The influence of presaccharification, fermentation temperature and yeast strain on ethanol production from sugarcane bagasse

Novel cyclic shifting of temperature strategy for simultaneous saccharification and fermentation for lignocellulosic bioethanol production

In the current study, a novel strategy using cyclic shifting of temperature was developed for simultaneous saccharification and fermentation (SSF) for bioethanol production from rice straw. The in-situ cellulase production, saccharification and fermentation was carried out using P. janthinellum and S. cerevisiae. Bioethanol titer of 14.98 g/l was obtained using base followed by acid pretreated rice straw by employing the cyclic shifting of temperature strategy "30 °C for 2 h to 40 °C for 2 h". The holding time was further tuned to increase the productivity and the tuned condition 30 °C(1.7 h) - 40 °C(2 h) improved the bioethanol titer to 15.9 g/l. Using this strategy, resulted 5.1-fold and 2.8-fold increment of bioethanol production compared to known approaches, SSF at mutual optimum temperature and prolong prehydrolysis followed by fermentation respectively. The application of cyclic shifting of temperature strategy can unleash a great potential in enhancing the yield and efficiency for a sustainable lignocellulosic bioethanol production.

Water extract from Ligusticum chuanxiong delays the aging of Saccharomyces cerevisiae via improving antioxidant activity

Ligusticum chuanxiong is a common traditional edible-medicinal herb that has various pharmacological activities. However, its effects on Saccharomyces cerevisiae (S. cerevisiae) remains unknown. In this study, we found that water extract of Ligusticum chuanxiong (abbreviated as WEL) exhibited excellent free radical scavenging ability in-vitro. Moreover, WEL treatment could delay the aging of S. cerevisiae, an important food microorganism sensitive to reactive oxygen species (ROS) stress. Biochemical analyses revealed that WEL significantly increased the activity of antioxidant enzymes in S. cerevisiae, including superoxide dismutase (SOD), catalase (CAT) and glutathione reductase (GR), as well as their gene expression. As a result, ROS level was significantly decreased and accompanied with the decline of malondialdehyde (MDA), which represented a state of low oxidative stress. The reduction of oxidative stress could elevate S. cerevisiae's ethanol fermentation efficiency. Taken together, WEL plays a protective role against S. cerevisiae aging via improving antioxidant activity.

Kluyveromyces marxianus as a microbial cell factory for lignocellulosic biomass valorisation

The non-conventional yeast Kluyveromyces marxianus is widely used for several biotechnological applications, mainly due to its thermotolerance, high growth rate, and ability to metabolise a wide range of sugars. These cell traits are strategic for lignocellulosic biomass valorisation and strain diversity prompts the development of robust chassis, either with improved tolerance to lignocellulosic inhibitors or ethanol. This review summarises bioethanol and value-added chemicals production by K. marxianus from different lignocellulosic biomasses. Moreover, metabolic engineering and process optimization strategies developed to expand K. marxianus potential are also compiled, as well as studies reporting cell mechanisms to cope with lignocellulosic-derived inhibitors. The main lignocellulosic-based products are bioethanol, representing 71% of the reports, and xylitol, representing 17% of the reports. K. marxianus also proved to be a good chassis for lactic acid and volatile compounds production from lignocellulosic biomass, although the literature on this matter is still scarce. The increasing advances in genome editing tools and process optimization strategies will widen the K. marxianus-based portfolio products.

