Biorefinery concept of simultaneous saccharification and co-fermentation: Challenges and improvements

doi:10.1016/j.cep.2021.108634

Chemical Engineering and Processing - Process Intensification

Volume 169, December 2021, 108634

https://doi.org/10.1016/j.cep.2021.108634 Get rights and content

Highlights

•
Process intensification of SSCF for commercial use.
•
Engineering of microbes for higher saccharification rate.
•
Impeller designing and pathway engineering for co-fermentation improvement.
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Role of conditional factors and challenges for optimizing the process.

Abstract

Bioethanol is an alternative to motor fuel, generated through the fermentation of sugars released from the hydrolyzed cellulosic material. For efficient ethanol production, total utilization of sugars needs an improved fermentation approach i.e. Simultaneous Saccharification and Co-fermentation (SSCF). SSCF is the current, advanced, close to commercialization approach that is being continuously improved for high ethanol titer, total sugar (hexose + pentose) utilization, high mass transfer, reducing the feedback inhibition, and one-pot conversion strategies. The major improvement strategies such as enhancing the saccharification rate, engineering microbes for co-fermentation, enhancing mass transfer through impeller designing, and the role of conditional factors on the SSCF process are reviewed in this study.

Graphical abstract

Introduction

Since decades, non-renewable fossil resources have been used for energy and power. The limited extent, hazardous to the environment, and high carbon credits of non-renewable energy resources made them a potential limitation in the recent global scenario. Bio-ethanol production from energy crops, forestry, and agricultural residues, possibly can be used in expanding energy security, decreasing trade deficits, and also in reducing air pollution [1]. Agricultural waste is composed of lignocellulosic material, which is a kind of renewable, abundant, and inexpensive bio-energy substrate [2]. Feedstock like sugarcane used for first-generation bioethanol production is unable to fulfill ethanol demand because it majorly contributes to the edible part used in processing food items such as commercial sugar. The first-generation ethanol produced from edible feedstock generates an impractical gap that can be fulfilled by employing non-edible sources such as crop residues [3]. A promising approach to produce bioethanol from agricultural residues involves pretreatment, enzymatic hydrolysis, sugar fermentation, and purification. Initial pre-treatment strategies improve sugar release during hydrolysis and further improve the efficiency of the whole process [4]. With the growing advances in the field of bioenergy, new technologies are implicating their role in process enhancement. Simultaneous saccharification and co-fermentation (SSCF) is a consolidated next-generation process used for advancing bioethanol production by hydrolyzing cellulose and fermenting sugars at the same time.

Section snippets

Diversification of lignocellulosic biomass and its constituents

The non-edible material leftover after crop harvest is termed as agricultural residue. Crops generally include wheat, rice, sugarcane, maize, and cotton. Agricultural residue includes straw, husk, leaves, stems, and stalks. Every year, millions of tons of agricultural residues are generated. Traditionally in rural areas, the not palatable and non-digestible residue is used as firewood in the kitchen. In villages, agricultural waste such as wheat straw is used as animal feed, while rice straw

Saccharification and improvements

Cellulose is made up of β-D-glucopyranose that are linked by β-1,4 glycosidic bonds and each glucan chain of the cellulose polymer can extend up to 25,000 glucose residues [23]. Cellulases are the enzymes specifically used for the generation of glucose, cellobiose, and cello-dextrin from cellulose [24]. Biomass conversion from different cellulose mixtures relies on multiple factors like specificity, stability, the extent of product inhibition, and the coordination between the various enzymes

Fermentation challenges and improvements

Fermentation is directly proportional to the saccharification i.e., more the hydrolysis is the ethanol yield. During pretreatment, most of the hemicellulose (a major source of xylose sugar) is broken down into xylose. These are weakly bound intermolecular pentose sugars which easily released out at the time of pretreatment. Then the cellulosic part (source of hexose sugar) needs to be hydrolyzed by enzymes. During hydrolysis, cellulose is converted into free glucose which is further utilized by

Conclusion

SSCF is an advanced method used for one-pot bioethanol production. The simultaneous saccharification and fermentation reduces the load of substrate feedback inhibition on enzymes and gives higher yield by the combined utilization of C6 and C5 sugars. To achieve high solid loading, the impeller designing plays a significant role. A xylose-fermenting engineered microbe is preferred to cut the cost of making separate inoculums which reduces the competitive utilization of sugar for growth and

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgment

The authors are grateful to Bennett University for providing support to complete this work.

