a high fermentation efficiency of 95.1 % after 48 h when raw corn flour was used as the substrate. An ethanol yield of 57.0 g/L and 93.5 % of fermentation efficiency were achieved with raw cassava flour after 36 h. In addition, the starch-binding domain deletion analysis revealed that SBD plays a very important role in raw starch hydrolysis by the enzyme PoGA15A.ConclusionsA novel raw starch-digesting glucoamylase from P. oxalicum, with high enzymatic activity, was biochemically, molecularly, and genetically identified. Its efficient hydrolysis of raw starches and its high efficiency during the direct conversion of raw corn and cassava flours via simultaneous saccharification and fermentation to ethanol suggests that the enzyme has a number of potential applications in industrial starch processing and starch-based ethanol production.Electronic supplementary materialThe online version of this article (doi:10.1186/s13068-016-0636-5) contains supplementary material, which is available to authorized users.

Variation of fermentation redox potential during cell-recycling continuous ethanol operation

Fermentation redox potential was monitored during cell-recycling continuous ethanol operation. The cell-recycling system (CRS) was operated using two hollow fibre (HF) membranes (pore sizes 0.20 and 0.65μm) at three dilution rates (0.02, 0.04 and 0.08h−1). Saccharomyces cerevisiae NP 01 were recycled in the fermenter at a recycle ratio of 0.625. Aeration was provided at 2.5vvm for the first 4h and then further supplied continuously at 0.25vvm. As steady state was established, results showed that the fermentation redox potential was lower for processes employing CRS than those without. At the same dilution rates, the sugar utilization and ethanol production with CRS were higher than those without CRS. The highest fermentation efficiency (87.94g/l of ethanol, ∼90% of theoretical yield) was achieved using a 0.2-μm HF membrane CRS at a dilution rate of 0.02h−1. It was found that 7.53–10.07% of the carbon derived from glucose was incorporated into the yeast. Further, at the same dilution rates, yeast in the processes with CRS incorporated less carbon into ethanol than in those grown without CRS. This result suggests that processes involving CRS utilize more carbon for metabolite synthesis than biomass formation. This indicated that the processes with CRS could utilize more carbon for metabolite synthesis than biomass formation.

Bioethanol production from potato starch by a novel vertical mass-flow type bioreactor with a co-cultured-cell strategy

The purpose of this study was to develop a vertical mass-flow type bioreactor (VMFB) consisting of both aerobic and anaerobic zones to produce ethanol by Simultaneous Saccharification and Fermentation (SSF). Potato starch and microorganisms of Aspergillus awamori, Rhizopus japonicus, and Zymomonas mobilis were used. The highest ethanol concentration with 6.0 % (w/v) initial starch were 2.99g/L by traditional two-stage Separate Hydrolysis and Fermentation (SHF, co-suspension culture, flask, 48 h), 17.71g/L (SHF, co-immobilization culture, flask, 84h), and 21.68g/L (SSF, co-cultured in VMFB, 27 h), respectively. In the repeated-batch process (3 cycles) using the VMFB, stable ethanol production was 95.97g/L after 72h. Increase of the starch concentration from 6.0% to 12.0% (w/v) resulted in higher ethanol production of 60.18g/L after 51h. Another type of starch was tested, and 27.91g/L of ethanol was obtained from 6.0% (w/v) cassava starch. The results showed that the performance of VMFB cannot only enhance the ethanol yield, but also has great benefit for high fermentation efficiency and less cost.

Lignocellulosic ethanol production by starch-base industrial yeast under PEG detoxification.

Cellulosic ethanol production from lignocellulosic biomass offers a sustainable solution for transition from fossil based fuels to renewable alternatives. However, a few long-standing technical challenges remain to be addressed in the development of an economically viable fermentation process from lignocellulose. Such challenges include the needs to improve yeast tolerance to toxic inhibitory compounds and to achieve high fermentation efficiency with minimum detoxification steps after a simple biomass pretreatment. Here we report an in-situ detoxification strategy by PEG exo-protection of an industrial dry yeast (starch-base). The exo-protected yeast cells displayed remarkably boosted vitality with high tolerance to toxic inhibitory compounds, and with largely improved ethanol productivity from crude hydrolysate derived from a pretreated lignocellulose. The PEG chemical exo-protection makes the industrial S. cerevisiae yeast directly applicable for the production of cellulosic ethanol with substantially improved productivity and yield, without of the need to use genetically modified microorganisms.