Bioenergy production in Pakistan: Potential, progress, and prospect

Pakistan is a developing country with a rapidly growing population. It is currently facing serious economic and energy challenges. Pakistan's energy demand is increasing by the day, and it now stands at 84 MTOE. Currently, the use of fossil fuels dominates Pakistan's energy sector. Conversely, indigenous fossil fuel resources are rapidly depleting and will be unable to meet rising energy demands in the future. Therefore, to withstand its energy needs, the country will need to explore alternative energy production methods. Biomass is one of the alternatives that has enormous potential to help Pakistan combat its growing energy crisis. In this review, we first present an overview of bioenergy, biomass resources, and biomass conversion technologies. We then discuss in detail the current state of the energy mix of Pakistan. Subsequently, we show that annual production of about 121 MT of agricultural residues, 427 MT of animal manure, and 7.5 MT of MSW in Pakistan offer a variety of bioenergy options ranging from biofuels to bio-electricity production. Overall, these biomass resources in Pakistan have the potential to generate 20,709 MW of bio-electricity and 12,615 million m³ of biogas annually in Pakistan. Though these resources hold promising potential for bioenergy production in the country, however, there are some critical challenges that need to be considered, and some of which are extremely difficult to overcome for a developing country like Pakistan. This work is expected to provide a useful basis for biomass management and utilization in Pakistan to harvest eco-friendly and sustainable green energy locally.

Physiological comparisons among Spathaspora passalidarum, Spathaspora arborariae, and Scheffersomyces stipitis reveal the bottlenecks for their use in the production of second-generation ethanol

The microbial conversion of pentoses to ethanol is one of the major drawbacks that limits the complete use of lignocellulosic sugars. In this study, we compared the yeast species Spathaspora arborariae, Spathaspora passalidarum, and Sheffersomyces stipitis regarding their potential use for xylose fermentation. Herein, we evaluated the effects of xylose concentration, presence of glucose, and temperature on ethanol production. The inhibitory effects of furfural, hydroxymethylfurfural (HMF), acetic acid, and ethanol were also determined. The highest ethanol yield (0.44 g/g) and productivity (1.02 g/L.h) were obtained using Sp. passalidarum grown in 100 g/L xylose at 32 °C. The rate of xylose consumption was reduced in the presence of glucose for the species tested. Hydroxymethylfurfural did not inhibit the growth of yeasts, whereas furfural extended their lag phase. Acetic acid inhibited the growth and fermentation of all yeasts. Furthermore, we showed that these xylose-fermenting yeasts do not produce ethanol concentrations greater than 4% (v/v), probably due to the inhibitory effects of ethanol on yeast physiology. Our data confirm that among the studied yeasts, Sp. passalidarum is the most promising for xylose fermentation, and the low tolerance to ethanol is an important aspect to be improved to increase its performance for second-generation (2G) ethanol production. Our molecular data showed that this yeast failed to induce the expression of some classical genes involved in ethanol tolerance. These findings suggest that Sp. passalidarum may have not activated a proper response to the stress, impacting its ability to overcome the negative effects of ethanol on the cells.

Efficient short-time hydrothermal depolymerization of sugarcane bagasse in one-pot for cellulosic ethanol production without solid-liquid separation, water washing, and detoxification

In these studies, a low-cost and energy efficiency production of cellulosic ethanol from sugarcane bagasse (SCB) using one-pot without solid-liquid separation, water washing, and detoxification was performed. Firstly, SCB was pretreated using liquid hot water as the only reagent at 210 °C for a short time (0 min), and the solid liquid ratio (SLR) was 1:20 (w/v). Then, the whole slurry of pretreated SCB was enzymatically hydrolyzed and fermented for cellulosic ethanol in one-pot. The results indicated that the one-pot preparation for ethanol achieved a high total fermentable sugar conversion of 84.52 ± 1.24%, containing 88.61 ± 1.57% of glucose and 78.01 ± 1.63% of xylose. Moreover, the ethanol yield reached 257 ± 5.51 mg/g _SCB, which was 77.56 ± 1.64% of the theoretical ethanol conversion from SCB. Importantly, there was no wastewater discharge in the whole process. Overall, the present work provides an economically feasible method for ethanol production.

