Effect of fermentation inhibitors in the presence and absence of activated charcoal on the growth of Saccharomyces cerevisiae

Highly Efficient Production of Cellulosic Ethanol from Poplar Using an Optimal C6/C5 Co-Fermentation Strain of Saccharomyces cerevisiae

Cellulosic ethanol is the key technology to alleviate the pressure of energy supply and climate change. However, the ethanol production process, which is close to industrial production and has a high saccharification rate and ethanol yield, still needs to be developed. This study demonstrates the effective conversion of poplar wood waste into fuel-grade ethanol. By employing a two-step pretreatment using sodium chlorite (SC)-dilute sulfuric acid (DSA), the raw material achieved a sugar conversion rate exceeding 85% of the theoretical value. Under optimized conditions, brewing yeast co-utilizing C6/C5 enabled a yield of 35 g/L ethanol from 10% solid loading delignified poplar hydrolysate. We increased the solid loading to enhance the final ethanol concentration and optimized both the hydrolysis and fermentation stages. With 20% solid loading delignified poplar hydrolysate, the final ethanol concentration reached 60 g/L, a 71.4% increase from the 10% solid loading. Our work incorporates the pretreatment, enzymatic hydrolysis, and fermentation stages to establish a simple, crude poplar waste fuel ethanol process, expanding the range of feedstocks for second-generation fuel ethanol production.

Optimal preparation of food waste to increase its utility for sophorolipid production by Starmerella bombicola

Secondary feedstocks, such as food waste (FW), have been used for yeasts (e.g. Starmerella bombicola) to produce sophorolipids (SLs), which are commercially available biosurfactants. However, the quality of FW varies by location and season and may contains chemicals that inhibit SLs production. Therefore, it is crucial to identify such inhibitors and, if possible, remove them, to ensure efficient utilization. In this study, large scale FW was first analysed to determine the concentration of potential inhibitors. Lacticacid, acetic acid and ethanol were identified and found to be inhibitors of the growth of S. bombicola and its SLs production. Various methods were then evaluated for their ability to remove these inhibitors. Finally, a simple and effective strategy for removing inhibitors from FW was developed that complied with the 12 principles of green chemistry and could be adopted by industry for high SLs production.

A Detoxification-Free Process for Enhanced Ethanol Production From Corn Fiber Under Semi-Simultaneous Saccharification and Fermentation

Corn fiber, a by-product from the corn-processing industry, is an attractive feedstock for cellulosic ethanol because of its rich carbohydrate content (mainly residual starch, cellulose, and hemicellulose), abundant reserves, easy collection, and almost no transportation cost. However, the complex structure and components of corn fiber, especially hemicellulose, make it difficult to be effectively hydrolyzed into fermentable sugars through enzymatic hydrolysis. This study developed a simple and easy industrialized process without detoxification treatment for high-yield ethanol produced from corn fiber. Corn fiber was pretreated by dilute acid under the conditions optimized by Box-Behnken design (0.5% H₂SO₄ at 105°C for 43 min), and 81.8% of total sugars, including glucose, xylose, and arabinose, could be recovered, then the mixture (solid and hydrolysates) was directly used for semi-simultaneous saccharification and fermentation without detoxification, and ethanol yield reached about 81% of the theoretical yield.

Production of single cell oil by Yarrowia lipolytica JCM 2320 using detoxified desiccated coconut residue hydrolysate

