Bioethanol production from rice straw by a sequential use of Saccharomyces cerevisiae and Pichia stipitis with heat inactivation of Saccharomyces cerevisiae cells prior to xylose fermentation

Sequential and consolidated bioprocessing of biogenic municipal solid waste: A strategic pairing of thermophilic anaerobe and mesophilic microaerobe for ethanol production

Feedstock availability and its pretreatment, high process economics and insufficient ethanol (H_Et) titres necessitated the bioprocesses that are sustainable. The advanced consolidated bioprocessing (CBP_Seq) strategy presently considered for improved H_Et production involves, sequential coupling of CBP thermophile, Clostridium thermocellum ATCC-27405 with mesophilic microaerobe, Pichia stipitis NCIM-3498. Biogenic municipal solid waste (BMSW) pretreated with 0.5% NaOH (CS_PBMSW) served as the sole carbon source. CBP_Seq (23.99 g/L) fared better than CBP standalone (18.10 g/L) wherein 1.32-folds improvement in H_Et titre was recorded. Considering insufficient xylanase titre in cellulosome complex of C. thermocellum, CBP_Seq was performed employing exogenous xylanases (CBP_SeqE) to improve xylan digestibility and H_Et yield. CBP_SeqE-II biosystem at pH 5 showed maximum H_Et titre of 36.90 g/L which corresponds to yield of 0.26 g H_Et/ g CS_PBMSW. This study substantiates efficacy of CBP_SeqE-II biosystem in sustainable bioethanol production from BMSW in a single reactor without laborious steps.

Evaluating novel fungal secretomes for efficient saccharification and fermentation of composite sugars derived from hydrolysate and molasses into ethanol

This paper evaluates the ability of secretome from two thermotolerant fungal strains (Aspergillus terreus 9DR and Achaetomium strumarium 10DR) for boosting the hydrolytic efficiency of benchmark cellulolytic preparation (Cellic CTec2). Further we report enhanced saccharification of different agro-residues under semi-aerobic when compared to aerobic conditions. The mass spectroscopic analysis of the hydrolysates indicates the role of auxiliary oxidative enzymes present in A. terreus and A. strumarium secretomes for enhancing the capability of the cellulolytic cocktails. The paper further demonstrate positive effect of using the cocktails for enhanced saccharification and subsequent fermentation to ethanol of acid pre-treated rice straw, corn residues and sugarcane bagasse at higher substrate loading rates (20% w/v). The paper also reports co-utilization of composite sugars derived from molasses and enzymatic hydrolysate obtained from agnostic lignocellulosics for efficient bioconversion to ethanol applicable for developing BOLT-ON technology.

Profiling and production of hemicellulases by thermophilic fungus Malbranchea flava and the role of xylanases in improved bioconversion of pretreated lignocellulosics to ethanol

This study reports thermophilic fungus Malbranchea flava as a potent source of xylanase and xylan-debranching accessory enzymes. M. flava produced high levels of xylanase on sorghum straw containing solidified culture medium. The optimization of culture conditions for production of hemicellulases was carried out using one factor at a time approach and Box–Behnken design of experiments with casein (%), inoculum age (h) and inoculum level (ml) as process variables and xylanase, β-xylosidase, acetyl esterases and arabinofuranosidase as response variables. The results showed that casein concentration between 3.0 and 3.5 %, inoculum age (56–60 h) and inoculum level (2–2.5 ml) resulted in production of 16,978, 10.0, 67.7 and 3.8 (U/gds) of xylanase, β-xylosidase, acetyl esterase and α-l-arabinofuranosidase, respectively. Under optimized conditions M. flava produced eight functionally diverse xylanases with distinct substrate specificity against different xylan types. The peptide mass fingerprinting of 2-D gel electrophoresis resolved proteins indicated to the presence of cellobiose dehydrogenase and glycosyl hydrolases suggesting the potential of this strain in oxidative and classical cellulase-mediated hydrolysis of lignocellulosics. Addition of xylanase (300 U/g substrate) during saccharification (at 15 % substrate loading) of different pretreated (acid/alkali) substrates (cotton stalks, wheat straw, rice straw, carrot grass) by commercial cellulase (NS28066) resulted in 9–36 % increase in saccharification and subsequent fermentation to ethanol when compared to experiment with commercial enzyme only. High ethanol level 46 (g/l) was achieved with acid pretreated cotton stalk when M. flava xylanase was supplemented as compared to 39 (g/l) with xylanase without xylanase addition.