Experimental optimization and techno-economic analysis of bioethanol production by simultaneous saccharification and fermentation process using sugarcane straw

The present work aims to determine a suitable yield-productivity balance in bioethanol production from hydrothermally pretreated sugarcane straw via pre-saccharification (PS) and simultaneous saccharification and fermentation (SSF). PS experiments were carried out evaluating effects of enzymatic dosage, biomass loading, and PS time. The performance of the whole process (PSSSF) was evaluated based on overall ethanol yield and productivity considering a simultaneous optimization (desirability function) of both variables. The multi-criteria optimization enabled to reach 5.7% w/w ethanol concentration yielding 290 L of ethanol per ton of pretreated sugarcane straw within 45 h of total processing time. Furthermore, a techno-economic analysis was performed under optimized conditions (14.5 FPU/g_cellulose, 19.3% w/v biomass loading and 33 h PS time). This process was integrated into a first-generation plant. Although the economic evaluation exhibited a negative performance, a sensitivity analysis indicated that a decrease of 23.3% in operational expenditure would be enough to achieve feasibility.

Optimization of pre-saccharification time during dSSF process in oat-hull bioethanol technology

This study suggests a mathematical description and the optimization of the pre-saccharification time during simultaneous saccharification and fermentation with delayed yeast inoculation (dSSF) to ensure the fastest and fullest possible conversion of a substrate into the target product-bioethanol. A pulp derived by alkaline delignification of oat hulls was used as a substrate. The pre-saccharification step of oat-hull pulp was performed at a solid loading of 60 g/L, at 46 ± 2 °C, using mixed enzymes CelloLux-A and BrewZyme BGX, the pre-saccharification time was 8, 15, 24, 39, 48 and 72 h. Afterwards, the reaction mixture was cooled to 28 °C, a 10% inoculum of Saccharomyces cerevisiae Y-1693 was seeded, and fermentation combined with saccharification. The optimum pre-saccharification time (inoculation time) under these conditions was found to be 24 h, thus providing the maximum hydrolysis of cellulose and hemicelluloses and the highest yield of bioethanol. The procedure suggested herein for determining the optimum pre-saccharification time can be used for other model substrates from lignocellulosic feedstocks.

Enhancing Hydrogen Production from Chlorella sp. Biomass by Pre-Hydrolysis with Simultaneous Saccharification and Fermentation (PSSF)

Simultaneous saccharification and fermentation (SSF) and pre-hydrolysis with SSF (PSSF) were used to produce hydrogen from the biomass of Chlorella sp. SSF was conducted using an enzyme mixture consisting of 80 filter paper unit (FPU) g-biomass−1 of cellulase, 92 U g-biomass−1 of amylase, and 120 U g-biomass−1 of glucoamylase at 35 °C for 108 h. This yielded 170 mL-H2 g-volatile-solids−1 (VS), with a productivity of 1.6 mL-H2 g-VS−1 h−1. Pre-hydrolyzing the biomass at 50 °C for 12 h resulted in the production of 1.8 g/L of reducing sugars, leading to a hydrogen yield (HY) of 172 mL-H2 g-VS−1. Using PSSF, the fermentation time was shortened by 36 h in which a productivity of 2.4 mL-H2 g-VS−1 h−1 was attained. To the best of our knowledge, the present study is the first report on the use of SSF and PSSF for hydrogen production from microalgal biomass, and the HY obtained in the study is by far the highest yield reported. Our results indicate that PSSF is a promising process for hydrogen production from microalgal biomass.

Isolation and identification of aroma producing strain with esterification capacity from yellow water

Kaoliang is a refreshing fragranced type of Chinese spirits with slight apple fragrance that comes from ethyl acetate (EA). Special aromas are produced by esterification microorganisms, which affect the taste and quality of the wine. In this study, new yeast strains were isolated from yellow water, a by-product during fermentation process. Meanwhile, the optimal culture condition was determined for its growth and EA production. Three new strains, Kazachstaniaexigua, Candida humilis and Saccharomyces cerevisiae were identified from yellow water. Among these strains, S. cerevisiae S5 was the new and dominant strain. Results from response surface methodology showed that S. cerevisiae S5 produced 161.88 ppm of EA, in the medium with 4.91% yeast extract, 9.82% peptone, and 20.91% glucose after 96 hours of cultivation at 27.53°C. GC analysis showed that aroma compounds, such as EA, isoamyl acetate and 2-phenylethanol increased from the sample of optimal condition when compared to the one from initial fermentation condition.

