Biorefinery concept of simultaneous saccharification and co-fermentation: Challenges and improvements
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.
References (134)
Ethanol from lignocellulosic biomass: technology, economics, and opportunities
Bioresour. Technol.
(1994)- et al.
Agro-industrial lignocellulosic biomass a key to unlock the future bio-energy: a brief review
J. Radiat. Res. Appl. Sci.
(2014) - et al.
Second generation bioethanol production: a critical review
Renew. Sustain. Energy Rev.
(2016) - et al.
Sustainable bio-ethanol production from agro-residues: a review
Renew. Sustain. Energy Rev.
(2015) - et al.
Property analysis and pretreatment of rice straw for energy use in grain drying: a review
Agric. Biol. J. North Am.
(2010) - et al.
Oyster mushroom cultivation with rice and wheat straw
Bioresour. Technol.
(2002) - et al.
Bioconversion of rice straw and certain agro-industrial wastes to amendments for organic farming systems: 1. Composting, quality, stability and maturity indices
Bioresour. Technol.
(2010) - et al.
Characterization of North American lignocellulosic biomass and biochars in terms of their candidacy for alternate renewable fuels
Bioenergy Res.
(2013) - et al.
Lignocellulosic biomass: a review of conversion technologies and fuel products
Curr. Biochem. Eng.
(2015) - et al.
Realistic approach for full-scale bioethanol production from lignocellulose: a review
J. Sci. Ind. Res.
(2008)