Evaluation of pretreatment methods for enzymatic saccharification of wheat straw for bioethanol production

From wheat straw to bioethanol: integrative analysis of a separate hydrolysis and co-fermentation process with implemented enzyme production

BACKGROUND

Lignocellulosic ethanol has a high potential as renewable energy source. In recent years, much research effort has been spent to optimize parameters involved in the production process. Despite that, there is still a lack of comprehensive studies on process integration. Single parameters and process configurations are, however, heavily interrelated and can affect the overall process efficiency in a multitude of ways. Here, we present an integrative approach for bioethanol production from wheat straw at a representative laboratory scale using a separate hydrolysis and co-fermentation (SHCF) process. The process does not rely on commercial (hemi-) cellulases but includes enzyme production through Hypocrea jecorina (formerly Trichoderma reesei) on the pre-treated feedstock as key unit operation. Hydrolysis reactions are run with high solid loadings of 15% dry mass pre-treated wheat straw (DM WS), and hydrolyzates are utilized without detoxification for mixed glucose-xylose fermentation with the genetically and evolutionary engineered Saccharomyces cerevisiae strain IBB10B05.

RESULTS

Process configurations of unit operations in the benchtop SHCF were varied and evaluated with respect to the overall process ethanol yield (Y Ethanol-Process). The highest Y Ethanol-Process of 71.2 g ethanol per kg raw material was reached when fungal fermentations were run as batch, and the hydrolysis reaction was done with an enzyme loading of 30 filter paper units (FPU)/gDM WS. 1.7 ± 0.1 FPU/mL were produced, glucose and xylose were released with a conversion efficiency of 67% and 95%, respectively, and strain IBB10B05 showed an ethanol yield of 0.4 g/gGlc + Xyl in 15% hydrolyzate fermentations. Based on the detailed process analysis, it was further possible to identify the enzyme yield, the glucose conversion efficiency, and the mass losses between the unit operations as key process parameters, exhibiting a major influence on Y Ethanol-Process.

CONCLUSIONS

Y Ethanol-Process is a measure for the efficiency of the lignocellulose-to-bioethanol process. Based on mass balance analysis, the correlations between single process parameters and Y Ethanol-Process were elucidated. The optimized laboratory scale SHCF process showed efficiencies similar to pilot scale plants. The herein presented process analysis can serve as effective and simple tool to identify key process parameters, bottlenecks, and future optimization targets.

Augmented digestion of lignocellulose by steam explosion, acid and alkaline pretreatment methods: a review

Lignocellulosic materials can be explored as one of the sustainable substrates for bioethanol production through microbial intervention as they are abundant, cheap and renewable. But at the same time, their recalcitrant structure makes the conversion process more cumbersome owing to their chemical composition which adversely affects the efficiency of bioethanol production. Therefore, the technical approaches to overcome recalcitrance of biomass feedstock has been developed to remove the barriers with the help of pretreatment methods which make cellulose more accessible to the hydrolytic enzymes, secreted by the microorganisms, for its conversion to glucose. Pretreatment of lignocellulosic biomass in cost effective manner is a major challenge to bioethanol technology research and development. Hence, in this review, we have discussed various aspects of three commonly used pretreatment methods, viz., steam explosion, acid and alkaline, applied on various lignocellulosic biomasses to augment their digestibility alongwith the challenges associated with their processing.

Combined pretreatment using alkaline hydrothermal and ball milling to enhance enzymatic hydrolysis of oil palm mesocarp fiber

Hydrothermal pretreatment of oil palm mesocarp fiber was conducted in tube reactor at treatment severity ranges of log Ro = 3.66-4.83 and partial removal of hemicellulose with migration of lignin was obtained. Concerning maximal recovery of glucose and xylose, 1.5% NaOH was impregnated in the system and subsequent ball milling treatment was employed to improve the conversion yield. The effects of combined hydrothermal and ball milling pretreatments were evaluated by chemical composition changes by using FT-IR, WAXD and morphological alterations by SEM. The successful of pretreatments were assessed by the degree of enzymatic digestibility of treated samples. The highest xylose and glucose yields obtained were 63.2% and 97.3% respectively at cellulase loadings of 10 FPU/g-substrate which is the highest conversion from OPMF ever reported.

Modulation of xylanase production from alkaliphilic Bacillus pumilus VLK-1 through process optimization and temperature shift operation

This study was aimed at enhancing the production of xylanase from an alkaliphilic Bacillus pumilus VLK-1 in submerged fermentation using wheat bran, a cheap and abundantly available agro-residue, through process optimization and to monitor the effect of temperature shift operation on it. The potential of xylanase in saccharification of wheat straw was also investigated. The results showed that optimization of the fermentation process by one variable approach increased the enzyme yield from 402 to 4,986 IU/ml. Subsequently, optimization of nitrogen and carbon sources through response surface methodology led to high level xylanase production (7,295 IU/ml) which was 1.46-fold greater than one variable approach after 56 h of cultivation at 30 °C. Temperature shift operation during fermentation resulted in maximum xylanase production in lesser duration (48 h instead of 56 h). Enzymatic hydrolysis of the alkali pre-treated wheat straw with 500 IU xylanase alone released 173 ± 8 mg sugars/g whereas in combination with cellulase and β-glucosidase released 553 ± 12 mg sugars/g dry substrate in 6 h, indicating its potential in saccharification of the lignocellulosic substrate. Temperature shift operation is likely to be attractive for large scale industrial fermentation due to significant reduction in the operating cost. To our knowledge, this is the first report which showed the effect of temperature shift operation on xylanase production from bacteria. The xylanase production from Bacillus sp. in the present study is close to the highest titre reported in the literature. An enhanced xylanase production using wheat bran, a cheap and abundantly available agro-residue, will apparently reduce the enzyme cost, which would be beneficial for industry.

Bamboo: a new source of carbohydrate for biorefinery

Bamboo is perennial woody grass, which distributed widely in the world and belonged to the Gramineae family and Bambuseae subfamily. It may be consider as a candidate lignocellulosic substrate for bio-ethanol production for its environmental benefits and higher annual biomass yield. The conversion of bamboo into bio-ethanol, bio-methane, natural food, flavonoids, and functional xylo-oligosaccharides production were reviewed in this paper. Future prospects for research include pretreatment, enzymatic hydrolysis and fermentation will also be performed to improve the whole process of ethanol production more economical. And revealing the molecular regulation mechanism of the fast growth of bamboo will provide chance for improving bamboo or other energy plants biomass yield through genetic engineering.