Effect of Reducing Sugar and Total Nitrogen to Ethanol Production from Molasses by Saccharomyces cerevisiae

Indonesia's oil production has declined, while demand for derivative products is increasing. Objective of this research are to understand effect of reducing sugar and total nitrogen variation to ethanol production and fermentation efficiency, cell viability, acidity, temperature, dissolved oxygen with molasses by Saccharomyces cerevisiae (SAF Instant). Step of this research consist of determination of reducing sugar, ethanol fermentation, total nitrogen determination, ethanol determination and data analysis. Treatment of reducing sugar (GR) and total nitrogen (N) (g.L -1 ) that are GR 100 N 0, GR 100 N 6, GR 100 N 10, GR 125 N 0, GR 125 N 6 and GR 125 N 10. Fermentation was carried out for 72 hours with three replications. Observation parameters every 24 hours are ethanol and reducing sugar concentration, temperature, acidity and dissolved oxygen. Highest ethanol resulted from GR 125 N 6 (3.68 g.L -1 ) and GR 100 N 6 (3.53 g.L -1 ). Low reducing sugar consumption inhibited by by-product of yeast metabolism and molasses chemical compound, lead leaves high sugar concentration (> 80 g.L -1 ). GR 100 N 6 and GR 125 N 6 have highest fermentation efficiency (69 and 57 %). There was no increase in temperature and decrease in pH significantly (α>0.05). Dissolved oxygen decreased significantly (α>0.05) at the early of fermentation and decrease until the end of fermentation. Total nitrogen 6 g g.L -1 has the highest fermentation efficiency. Keywords: ethanol, molasses, reducing sugar, Saccharomyces cerevisiae , total nitrogen

Evaluation of ethanol fermentation parameters for bioethanol production from sugar beet pulp and juice

The aim of this study was to investigate the efficient utilization of sugar beet pulp, as well as raw, concentrated raw and thick sugar beet juice, for bioethanol production. Different fermentation conditions were examined. The influence of raw material pre-treatment (pasteurization or sterilization), type of batch culture process (stationary or shaken) as well as the type of Saccharomyces cerevisiae yeast preparation on the yields of the process were studied. Moreover, the fermentation process effectiveness was examined in connection with the quality of the obtained distillates. Sterilization, stationary batch culture and Safdistil C-70 yeast preparation were identified as the most profitable factors for sugar beet pulp fermentation, providing a high fermentation efficiency and ethanol yield (87.7% of theoretical ethanol yield). Concentrated raw beet juice resulted in a value of 94.2% of theoretical yield, and thick juice a 92.6% yield. The results suggest that bioethanol production from sugar beet pulp and sugar beet juice has promise as an alternative fuel. The raw spirits obtained from the sugar beet juice were characterized as having the lowest quantity of volatile by-products. Copyright © 2014 The Institute of Brewing & Distilling

Mutation Breeding of Salt-tolerant and Ethanol-producing Strain <em>S. cerevisiae</em> H058 by Low-energy Ion Implantation

To obtain an industrial strain with high ethanol fermentation efficiency under salted conditions, the wild strain H058 of Saccharomyces cerevisiae was mutated by means of nitrogen ions implantation. Mutagenic effects of strain H058 by low energy N + ion implantation were studied. A similar saddle shape survival curve due to ion beam irradiation appeared again in this study. By repeated screening, a high salt-tolerant and ethanol-producing strain M158 was obtained. Results showed that in medium contained 0, 1.5, 3.0, 4.5, 6.0% NaCl, M158 produced maximal ethanol of 98.3, 97.2, 96.4, 95.6 and 78.3 g/L at 54, 54, 54 and 72 h, respectively. However, the original strain H058 maximal ethanol of 95.2, 90.9, 84.8, 79.4 and 67.5 g/L at 60, 60, 66 and 72 h, respectively. In addition, the ethanol yield (g/g) in all of the NaCl concentrations for M158 is 0.492, 0.486, 0.482, 0.48 and 0.392 g/g, respectively, which were higher than those (0.476, 0.455, 0.424, 0.397 and 0.338 g/g, respectively) of the original strain H058. The higher production and shorter fermentation period suggest that strain M158 is a good salt-tolerant and ethanol-producing strain.