Nowadays, the replacement of petro-diesel with biodiesel has raised the concern among the community for the utilization of improper feedstocks and the cost involved. However, these issues can be solved by producing single cell oil (SCO) from lignocellulosic biomass hydrolysates by oleaginous microorganisms. This study introduced Yarrowia lipolytica JCM 2320 with a desiccated coconut residue (DCR) hydrolysate (obtained from the 2% dilute sulphuric acid pretreatment) as a carbon source in generating SCO. However, common inhibitors formed during acid pretreatment of biomass such as five-hydroxymethylfurfural (HMF), furfural, acetic acid and levulinic acid resulting from the sugar degradations may have detrimental effects towards the fermentation process. To visualize the effect of inhibitors on Y. lipolytica, an inhibitory study was conducted by adding 0.5-5.0 g/L of potential inhibitors to the YPD (yeast, peptone and D-glucose) medium. It was found that the presence of furfural at 0.5 g/L would increase the lag phase, which beyond that was detrimental to Y. lipolytica. Furthermore, increasing the five-hydroxymethylfurfural (HMF) concentration would increase the lag phase of Y. lipolytica, whereas, for acetic acid and levulinic acid, it showed a negligible effect. Detoxification was hence conducted to remove the potential inhibitors from the DCR hydrolysate prior its utilization in the fermentation. To examine the possibility of using adsorption resins for the detoxification of DCR hydrolysate, five different resins were tested (Amberlite® XAD-4, Amberlite® XAD-7, Amberlite® IR 120, Amberlite® IRA 96 and Amberlite® IRA 402) with five different concentrations of 1%, 3%, 5%, 10% and 15% (w/v), respectively. At resin concentration of 10%, Amberlite® XAD-4 recorded the highest SCO yield, 2.90 ± 0.02 g/L, whereas the control and the conventional overliming detoxification method, recorded only 1.29 ± 0.01 g/L and 1.27 ± 0.02 g/L SCO accumulation, respectively. Moreover, the fatty acid profile of the oil produced was rich in oleic acid (33.60%), linoleic acid (9.90%), and palmitic acid (14.90%), which indicates the potential as a good biodiesel raw material.

Punica granatum waste to ethanol valorisation employing optimized levels of saccharification and fermentation

Pomegranate peels (PPW) as municipal waste is inexpensive biomass that could be a renewable source of sugars particularly rich in hemicellulosic contents. The subsequent conversion of available sugars in PPW can provide prospective strategy for cost-effective bioenergy production. In this study, an experimental setup based on CCD was implemented with the aim of bioconversion of biomass into bioethanol. The factors considered were Hydrochloric acid concentration (X1), the hydrolysis temperature (X2) and time (X3) for optimization with dilute Hydrochloric acid (HCl) saccharification. The present study investigates the optimised level of bioethanol synthesis from acid pre-treated PPW explained by RSM. Subsequently, three yeasts viz. Saccharomyces cerevisiae K7, Metschnikowia sp. Y31 and M. cibodasensis Y34 were utilized for fermentation of acid hydrolysed and detoxified feed stocks. Optimum values of reducing sugars 48.02 ± 0.02 (gL-1) and total carbohydrates 205.88 ± 0.13 (gL-1) were found when PPW was hydrolyzed with 1% HCl concentration at 100˚C of temperature for 30 min. Later on, fermentation of PPWH after detoxification with 2.5% activated charcoal. The significant ethanol (g ethanol/g of reducing sugars) yields after fermentation with Metschnikowia sp. Y31 and M. cibodasensis Y34 found to be 0.40 ± 0.03 on day 5 and 0.41 ± 0.02 on last day of experiment correspondingly. Saccharomyces cerevisiae K7 also produce maximum ethanol 0.40 ± 0.00 on last day of incubation utilizing the PPWH. The bioconversion of commonly available PPW into bioethanol as emphasize in this study could be a hopeful expectation and also cost-effective to meet today energy crisis.

Semi-batch cultivation of Chlorella sorokiniana AK-1 with dual carriers for the effective treatment of full strength piggery wastewater treatment

In this study, process optimization for the microalgae-based piggery wastewater treatment was carried out by growing Chlorella sorokiniana AK-1 on untreated piggery wastewater with efficient COD/BOD/TN/TP removal and high biomass/protein productivities. Integration of the immobilization carriers (sponge, activated carbon) and semi-batch cultivation resulted in the effective treatment of raw untreated piggery wastewater. With 100% wastewater, 0.2% sponge and 2% activated carbon, the semi-batch cultivation (90% media replacement every 6 days) exhibited a COD, BOD, TN and TP removal efficiency of 95.7%, 99.0%, 94.1% and 96.9%, respectively. The maximal protein content, protein productivity, lutein content, and lutein productivity of the obtained microalgal biomass was 61.1%, 0.48 g/L/d, 4.56 mg/g, and 3.56 mg/L/d, respectively. The characteristics of the treated effluent satisfied Taiwan Piggery Wastewater Discharge Standards (COD < 600 mg/L, BOD < 80 mg/L). This innovative approach demonstrated excellent performance for simultaneous piggery wastewater treatment and microalgal biomass production.

