Abstract
To mitigate the current global energy and the environmental crisis, biofuels such as bioethanol have progressively gained attention from both scientific and industrial perspectives. However, at present, commercialized bioethanol is mainly derived from edible crops, thus raising serious concerns given its competition with feed production. For this reason, lignocellulosic biomasses (LCBs) have been recognized as important alternatives for bioethanol production. Because LCBs supply is sustainable, abundant, widespread, and cheap, LCBs-derived bioethanol currently represents one of the most viable solutions to meet the global demand for liquid fuel. However, the cost-effective conversion of LCBs into ethanol remains a challenge and its implementation has been hampered by several bottlenecks that must still be tackled. Among other factors related to the challenging and variable nature of LCBs, we highlight: (i) energy-demanding pretreatments, (ii) expensive hydrolytic enzyme blends, and (iii) the need for microorganisms that can ferment mixed sugars. In this regard, thermophiles represent valuable tools to overcome some of these limitations. Thus, the aim of this review is to provide an overview of the state-of-the-art technologies involved, such as the use of thermophilic enzymes and microorganisms in industrial-relevant conditions, and to propose possible means to implement thermophiles into second-generation ethanol biorefineries that are already in operation.
Highlights
One of the possible countermeasures to decrease the atmospheric levels of carbon dioxide is to replace fossil fuels with carbon-neutral alternatives, such as bioethanol
A possible solution to the food vs. fuel debate is the use of non-edible lignocellulosic biomasses (LCBs) as a feedstock to produce second-generation (2G) bioethanol
LCBs are abundant, widespread, and cheap, they are significantly more resistant to the physical, chemical, and enzymatic treatments used to break down them into simple sugars that can be fermented to bioethanol
Summary
Carbon dioxide is one of the major greenhouse gases (GHGs) responsible for global warming; its atmospheric levels have significantly and uncontrollably increased since the Industrial Revolution to roughly 420 ppm, as of May 2021 [1]. The prospect of utilizing environmentally friendly feedstocks In this regard, lignocellulosic biomasses (LCBs) are non-food feedstocks to produce so-called second-generation (2G) bioethanol is of signifithecant most abundant and cheap type of available, representing a interest. Date, LCBs-derived bioethanol is driving the need for capable of utilizingTo renewable and environmentally represents one of the In most solutionsbiomasses to meet the global liquid fuel, friendly feedstocks. Hemicellulose(s), the second most abundant components of LCBs (20 to 35% of dry biomass weight), are a family of branched and heterogeneous polymers, with a chemical composition that varies among different plant tissues and species It consists of C5 sugars (i.e., xylose and arabinose) and/or C6 sugars (i.e., glucose, mannose, and galactose). Further research is needed to improve the yields of all biofuel generations, considering the advantages and disadvantages of each generation, the most likely future of biofuel production may consist of the integration of some or all four generations [33]
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