The use of plant cell wall–degrading enzymes from newly isolated Penicillium ochrochloron Biourge for viscosity reduction in ethanol production with fresh sweet potato tubers as feedstock

Penicillium ochrochloron Biourge, which was isolated from rotten sweet potato, can produce plant cell wall-degrading enzymes (PCWDEs) with high viscosity reducing capability for ethanol production using fresh sweet potato tubers as feedstock. The enzyme preparation was characterized by a broad enzyme spectrum including 13 kinds of enzymes with the activity to hydrolyze cellulose, hemicellulose, pectin, starch, and protein. The maximum viscosity-reducing capability was observed when the enzyme preparation was obtained after 5 days of fermentation using 20 g/L corncob as a sole carbon source, 4.5 g/L NH4 NO3 as a sole nitrogen source, and an initial medium pH of 6.5. The sweet potato mash treated with the enzyme preparation exhibited much higher fermentation efficiency (92.58%) compared with commercial cellulase (88.06%) and control (83.5%). The enzyme production was then scaled up to 0.5, 5, and 100 L, and the viscosity-reducing rates were found to be 85%, 90%, and 91%, respectively. Thus, P. ochrochloron Biourge displays potential viscosity-reducing capability for ethanol production.

Improvement of oxidative stress tolerance in Saccharomyces cerevisiae through global transcription machinery engineering

Excessive oxidative stress poses significant damage to yeast cells during fermentation process, and finally affects fermentation efficiency and the quality of products. In this paper, global transcription machinery engineering was employed to elicit Saccharomyces cerevisiae phenotypes of higher tolerance against oxidative stress caused by H2O2. Two strains from two plasmid-based mutagenesis libraries (Spt15 and Taf25), which exhibited significant increases in oxidative stress tolerance, were successfully isolated. At moderate H2O2 shock (≤3.5 mM), a positive correlation was found between the outperformance in cell growth of the oxidation-tolerate strains and H2O2 concentration. Several mutations were observed in the native transcription factors, which resulted in a different transcriptional profile compared with the control. Catalase and superoxide dismutase activities of the two mutants increased under H2O2 stress conditions. Fermentation experiments revealed that the mutant strain taf25-3 has a shorter lag phase compared to the control one, indicating that taf25-3 had improved adaptation ability to H2O2-induced oxidative stress and higher fermentation efficiency. Our study demonstrated that several amino acid substitutions in general transcription factors (Spt15 and Taf25) could modify the cellular oxidation defense systems and improve the anti-oxidation ability of S. cerevisiae. It could make the industrial ethanol fermentation more efficient and cost-effective by using the strain of higher stress tolerance.

Impacts of Kafirin Allelic Diversity, Starch Content, and Protein Digestibility on Ethanol Conversion Efficiency in Grain Sorghum

ABSTRACTSeed protein and starch composition determine the efficiency of the fermentation process in the production of grain‐based ethanol. Sorghum, a highly water‐ and nutrient‐efficient plant, provides an alternative to fuel crops with greater irrigation and fertilizer requirements, such as maize. However, sorghum grain is generally less digestible because of extensive disulfide cross‐linking among sulfur‐rich storage proteins in the protein– starch matrix. Thus, the fine structure and composition of the seed endosperm directly impact grain end use, including fermentation performance. To test the hypothesis that kafirin (prolamin) seed storage proteins specifically influence the efficiency of ethanol production from sorghum, 10 diverse genetic lines with allelic variation in the β‐, γ‐, and (δ‐kafirins, including three β‐kafirin null mutants, were tested for ethanol yield and fermentation efficiency. Our selected lines showed wide variation in grain biochemical features, including total protein (9.96–16.47%), starch (65.52–74.29%), and free amino nitrogen (FAN) (32.84–73.51 mg/L). Total ethanol yield (ranging from 384 to 426 L/metric ton), was positively correlated to starch content (R2 = 0.74), and there was a slight positive correlation between protein digestibility and ethanol yield (R2 = 0.52). Increases in FAN content enhanced fermentation efficiency (R2 = 0.65). The highest ethanol producer was elite staygreen breeding line B923296, and the line with the highest fermentation efficiency at the 72 h time point was inbred BT×623. A large‐seeded genotype, KS115, carrying a novel γ‐kafirin allele, was rich in FAN and exhibited excellent short‐term fermentation efficiency at 85.68% at the 20 h time point. However, the overall ethanol yield from this line was comparatively low at 384 L/metric ton, because of insufficient starch, low digestibility, and high crude protein. Multivariate analysis indicated an association between the β‐kafirin allele and variation in grain digestibility (P = 0.042) and FAN (P = 0.036), with subsequent effects on ethanol yield. Reversed‐phase HPLC profiling of the alcohol‐soluble kafirin protein fraction revealed diversity in protein content and composition across the lines, with similarities in peak distribution profiles among β‐kafirin null mutants compared with normal lines.