Simultaneous pretreatment and saccharification process for fermentable sugars production from Casuarina equisetifolia biomass using transgenic Trichoderma atroviride

With the increase in the cognizance toward the wide and abundant lignocellulosic biomass, a great interest has been garnered toward the production of value-added products from the biomass. Hence, by capitalizing the Casuarina equisetifolia biomass, the current work developed a simultaneous pre-treatment and saccharification (SPS) process using transgenic Trichoderma atroviride. The ability of T. atroviride to produce lignolytic and cellulolytic enzymes was enhanced by optimizing key process parameters. Under the optimized conditions, a maximum of 1245.6 U/kg of cellulase and 1203.36 U/kg of xylanase, 183.2 U/kg of laccase along with 392.36 g/kg of fermentable sugars were obtained. On comparing with acid and alkaline pre-treatment methods, the T. atroviride -mediated SPS process exhibited trace formation of fermentative inhibitors, which resulted in a minimal inhibition of Escherichia coli. Overall, the current work implements the biorefinery concept on Casuarina equisetifolia biomass by advocating circular economy. Implications: Valorization of lignocellulosic waste biomass into value added compound and as biofuel is considered as a promising alternative resource, owing to its availability and low production cost. However, the presence of chemically resistant lignin demands an intensive treatment process, which sometimes leads to the formation of fermentative inhibitors. Casuarina equisetifolia is a deciduous commercial plant, and an average of 125 tonnes/hector of waste is generated annually in India. By considering the demerit of delignification and the wide availability of Casuarina equisetifolia biomass (CB), the current work aimed at the development of a single-pot simultaneous pre-treatment and saccharification (SPS) of CB by transgenic Trichoderma atroviride.

Engineered microbial host selection for value-added bioproducts from lignocellulose

Lignocellulose is a rich and sustainable globally available carbon source and is considered a prominent alternative raw material for producing biofuels and valuable chemical compounds. Enzymatic hydrolysis is one of the crucial steps of lignocellulose degradation. Cellulolytic and hemicellulolytic enzyme mixes produced by different microorganisms including filamentous fungi, yeasts and bacteria, are used to degrade the biomass to liberate monosaccharides and other compounds for fermentation or conversion to value-added products. During biomass pretreatment and degradation, toxic compounds are produced, and undesirable carbon catabolic repression (CCR) can occur. In order to solve this problem, microbial metabolic pathways and transcription factors involved have been investigated along with the application of protein engineering to optimize the biorefinery platform. Engineered Microorganisms have been used to produce specific enzymes to breakdown biomass polymers and metabolize sugars to produce ethanol as well other biochemical compounds. Protein engineering strategies have been used for modifying lignocellulolytic enzymes to overcome enzymatic limitations and improving both their production and functionality. Furthermore, promoters and transcription factors, which are key proteins in this process, are modified to promote microbial gene expression that allows a maximum performance of the hydrolytic enzymes for lignocellulosic degradation. The present review will present a critical discussion and highlight the aspects of the use of microorganisms to convert lignocellulose into value-added bioproduct as well combat the bottlenecks to make the biorefinery platform from lignocellulose attractive to the market.

Effect of Lyophilization on Survivability and Growth Kinetic of Trichoderma Strains Preserved on Various Agriculture By-Products

Growth of four Trichoderma strains were tested on lignocellulosic by-products in solid state fermentation (SSF). The strains were also analyzed for their survival rate and growth after lyophilization on these carriers. All applied monocomponent and bicomponent media were substrates for the production and preservation of Trichoderma biomass. However, the maximum number of colony forming units (CFU/g dm) was acquired on bicomponent media based on dried grass and beet pulp or grass with corn cobs, when compared to monocomponent media. Although the process of lyophilization reduced the survival rate by 50%-60%, the actual number of viable cells in obtained biopreparations remained relatively high (0.58 × 108-1.68 × 108 CFU/g dm). The studied strains in the preserved biopreparations were characterized by a high growth rate, as evaluated in microcultures using the Bioscreen C system.