SELECTION OF THE COMPLEX OF ENZYME PREPARATIONS FOR THE HYDROLYSIS OF GRAIN CONSTITUENTS DURING THE FERMENTATION OF THE WORT OF HIGH CONCENTRATION

In this paper, an optimal complex is selected of enzyme preparations for hydrolysis of the components of grain raw materials during fermentation of high concentration wort. When selecting enzyme systems, their effect on the technical and chemical parameters of the fermented wash during the fermentation of wort is investigated. For the research, maize grain with a starch content of 69.0 % was used. Fermentation was carried out with 18–30% of dry matters (DM) in the wort, using the osmophilic yeast strain Saccharomyces cerevisiae DO-16.The recommended concentration of the enzyme preparation Amylex 4 T (the source of the α-amylase enzyme) – 0.4–0.6 units of α-amylase ability/g of starch – is optimal for the concentration 18–27% of DS in the wort. For 30 % of DS, it is practical to use 0.6 units of α-amylase ability/g of starch. With the use of the enzyme preparation Diazyme TGA (the source of the enzyme glucoamylase), the value is 7.5 units of glucoamylase ability/g of starch, alcohol accumulation in fermented washes was 10.51, 13.35, 15.78% vol., according to the wort concentrations 18, 27, 30 %, respectively. It has been established that with the application of the cytolytic enzyme Laminex 750, the concentrations of dissolved carbohydrates and non-dissolved starch have a tendency to decrease. In the samples where the proteolytic enzyme preparation Alphalase AFP was added at a concentration of 0.05 units of proteolytic ability/g of raw materials, there was an increase in the accumulation of yeast cells by 6.5% compared with the reference sample. The recommended concentration of Deltazyme VR XL (the source of β-glucanase and xylanase) is 0.05 units β-glucose/g of raw materials. The addition of a cytolytic and proteolytic enzyme preparation in combination with β-glucanase and xylanase contributed to an increase in the accumulation of ethanol in the washes by 1.7 % compared with the reference sample, and to an almost 33 % decrease in the concentration of dissolved carbohydrates and non-dissolved starch. On the basis of experimental studies, it has been found that using a complex of enzyme preparations – amylolytic (Amylex 4T), saccharifying (Diazyme TGA), proteolytic (Alphalase AFP), cytolytic (Laminex 750), and complex AF β-glucanase and xylanase (Deltazyme VR XL), in various combinations of their concentrations, – contributed to the intensification of the fermentation process of the wort and increased accumulation of the target product, ethanol, by 0.8–1.4 %, depending on the wort concentration. The highest amount of ethanol accumulated at the maximum dosage of additional enzyme preparations.

Process alternatives for bioethanol production from mango stem bark residues

Three alternatives for bioethanol production from pretreated mango stem bark after maceration (MSBAM) were evaluated as a biorefinery component for the mango agroindustry. These included separate hydrolysis and fermentation (SHF), simultaneous saccharification and fermentation (SSF), and pre-saccharification followed by simultaneous saccharification and fermentation (PSSF). The effects on ethanol concentration, yield and productivity of pretreated MSBAM solids loading, Tween 20 addition, and temperature were used for process comparisons. The highest yields for the SHF, SSF, and PSSF process alternatives were 58.8, 81.6, and 84.5%, respectively. Since saccharification and fermentation are carried out in the same vessel in the SSF alternative, and no significant SSF and PSSF differences in ethanol concentration were observed, SSF is recommended as the best process configuration.