Ethanol production from glucose and xylose obtained from steam exploded water-extracted olive tree pruning using phosphoric acid as catalyst

In this work, the effect of phosphoric acid (1% w/w) in steam explosion pretreatment of water extracted olive tree pruning at 175°C and 195°C was evaluated. The objective is to produce ethanol from all sugars (mainly glucose and xylose) contained in the pretreated material. The water insoluble fraction obtained after pretreatment was used as substrate in a simultaneous saccharification and fermentation (SSF) process by a commercial strain of Saccharomyces cerevisiae. The liquid fraction, containing mainly xylose, was detoxified by alkali and ion-exchange resin and then fermented by the xylose fermenting yeast Scheffersomyces stipitis. Ethanol yields reached in a SSF process were close to 80% when using 15% (w/w) substrate consistency and about 70% of theoretical when using prehydrolysates detoxified by ion-exchange resins. Considering sugars recovery and ethanol yields about 160g of ethanol from kg of water extracted olive tree pruning could be obtained.

Pichia anomala 29X: a resistant strain for lignocellulosic biomass hydrolysate fermentation

To efficiently use lignocellulosic biomass hydrolysates as fermentation media for bioethanol production, besides being capable of producing significant amount of ethanol, the fermenting host should also meet the following two requirements: (1) resistant to the inhibitory compounds formed during biomass pretreatment process, (2) capable of utilizing C5 sugars, such as xylose, as carbon source. In our laboratory, a screening was conducted on microorganisms collected from environmental sources for their tolerance to hydrolysate inhibitors. A unique resistant strain was selected and identified as Pichia anomala (Wickerhamomyces anomalus), deposited as CBS 132101. The strain is able to produce ethanol in various biomass hydrolysates, both with and without oxygen. Besides, the strain could assimilate xylose and use nitrate as N source. These physiological characteristics make P. anomala an interesting strain for bioethanol production from lignocellulosic biomass hydrolysates.

Alcohol dehydrogenases from Scheffersomyces stipitis involved in the detoxification of aldehyde inhibitors derived from lignocellulosic biomass conversion

Aldehyde inhibitors such as furfural and 5-hydroxymethylfurfural (HMF) are generated from biomass pretreatment. Scheffersomyces stipitis is able to reduce furfural and HMF to less toxic furanmethanol and furan-2,5-dimethanol; however, the enzymes involved in the reductive reaction still remain unknown. In this study, transcription responses of two known and five putative alcohol dehydrogenase genes from S. stipitis were analyzed under furfural and HMF stress conditions. All the seven alcohol dehydrogenase genes were also cloned and overexpressed for their activity analyses. Our results indicate that transcriptions of SsADH4 and SsADH6 were highly induced under furfural and HMF stress conditions, and the proteins encoded by them exhibited NADH- and/or NADPH-dependent activities for furfural and HMF reduction, respectively. For furfural reduction, NADH-dependent activity was also observed in SsAdh1p and NAD(P)H-dependent activities were also observed in SsAdh5p and SsAdh7p. For HMF reduction, NADPH-dependent activities were also observed in SsAdh5p and SsAdh7p. SsAdh4p displayed the highest NADPH-dependent specific activity and catalytic efficiency for reduction of both furfural and HMF among the seven alcohol dehydrogenases. Enzyme activities of all SsADH proteins were more stable under acidic condition. For most SsADH proteins, the optimum temperature for enzyme activities was 30 °C and more than 50 % enzyme activities remained at 60 °C. Reduction activities of formaldehyde, acetaldehyde, isovaleraldehyde, benzaldehyde, and phenylacetaldehyde were also observed in some SsADH proteins. Our results indicate that multiple alcohol dehydrogenases in S. stipitis are involved in the detoxification of aldehyde inhibitors derived from lignocellulosic biomass conversion.

