Anita Singh

Second-generation bioethanol, derived from lignocellulosic biomass, offers a promising renewable energy option for expanding global population. However, there are various challenges and drawbacks in this process which limit the potential of second-generation bioethanol in energy sector. These challenges include the generation of inhibitory compounds during the biomass pretreatment, and the restricted conversion of pentose sugars. Detoxification, neutralization, desalination, and a number of other procedures are among the many technological initiatives and interventions that have been made in an attempt to address and resolve these challenges. Nevertheless, these interventions come with a high net production cost rendering second-generation bioethanol economically unfeasible. The development of new strains offers a workable answer to a good portion of these problems that are associated with second-generation bioethanol. This review delves into the advancements in strain development through genetic engineering, mutagenesis, and adaptive evolution, both individually and in combination, and its potential to provide a breakthrough in the economic feasibility of second-generation bioethanol. Specifically, we focus on how these processes can be used to create strains that are more resistant to the effects of environmental stressors and have higher substrate utilization. Briefly, we also discuss about the future prospect of the strain development for second-generation bioethanol production. Graphical Abstract

Bioethanol is an appropriate alternate energy option due to its renewable, nontoxic, environmentally friendly, and carbon-neutral nature. Depending upon various feedstocks, bioethanol is classified in different various generations. First-generation ethanol created a food vs fuel problem, which was overcome by second-generation, third-generation and fourth-generation ethanol. The considerable availability of lignocellulosic biomass makes it a suitable feedstock, however, its recalcitrant nature is the main hurdle in converting it to bioethanol. The present study gives a comprehensive assessment of global biofuel policies and the current status of ethanol production. Feedstocks for first-generation (sugar and starch-based), second-generation (lignocellulosic biomass and energy crops), third-generation (algal-based) and fourth-generation (genetically modified algal biomass or crops) are discussed in detail. The study also assessed the process for ethanol production from various feedstocks, besides giving a holestic background knowledge on the bioconversion process, factors affecting bioethanol production, and various microorganisms involved in the fermentation process. Biotechnological tools also play a pivotal role in enhancing process efficiency and product yield. In adddition, most significant development in the field of genetic engineering and adaptive evolution are also highlighted.

Lignocellulosic biomass generated from different sectors (agriculture, forestry, industrial) act as biorefinery precursor for production of second-generation (2G) bioethanol and other biochemicals. The integration of various conversion techniques on a single platform under biorefinery approach for production of biofuel and industrially important chemicals from LCB is gaining interest worldwide. The waste generated on utilization of bio-resources is almost negligible or zero in a biorefinery along with reduced greenhouse gas emissions, which supports the circular bioeconomy concept. The economic viability of a lignocellulosic biorefinery depends upon the efficient utilization of three major components of LCB—cellulose, hemicellulose and lignin. The heterogeneous structure and recalcitrant nature of LCB is main obstacle in its valorization into bioethanol and other value-added products. The success of bioconversion process depends upon methods used during pre-treatment, hydrolysis and fermentation processes. The cost involved in each step of the bioconversion process affects the viability of cellulosic ethanol. The lignocellulose biorefinery has ample scope, but much-focused research is required to fully utilize major parts of lignocellulosic biomass with zero wastage. The present review entails lignocellulosic biomass valorization for ethanol production, along with different steps involved in its production. Various value-added products produced from LCB components were also discussed. Recent technological advances and significant challenges in bioethanol production are also highlighted in addition to future perspectives. Graphical abstract