Production of cellulosic ethanol from sugarcane bagasse by steam explosion: Effect of extractives content, acid catalysis and different fermentation technologies

The production of cellulosic ethanol was carried out using samples of native (NCB) and ethanol-extracted (EECB) sugarcane bagasse. Autohydrolysis (AH) exhibited the best glucose recovery from both samples, compared to the use of both H3PO4 and H2SO4 catalysis at the same pretreatment time and temperature. All water-insoluble steam-exploded materials (SEB-WI) resulted in high glucose yields by enzymatic hydrolysis. SHF (separate hydrolysis and fermentation) gave ethanol yields higher than those obtained by SSF (simultaneous hydrolysis and fermentation) and pSSF (pre-hydrolysis followed by SSF). For instance, AH gave 25, 18 and 16 g L(-1) of ethanol by SHF, SSF and pSSF, respectively. However, when the total processing time was taken into account, pSSF provided the best overall ethanol volumetric productivity of 0.58 g L(-1) h(-1). Also, the removal of ethanol-extractable materials from cane bagasse had no influence on the cellulosic ethanol production of SEB-WI, regardless of the fermentation strategy used for conversion.

Comparative study of simultaneous saccharification and fermentation byproducts from sugarcane bagasse using steam explosion, alkaline hydrogen peroxide and organosolv pretreatments

Most of the hemicelluloses were removed and more acetyl groups were generated after steam pretreatment, and a high acetic acid concentration was observed during SSF.

Saccharification behavior of cellulose acetate during enzymatic processing for microbial ethanol production

This study was conducted to realize the potential application of cellulose acetate to enzymatic processing, followed by microbial ethanol fermentation. To eliminate the effect of steric hindrance of acetyl groups on the action of cellulase, cellulose acetate was subjected to deacetylation in the presence of 1N sodium hydroxide and a mixture of methanol/acetone, yielding 88.8-98.6% at 5-20% substrate loadings during a 48h saccharification at 50°C. Ethanol fermentation using Saccharomyces cerevisiae attained a high yield of 92.3% from the initial glucose concentration of 44.2g/L; however, a low saccharification yield was obtained at 35°C, decreasing efficiency during simultaneous saccharification and fermentation (SSF). Presaccharification at 50°C prior to SSF without increasing the total process time attained the ethanol titers of 19.8g/L (5% substrate), 38.0g/L (10% substrate), 55.9g/L (15% substrate), and 70.9g/L (20% substrate), which show a 12.0-16.2% improvement in ethanol yield.

Physiological characterization of thermotolerant yeast for cellulosic ethanol production

The conversion of lignocellulose into fermentable sugars is considered a promising alternative for increasing ethanol production. Higher fermentation yield has been achieved through the process of simultaneous saccharification and fermentation (SSF). In this study, a comparison was performed between the yeast species Saccharomyces cerevisiae and Kluyveromyces marxianus for their potential use in SSF process. Three strains of S. cerevisiae were evaluated: two are widely used in the Brazilian ethanol industry (CAT-1 and PE-2), and one has been isolated based on its capacity to grow and ferment at 42 °C (LBM-1). In addition, we used thermotolerant strains of K. marxianus. Two strains were obtained from biological collections, ATCC 8554 and CCT 4086, and one strain was isolated based on its fermentative capacity (UFV-3). SSF experiments revealed that S. cerevisiae industrial strains (CAT-1 and PE-2) have the potential to produce cellulosic ethanol once ethanol had presented yields similar to yields from thermotolerant strains. The industrial strains are more tolerant to ethanol and had already been adapted to industrial conditions. Moreover, the study shows that although the K. marxianus strains have fermentative capacities similar to strains of S. cerevisiae, they have low tolerance to ethanol. This characteristic is an important target for enhancing the performance of this yeast in ethanol production.