Biochar-Rhizosphere Interactions – a Review

Biochar is a solid material of biological origin obtained from biomass carbonization, designed as a mean to reduce greenhouse gases emission and carbon sequestration in soils for a long time. Biochar has a wide spectrum of practical utilization and is applied as a promising soil improver or fertilizer in agriculture, or as a medium for soil or water remediation. Preparations of biochar increase plant growth and yielding when applied into soil and also improve plant growth conditions, mainly bio, physical and chemical properties of soil. Its physical and chemical properties have an influence on bacteria, fungi and invertebrates, both in field and laboratory conditions. Such effects on rhizosphere organisms are positive or negative depending on biochar raw material origin, charring conditions, frequency of applications, applications method and doses, but long term effects are generally positive and are associated mainly with increased soil biota activity. However, a risk assessment of biochar applications is necessary to protect food production and the soil environment. This should be accomplished by biochar production and characterization, land use implementation, economic analysis, including life cycle assessment, and environmental impact assessment.

Application of solid-acid catalyst and marine macro-algae Gracilaria verrucosa to production of fermentable sugars

In this study, the hydrolysis of marine macro-algae Gracilaria verrucosa with a solid-acid catalyst was investigated. To optimize the hydrolysis, four reaction factors, including liquid-to-solid ratio, catalyst loading, reaction temperature, and reaction time, were investigated. In the results, the highest total reducing sugar (TRS) yield, 61 g/L (51.9%), was obtained under the following conditions: 1:7.5 solid-to-liquid ratio, 15% (w/v) catalyst loading, 140 °C reaction temperature, and 150 min reaction time. Under these conditions, 10.7 g/L of 5-HMF and 2.5 g/L of levulinic acid (LA) were generated. The application of solid-acid catalyst and marine macro-algae resources shows a very high potential for production of fermentable sugars.

Current Trends in Bioethanol Production by Saccharomyces cerevisiae: Substrate, Inhibitor Reduction, Growth Variables, Coculture, and Immobilization

Bioethanol is one of the most commonly used biofuels in transportation sector to reduce greenhouse gases. S. cerevisiae is the most employed yeast for ethanol production at industrial level though ethanol is produced by an array of other yeasts, bacteria, and fungi. This paper reviews the current and nonmolecular trends in ethanol production using S. cerevisiae. Ethanol has been produced from wide range of substrates such as molasses, starch based substrate, sweet sorghum cane extract, lignocellulose, and other wastes. The inhibitors in lignocellulosic hydrolysates can be reduced by repeated sequential fermentation, treatment with reducing agents and activated charcoal, overliming, anion exchanger, evaporation, enzymatic treatment with peroxidase and laccase, in situ detoxification by fermenting microbes, and different extraction methods. Coculturing S. cerevisiae with other yeasts or microbes is targeted to optimize ethanol production, shorten fermentation time, and reduce process cost. Immobilization of yeast cells has been considered as potential alternative for enhancing ethanol productivity, because immobilizing yeasts reduce risk of contamination, make the separation of cell mass from the bulk liquid easy, retain stability of cell activities, minimize production costs, enable biocatalyst recycling, reduce fermentation time, and protect the cells from inhibitors. The effects of growth variables of the yeast and supplementation of external nitrogen sources on ethanol optimization are also reviewed.

Development of a phenotypic assay for characterisation of ethanologenic yeast strain sensitivity to inhibitors released from lignocellulosic feedstocks

Inhibitors released by the breakdown of plant cell walls prevent efficient conversion of sugar into ethanol. The aim of this study was to develop a fast and reliable inhibitor sensitivity assay for ethanologenic yeast strains. The assay comprised bespoke 96-well plates containing inhibitors in isolation or combination in a format that was compatible with the Phenotypic Microarray Omnilog reader (Biolog, hayward, CA, USA). A redox reporter within the assay permits analysis of inhibitor sensitivity in aerobic and/or anaerobic conditions. Results from the assay were verified using growth on spot plates and tolerance assays in which maintenance of viability was assessed. The assay allows for individual and synergistic effects of inhibitors to be determined. It was observed that the presence of both acetic and formic acid significantly inhibited the yeast strains assessed, although this impact could be partially mitigated by buffering to neutral pH. Scheffersomyces stipitis, Candida spp., and Pichia guilliermondii demonstrated increased sensitivity to short chain weak acids at concentrations typically present in lignocellulosic hydrolysates. S. cerevisiae exhibited robustness to short chain weak acids at these concentrations. However, S. stipitis, Candida spp., and P. guilliermondii displayed increased tolerance to HMF when compared to that observed for S. cerevisiae. The results demonstrate that the phenotypic microarray assay developed in the current study is a valuable tool that can be used to identify yeast strains with desirable resistance to inhibitory compounds found in lignocellulosic hydrolysates.