Oxidative stress response and nitrogen utilization are strongly variable in Saccharomyces cerevisiae wine strains with different fermentation performances

We used RNA-sequencing (RNA-seq) to analyze the expression profile of four vineyard strains of Saccharomyces cerevisiae having different fermentation performances. The expression profiles obtained in two steps of the fermentation process were compared with those obtained for the industrial wine strain EC1118 and for the laboratory strain S288c. The two strains with low fermentation efficiency, namely, S288c and the vineyard strain R103, exhibited markedly different expression profiles when compared to the other four strains. We also found that the vineyard strains P283 and P301 are characterized by a high expression of the transcription factor Met32p in the first step of the fermentation. Met32p, in coordination with the Hap4p transcription factor, determined the over-expression of the genes involved in the respiration processes, in the response to oxidative stress and in the sulfur amino acids biosynthesis. These combined actions are likely to increase the level of antioxidants whose protective effect could contribute to improve the fermentation process. Gene expression and phenotypic data revealed that the vineyard strain P301 has low nitrogen utilization in comparison to the other wine strains, combined with high fermentation efficiency. Analysis of the genes involved in fermentation stress response revealed a lower expression in strains characterized by low fermentation efficiency, particularly in the first fermentation phase. These findings evidenced the high variability of transcriptional profiles among different wine yeast strains and clarify their connection with complex phenotypic traits, such as the fermentation efficiency and the nitrogen sources utilization.

Yeast Isolation for Bioethanol Production

We have isolated 12 yeast isolates from five different rotten fruits by using a yeast glucose chloramphenicol agar (YGCA) medium supplemented with tetracycline. From pre-screening assay, four isolates exhibited higher substrate (glucose-xylose) consumption efficiency in the reaction tube fermentation compared to Saccharomyces cerevisiae dan Saccharomyces ellipsoids as the reference strains. Based on the fermentation process in gooseneck flasks, we observed that two isolates (K and SB) showed high fermentation efficiency both in sole glucose and mixed glucose-xylose substrate. Moreover, isolates K and SB produced relatively identical level of ethanol concentration compared to the reference strains. Isolates H and MP could only produce high levels of ethanol in glucose fermentation, while only half of that amount of ethanol was detected in glucose-xylose fermentation. Isolate K and SB were identified as Pichia kudriavzeevii (100%) based on large sub unit (LSU) ribosomal DNA D1/D2 region.

Alcohol production from sterilized and non-sterilized molasses by Saccharomyces cerevisiae immobilized on brewer's spent grains in two types of continuous bioreactor systems

Abstract In this work an integrated cost effective system for continuous alcoholic fermentation of a cheap raw material (molasses) is described, involving yeast immobilized by a simple method on brewer's spent grains, able to ferment in the temperature range 30–40 °C, and two types of bioreactors, a Multistage Fixed Bed Tower (MFBT) and a Packed Bed reactor (PB). The MFBT bioreactor gave better results regarding ethanol concentration, productivity and conversion. Furthermore, the use of sterilized and non-sterilized molasses, fed in two similar MFBT bioreactors, showed that ethanol concentration (kg m −3 ) was significantly ( p −3 at 35 °C and 44.2–48.2 kg m −3 at 40 °C), compared to sterilized molasses, where ethanol concentration ranged from 35.6 to 46.6 kg m −3 at 35 °C and 30.8–44.2 kg m −3 at 40 °C. During 32 days of continuous operation using non-sterilized molasses no contamination was observed. Industrialization of the proposed system seems to have a potential, mainly due to its high fermentation efficiency and the obtained high operational stability.

A kinetic growth model for Saccharomyces cerevisiae grown under redox potential-controlled very-high-gravity environment

A growth kinetic model for redox potential-controlled very-high-gravity (VHG) fermentation was developed. The model is semi-empirical and two important VHG fermentation operating conditions including glucose feed and redox potential level were incorporated in the model. The model consists of a substrate inhibition term and a product toxification term. A two-step parameter-estimating strategy was proposed and implemented. In the first step, the first few experimentally collected data points were used to estimate parameters relating to the substrate inhibition term; in the second step, the complete data set was used to estimate parameters relating to the product toxification term. The developed growth model could fit the original data with an R 2 value >0.95. Three operating diagrams were constructed by using the results generated from the developed model. Each respective diagram could be used to select the best operating condition for the shortest fermentation time, the highest final ethanol concentration, or the highest fermentation efficiency.