Sequential incubation of Candida shehatae and ethanol-tolerant yeast cells for efficient ethanol production from a mixture of glucose, xylose and cellobiose

A mixture of 5% (w/v) glucose, 4% (w/v) xylose and 5% (w/v) cellobiose was fermented into ethanol using non-recombinant yeasts. Two series of experiments were carried out: (1) sequential fermentation with Candida shehatae D45-6 and Saccharomyces cerevisiae (Cs-Sc), and (2) sequential fermentation with C. shehatae D45-6 and Brettanomyces bruxellensis (Cs-Bb). C. shehatae D45-6 was initially used for glucose and xylose fermentation before adding highly ethanol-tolerant yeasts, either S. cerevisiae or B. bruxellensis, for cellobiose fermentation. For the sequential fermentation using S. cerevisiae, β-glucosidase was also included in the second step. In these two experiments, ethanol concentration reached 5.6-5.8% (w/v) and 99% sugar was consumed. Our results suggest that restricted glucose production from cellulose by saccharification could allow D45-6 to complete monosaccharide fermentation before the ethanol concentration exceeded its tolerance level.

Comparison of Scheffersomyces stipitis strains CBS 5773 and CBS 6054 with regard to their xylose metabolism: implications for xylose fermentation

The various strains of Scheffersomyces stipitis (Pichia stipitis) differ substantially with respect to their ability to ferment xylose into ethanol. Two P. stipitis strains CBS 5773 and CBS 6054 have been most often used in literature but comparison of their performance in xylose fermentation under identical conditions has not been reported so far. Conversion of xylose (22 g/L) by each of these P. stipitis strain was analyzed under anaerobic and microaerobic conditions. Ethanol yields of ∼0.41 g/g were independent of strain and conditions used. Glycerol and acetate were formed in constant yields of 0.006 g/g and 0.02 g/g, respectively. Xylitol formation decreased from ∼0.08 g/g to ∼0.05 g/g upon switch from anaerobic to microaerobic conditions. Specific activities of enzymes of the two-step oxidoreductive xylose conversion pathway (xylose reductase and xylitol dehydrogenase) matched for both strains within limits of error. When xylose was offered at 76 g/L under microaerobic reaction conditions, ethanol yields were still high (0.37-0.39 g/g) for both strains even though the xylitol yields (0.12-0.13 g/g) were increased as compared to the conditions of low xylose concentration. P. stipitis strains CBS 5773 and CBS 6054 are therefore identical by the criteria selected and show useful performance during conversion of xylose into ethanol, irrespective of the supply of oxygen.

Enzymatic hydrolysis optimization of microwave alkali pretreated wheat straw and ethanol production by yeast

Microwave alkali pretreated wheat straw was used for in-house enzyme production by Aspergillusflavus and Trichodermareesei. Produced enzymes were concentrated, pooled and assessed for the hydrolysis of pretreated wheat straw. Factors affecting hydrolysis were screened out by Placket-Burman design (PBD) and most significant factors were further optimized by Box-Behnken design (BBD). Under optimum conditions, 82% efficiency in hydrolysis yield was observed. After the optimization by response surface methodology (RSM), a model was proposed to predict the optimum value confirmed by the experimental results. The concentrated enzymatic hydrolyzate was fermented for ethanol production by Saccharomyces cerevisiae, Pichia stipitis and co-culture of both. The yield of ethanol was found to be 0.48 g(p)/g(s), 0.43 g(p)/g(s) and 0.40 g(p)/g(s) by S. cerevisiae, P. stipitis and by co-culture, respectively, using concentrated enzymatic hydrolyzate. During anaerobic fermentation 42.31 μmol/mL, 36.69 μmol/mL, 43.35 μmol/mL CO(2) was released by S. cerevisiae, P. stipitis and by co-culture, respectively.