Second Generation Bioethanol Production: The State of Art

Second-generation biorefineries: single platform for the conversion of lignocellulosic wastes to environmentally important biofuels

The continuously increasing demands for various fossil fuels to achieve the day-to-day needs of the human population are growing and causing adverse effects on the environment and leading to the depletion of their natural resources. To overcome such drastic problems and minimize the production of greenhouse gases, lignocellulose biomass, which is an abundant and bio-renewable source present on earth with excellent properties and composition, has been used for decades to develop biofuels that can easily take over the place of conventional fuels. Lignocellulose biomass comprises polymeric sugars, i.e., cellulose and hemicellulose, and aromatic polymer, lignin, which are responsible for producing various bio-based products. However, utilizing lignocellulosic wastes for such purposes is needed but their recalcitrant structure makes it difficult to achieve their full usage. For this, several pretreatment approaches are developed to loosen the complexity between sugars and lignin. In some way, few of the conventional pretreatment methods are expensive, non-eco-friendly, and produce undesired by-products, causing a lower yield and reusability of enzymes used in the reaction. Utilizing novel pretreatment strategies that are cost-effective, help in increasing the yield of products, and are environment-friendly is required. Thus, incorporating nanoparticles and nanomaterials in the development of pretreatment and other strategies for the production of bio-based products is currently thriving. This review is designed in such a way that the readers can easily get brief knowledge about the production of important biofuels developed within second-generation biorefineries using lignocellulosic biomass. It also summarizes the importance of nanotechnology in different steps of biofuel development.

The prospect of fruit wastes in bioethanol production: A review

Utilising agricultural byproducts specifically fruit wastes for bioethanol production offers a promising approach to sustainable energy production and waste mitigation. This approach focuses on assessing the biochemical composition of fruit wastes, particularly their sugar content, as a key aspect of bioethanol production. This study evaluates the potential of pineapple, mango, pawpaw and watermelon fruit wastes for bioethanol production, highlighting the substantial organic waste generated during fruit processing stages such as peeling and pulping. Various techniques, including enzymatic hydrolysis, fermentation, and distillation, are reviewed to optimise bioethanol yields while addressing challenges such as seasonal availability, substrate variability and process optimisation. Besides, the environmental benefits of bioethanol derived from fruit wastes, such as reduced environmental pollution, decreased reliance on fossil fuels, and promotion of sustainable agricultural practices, are emphasised. The study deployed a comprehensive literature review using keywords, specific research questions, and a search strategy that included academic databases, library catalogues, and Google Scholar. Search results were systematically screened and selected based on their relevance to the topic.

CRISPRi screen highlights chromatin regulation to be involved in formic acid tolerance in Saccharomyces cerevisiae

Formic acid is one of the main weak acids in lignocellulosic hydrolysates that is known to be inhibitory to yeast growth even at low concentrations. In this study, we employed a CRISPR interference (CRISPRi) strain library comprising >9000 strains encompassing >98% of all essential and respiratory growth-essential genes, to study formic acid tolerance in Saccharomyces cerevisiae. To provide quantitative growth estimates on formic acid tolerance, the strains were screened individually on solid medium supplemented with 140 mM formic acid using the Scan-o-Matic platform. Selected resistant and sensitive strains were characterized in liquid medium supplemented with formic acid and in synthetic hydrolysate medium containing a combination of inhibitors. Strains with gRNAs targeting genes associated with chromatin remodeling were significantly enriched for strains showing formic acid tolerance. In line with earlier findings on acetic acid tolerance, we found genes encoding proteins involved in intracellular vesicle transport enriched among formic acid sensitive strains. The growth of the strains in synthetic hydrolysate medium followed the same trend as when screened in medium supplemented with formic acid. Strains sensitive to formic acid had decreased growth in the synthetic hydrolysate and all strains that had improved growth in the presence of formic acid also grew better in the hydrolysate medium. Systematic analysis of CRISPRi strains allowed identification of genes involved in tolerance mechanisms and provided novel engineering targets for bioengineering strains with increased resistance to inhibitors in lignocellulosic hydrolysates.