Fed-batch semi-simultaneous saccharification and fermentation of reed pretreated with liquid hot water for bio-ethanol production using Saccharomyces cerevisiae

Reed was pretreated with liquid hot water (LHW) and then subjected to fed-batch semi-simultaneous saccharification and fermentation (S-SSF) to obtain high ethanol concentration and yield. Results show that water-insoluble solid (WIS) produced from reed pretreated at 180 and 210°C could be effectively converted to ethanol by using Saccharomyces cerevisiae. The optimum conditions for bio-ethanol production are as follows: fermentation temperature of 36°C, pH of 4.8 with cellulase loading of 40 filter paper activity units/g oven-dried WIS, and 18 h pre-hydrolysis at 50°C. Approximately 6.4% (w/v) fed-batch substrate was added after 6 h of the 18 h enzymatic pre-hydrolysis. The highest ethanol concentration of 39.4 g/L was achieved. The conversion of glucan in the WIS to ethanol reached 79.1% (180°C) and 75.1% (210°C) respectively. The ethanol yields per kg of oven-dried reed were 283 g/L at 180°C and 244 g/L at 210°C.

Production and application of an enzyme blend from Chrysoporthe cubensis and Penicillium pinophilum with potential for hydrolysis of sugarcane bagasse

Blending of the enzyme extracts produced by different fungi can result in favorable synergetic enhancement of the enzyme blend with regards to the main cellulase activities, as well as the inclusion of accessory enzymes that may not be as abundant in enzyme extracts produced by predominantly cellulase producing fungi. The Chrysoporthe cubensis:Penicillium pinophilum 50:50 (v/v) blend produced herein presented good synergy, especially for FPase and endoglucanase activities which were 76% and 48% greater than theoretical, respectively. This enzyme blend was applied to sugarcane bagasse previously submitted to a simple alkali pretreatment. Glucan hydrolysis efficiency reached an excess of 60% and xylan conversion exceeded 90%. Increasing the hydrolysis temperature from 45 to 50°C also resulted in a 16-20% increase in conversion of both glucan and xylan fractions. The blended enzyme extract obtained therefore showed great potential for application in the lignocellulose hydrolysis process.

Comparison of process configurations for ethanol production from acid- and alkali-pretreated corncob by Saccharomyces cerevisiae strains with and without β-glucosidase expression

β-Glucosidase was shown to have synergistic effects with commercial cellulase in the hydrolysis of acid- and alkali-pretreated corncob, especially at the dose of 5 U/g biomass and 5 or 10 FPU/g biomass. An integrating yeast strain 45# expressing β-glucosidase was constructed that utilized cellobiose quickly and efficiently. Process configurations were compared under conditions of 10% solid content, 10 FPU cellulase/g biomass, 5 U β-glucosidase/g biomass (only used for parental strain W303-1A), 1g/kg yeast loading and 3.3g/kg urea supplementation. While separate hydrolysis and fermentation was optimal for W303-1A and the ethanol titer and yield reached 3.22 g/100g and 75.6% (expressed as a percentage of the theoretical yield), respectively, simultaneous saccharification and fermentation was optimal for strain 45# and the ethanol titer and yield reached 3.31 g/100g and 77.7%, respectively. These results are valuable in optimization of the process configuration and improving the yeast strain selected for cellulosic ethanol production.

Dynamic Characteristics and Speed Control Strategy of Cellulose Hydrolysis Reactor at High Solids Loading

This article put emphasis on systematic research and development on cellulose hydrolysis reactor. The simultaneous saccharification and fermentation of lignocellulosic was carried out in a 5-L reactor at high solids loading of corn stover (25%, w/w). The dynamic characteristics of reactive system were examined. The experimental results showed that the lignocellulosic biomass experienced a series of morphology evolution and viscosity variation. This evolution was a typical event caused by solid reactive processing, and it constituted the basis of a new geometrical configuration of reactor (specifically hybrid impeller). The minimum rotational speed of the impeller was proposed based on the Coutte flow analogy and the yield stress concept. The macroscopic performance of the reactor changed significantly with time. In view of this effect, the operating strategy of the impeller was determined.