Funding

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

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Citation Excerpt :
Simultaneous Saccharification and co-fermentation is a favourable approach for bioethanol production under biorefinery strategy, in which enzymatic hydrolysis of the pretreated biomass occurs concurrently with co-fermentation of hexose and pentose (mainly glucose and xylose) by genetically engineered Saccharomyces cerevisiae, as it allows decreased investment cost, saving energy, and accomplishing higher ethanol productivity by decreasing end-products inhibition (Qin et al., 2018). The biggest challenge in SSCF is temperature because the ideal temperature for enzyme activity during the saccharification process is 50 °C whereas microorganisms (yeast or bacteria) works at 30–37 °C (Sharma et al., 2021b). Therefore, temperature in this process becomes the limiting factor.
There is an immediate global requirement for an ingenious strategy for food waste conversion to biofuels in order to replace fossil fuels with renewable resources. Food waste conversion to bioethanol could lead to a sustainable process having the dual advantage of resolving the issue of food waste disposal as well as meeting the energy requirements of the increasing population. Food waste is increasing at the rate of 1.3 billion tonnes per year, considered to be one-third of global food production. According to LCA studies discarding these wastes is detritus to the environment, therefore; it is beneficial to convert the food waste into bioethanol. The CO₂ emission in this process offers zero impact on the environment as it is biogenic. Among several pretreatment strategies, hydrothermal pretreatment could be a better approach for pretreating food waste because it solubilizes organic solids, resulting in an increased recovery of fermentable sugars to produce bioenergy.

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Biorefinery concept of simultaneous saccharification and co-fermentation: Challenges and improvements

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Diversification of lignocellulosic biomass and its constituents

Saccharification and improvements

Fermentation challenges and improvements

Conclusion

Declaration of Competing Interest

Acknowledgment

Funding

Bioresour. Technol.

J. Radiat. Res. Appl. Sci.

Renew. Sustain. Energy Rev.

Renew. Sustain. Energy Rev.

Bioresour. Technol.

Bioresour. Technol.

J. Mater. Res. Technol.

Energy Proc.

Direct Thermochem. Liq. Energy Appl.

Ind. Crops Prod.

Biomass Bioenergy

Renew. Sustain. Energy Rev.

Renew. Sustain. Energy Rev.

Biomass Bioenergy

Bioresour. Technol.

J. Biotechnol.

J. Radiat. Res. Appl. Sci.

J. Mol. Catal. B Enzym.

Process Biochem.

Enzyme Microb. Technol.

Biochem. Biophys. Rep.

Chem. Eng. J.

Enzyme Microb. Technol.

Renew. Energy

BMC Biotechnol.

Appl. Energy

Bioresour. Bioprocess.

Curr. Opin. Chem. Biol.

Bioresour. Technol.

Bioresour. Technol.

Metab. Eng.

Biotechnol. Adv.

Property analysis and pretreatment of rice straw for energy use in grain drying: a review

Agric. Biol. J. North Am.

Characterization of North American lignocellulosic biomass and biochars in terms of their candidacy for alternate renewable fuels

Bioenergy Res.

Lignocellulosic biomass: a review of conversion technologies and fuel products

Curr. Biochem. Eng.

Realistic approach for full-scale bioethanol production from lignocellulose: a review

J. Sci. Ind. Res.

Lignin biosynthesis and structure

Plant Physiol.

AE493/AE493: how ethanol is made from cellulosic biomass

Influence of steam explosion pretreatment on the composition and structure of wheat straw : BioResources

Bioresources

Composition of sugar cane, energy cane, and sweet sorghum suitable for ethanol production at Louisiana sugar mills

J. Ind. Microbiol. Biotechnol.

On the determination of crystallinity and cellulose content in plant fibres

Cellulose

Bioconversion of cellulosic materials by the action of microbial cellulases

Int. Res. J. Eng. Technol.

Continuous SSCF of AFEX™ pretreated corn stover for enhanced ethanol productivity using commercial enzymes and Saccharomyces cerevisiae 424A (LNH-ST)

Biotechnol. Bioeng.

Development of a low-cost cellulase production process using Trichoderma reesei for Brazilian biorefineries

Biotechnol. Biofuels

Differential regulation of the cellulase transcription factors XYR1, ACE2, and ACE1 in trichoderma reesei strains producing high and low levels of cellulase

Eukaryot. Cell

Screening of candidate regulators for cellulase and hemicellulase production in Trichoderma reesei and identification of a factor essential for cellulase production

Biotechnol. Biofuels

Transcriptional regulation of xyr1, encoding the main regulator of the xylanolytic and cellulolytic enzyme system in Hypocrea jecorina

Appl. Environ. Microbiol.

Enhancing cellulase production in Trichoderma reesei RUT C30 through combined manipulation of activating and repressing genes

J. Ind. Microbiol. Biotechnol.