Enhanced cellulosic ethanol production via fed-batch simultaneous saccharification and fermentation of sequential dilute acid-alkali pretreated sugarcane bagasse

This study reports high gravity fed-batch simultaneous saccharification and fermentation (FB-SSF) of sequentially pretreated sugarcane bagasse (SCB) for enhanced bioethanol by employing multiple inhibitor tolerant Kluyveromyces marxianusJKH5 C60. FB-SSF with intermittent feeding of SCB (total 20 % solid loading) and enzyme (total dose of 20 FPU/g) at 6 and 12 h resulted in superior bioethanol production at42 °C. Under optimizedlab-scaleFB-SSF, the maximum ethanoltiter, efficiency and productivities were73.4 ± 1.2 g/L,78 % and 3.0 g/L/h, respectively, after 72 h in presence of inhibitors (acetic acid, furfural, and vanillin at 3, 1, and 1 g/L, respectively). Furthermore, pentose rich dilute acid hydrolysate of SCB was subjected to fermentation by Pichia stipitis NCIM 3499, resulting in ethanol titer of 6.8 g/L. Overall ethanol yield during the developed process was 260.1 g/kg native SCB, which proves industrial potential of the developed bioethanol conversion process.

Microwave-Assisted Pyrolysis of Biomass with and without Use of Catalyst in a Fluidised Bed Reactor: A Review

Lignocellulosic biomass and waste, such as plastics, represent an abundant resource today, and they can be converted thermo-chemically into energy in a refinery. Existing research works on catalytic and non-catalytic pyrolysis performed in thermally-heated reactors have been reviewed in this text, along with those performed in microwave-heated ones. Thermally-heated reactors, albeit being the most commonly used, present various drawbacks such as superficial heating, high thermal inertia and slow response times. That is why microwave-assisted pyrolysis (MAP) appears to be a very promising technology, even if the process does present some technical drawbacks as well such as the formation of hot spots. The different types of catalysts used during the process and their impacts have also been examined in the text. More specifically, studies conducted in fluidised bed reactors (FBR) have been detailed and their advantages and drawbacks discussed. Finally, future prospects of MAP have been briefly presented.

An integrated approach to obtain xylo-oligosaccharides from sugarcane straw: From lab to pilot scale

Sugarcane straw (SS) is a widely available agricultural processing feedstock with the potential to produce 2nd generation bioethanol and bioproducts, in addition to the more conventional use for heat and/or electrical power generation. In this study, we investigated the operational parameters to maximize the production of xylo-oligosaccharides (XOS) using mild deacetylation, followed by hydrothermal pretreatment. From the laboratory to the pilot-scale, the optimized two-stage pretreatment promoted 81.5% and 70.5% hemicellulose solubilization and led to XOS yields up to 9.8% and 9.1% (w/w of initial straw), respectively. Moreover, different fungal xylanases were also tested to hydrolyze XOS into xylobiose (X2) and xylotriose (X3). GH10 from Aspergillus nidulans performed better than GH11 xylanases and the ratio of the desired products (X2 + X3) increased to 72% due to minimal monomeric sugar formation. Furthermore, a cellulose-rich fraction was obtained, which can be used in other high value-added applications, such as for the production of cello-oligomers.

A CRISPR activation and interference toolkit for industrial Saccharomyces cerevisiae strain KE6-12

Recent advances in CRISPR/Cas9 based genome editing have considerably advanced genetic engineering of industrial yeast strains. In this study, we report the construction and characterization of a toolkit for CRISPR activation and interference (CRISPRa/i) for a polyploid industrial yeast strain. In the CRISPRa/i plasmids that are available in high and low copy variants, dCas9 is expressed alone, or as a fusion with an activation or repression domain; VP64, VPR or Mxi1. The sgRNA is introduced to the CRISPRa/i plasmids from a double stranded oligonucleotide by in vivo homology-directed repair, allowing rapid transcriptional modulation of new target genes without cloning. The CRISPRa/i toolkit was characterized by alteration of expression of fluorescent protein-encoding genes under two different promoters allowing expression alterations up to ~ 2.5-fold. Furthermore, we demonstrated the usability of the CRISPRa/i toolkit by improving the tolerance towards wheat straw hydrolysate of our industrial production strain. We anticipate that our CRISPRa/i toolkit can be widely used to assess novel targets for strain improvement and thus accelerate the design-build-test cycle for developing various industrial production strains.