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

Sustainable Production of Shinorine from Lignocellulosic Biomass by Metabolically Engineered Saccharomyces cerevisiae

Mycosporine-like amino acids (MAAs) have been used in cosmetics and pharmaceuticals. The purpose of this work was to develop yeast strains for sustainable and economical production of MAAs, especially shinorine. First, genes involved in MAA biosynthetic pathway from Actinosynnema mirum were introduced into Saccharomyces cerevisiae for heterologous shinorine production. Second, combinatorial expression of wild and mutant xylose reductase was adopted in the engineered S. cerevisiae to facilitate xylose utilization in the pentose phosphate pathway. Finally, the accumulation of sedoheptulose 7-phosphate (S7P) was attempted by deleting transaldolase-encoding TAL1 in the pentose phosphate pathway to increase carbon flux toward shinorine production. In fed-batch fermentation, the engineered strain (DXdT-M) produced 751 mg/L shinorine in 71 h. Ultimately, 54 mg/L MAAs was produced by DXdT-M from rice straw hydrolysate. The results suggest that shinorine production by S. cerevisiae might be a promising process for sustainable production and industrial applications.

An integrated biorefinery process for co-production of xylose and glucose using maleic acid as efficient catalyst

This study aims to valorize wheat straw for xylose and glucose recovery using maleic acid in the pretreatment. The process conditions of maleic acid hydrolysis of wheat straw for xylose recovery were optimized by response surface methodology, through which the maximum xylose recovery of 77.12% versus minimum furfural yield of 1.61% were achieved using 70 g/L solid-to-liquid ratio and 0.1 mol/L maleic acid for 40 min at 150 °C. Moreover, 88.58% cellulose conversion was achieved by enzymatic hydrolysis of maleic acid-pretreated wheat straw. Results showed that maleic acid was an effective pretreatment solvent for sugars recovery: 19.88 g xylose and 30.89 g glucose were respectively obtained from 100 g wheat straw due to acidic and enzymatic hydrolysis, with only 0.37 g furfural produced. This study provides a strategy for hydrolyzing wheat straw to produce fermentable sugars with low amount of degradation product.

Pretreatment and enzymatic saccharification of oak at high solids loadings to obtain high titers and high yields of sugars

Production of high-titer sugar from lignocellulose is important in terms of process economics of bio-based product industry. In this study, to obtain high titers and yields of sugars, we combined pretreatment and saccharification steps, both at high solids loadings. First, pretreatment of oak was optimized at a 30% (w/w) solids loading. The whole slurry of the pretreated oak was subjected to a fed-batch saccharification step at the final solids loading of 30%, to minimize loss of fermentable sugars and simplify the processes. As a result, high-titer sugars (157.5 g/L) consisting of 120.2 g/L of glucose and 37.3 g/L of xylose were obtained at 75.9% and 58.6%, respectively, of theoretical maximum yields, based on the initial glucan and xylan contents. Thus, through proper optimization processes of oak, the combination of pretreatment and saccharification at high solids loadings was effective in obtaining both high titers and high yields of sugars from lignocellulose.

Overproduction of native endo-β-1,4-glucanases leads to largely enhanced biomass saccharification and bioethanol production by specific modification of cellulose features in transgenic rice

BACKGROUND

Genetic modification of plant cell walls has been implemented to reduce lignocellulosic recalcitrance for biofuel production. Plant glycoside hydrolase family 9 (GH9) comprises endo-β-1,4-glucanase in plants. Few studies have examined the roles of GH9 in cell wall modification. In this study, we independently overexpressed two genes from GH9B subclasses (OsGH9B1 and OsGH9B3) and examined cell wall features and biomass saccharification in transgenic rice plants.

RESULTS

Compared with the wild type (WT, Nipponbare), the OsGH9B1 and OsGH9B3 transgenic rice plants, respectively, contained much higher OsGH9B1 and OsGH9B3 protein levels and both proteins were observed in situ with nonspecific distribution in the plant cells. The transgenic lines exhibited significantly increased cellulase activity in vitro than the WT. The OsGH9B1 and OsGH9B3 transgenic plants showed a slight alteration in three wall polymer compositions (cellulose, hemicelluloses, and lignin), in their stem mechanical strength and biomass yield, but were significantly decreased in the cellulose degree of polymerization (DP) and lignocellulose crystalline index (CrI) by 21-22%. Notably, the crude cellulose substrates of the transgenic lines were more efficiently digested by cellobiohydrolase (CBHI) than those of the WT, indicating the significantly increased amounts of reducing ends of β-1,4-glucans in cellulose microfibrils. Finally, the engineered lines generated high sugar yields after mild alkali pretreatments and subsequent enzymatic hydrolysis, resulting in the high bioethanol yields obtained at 22.5% of dry matter.

CONCLUSIONS

Overproduction of OsGH9B1/B3 enzymes should have specific activity in the postmodification of cellulose microfibrils. The increased reducing ends of β-1,4-glucan chains for reduced cellulose DP and CrI positively affected biomass enzymatic saccharification. Our results demonstrate a potential strategy for genetic modification of cellulose microfibrils in bioenergy crops.

Feasibility of high-concentration cellulosic bioethanol production from undetoxified whole Monterey pine slurry

The economic feasibility of high-concentration cellulosic bioethanol production remains challenging because it requires easily available feedstock and low energy consumption process. Simultaneous saccharification and fermentation (SSF) of sulfite pretreated Momentary pine slurry at 20% (w/w) loadings increased ethanol concentration from 59.3 g/L to 68.5 g/L by washing strategy. Effects of inhibitors in pretreatment liquor were further investigated. Besides HMF, furfural and acetic acid, other inhibitors and/or their synergistic effects proved to be responsible for a lower fermentability. To bypass the inhibition and achieve high-efficient bioethanol concentration, a fermentation temperature of 28 °C was optimized for both cell growth and ethanol production. Under the optimal conditions with prehydrolyzed 25% (w/w) whole undetoxified slurry, a high ethanol concentration (up to 82.1 g/L) were produced with a yield of 205 kg/ton Monterey pine in the SSF. Thus, this high cellulosic bioethanol production from Monterey pine makes it a potential strategy for biofuel production.

Biomass pretreatments capable of enabling lignin valorization in a biorefinery process

Recent techno-economic studies of proposed lignocellulosic biorefineries have concluded that creating value from lignin will assist realization of biomass utilization into valuable fuels, chemicals, and materials due to co-valorization and the new revenues beyond carbohydrates. The pretreatment step within a biorefinery process is essential for recovering carbohydrates, but different techniques and intensities have a variety of effects on lignin. Acidic and alkaline pretreatments have been shown to produce diverse lignins based on delignification chemistry. The valorization potential of pretreated lignin is affected by its chemical structure, which is known to degrade, including inter-lignin condensation under high-severity pretreatment. Co-valorization of lignin and carbohydrates will require dampening of pretreatment intensities to avoid such effects, in spite of tradeoffs in carbohydrate production.

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, 20 min, 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.