Bioethanol is one of the most promising and eco-friendly alternatives to fossil fuels, which is produced from renewable sources. Although almost all the current fuel ethanol is generated from edible sources (sugars and starch), lignocellulosic biomass (LCB) has drawn much attention in recent times. However, the conversion efficiency as well as ethanol yield of the biomass differs greatly with respect to the source and nature of LCB, primarily due to the variation in lignocellulosic content. Two major polysaccharides in LCB, namely, cellulose and hemicellulose firmly link to lignin and form a complex lignocellulosic network, which is highly robust and recalcitrant to depolymerization. For this reason, generation of ethanol from LCB requires a complicated conversion process that has made it commercially non-competitive. As attempts to exploit LCBs into commercial ethanol production, recent research efforts have been devoted to the techno-economic improvements of the overall conversion process, in addition to screen out promising feedstocks. This review paper presents an overview on the diversity of biomass, technological approaches and microbial contribution to the conversion of LCB into ethanol.

Fuel ethanol production from lignocellulosic biomass: an overview on feedstocks and technological approaches.

Augmented digestion of lignocellulose by steam explosion, acid and alkaline pretreatment methods: a review.

The current issues of the depletion of fossil fuels reserve and environmental changes have increased the concern for the hunt of sustainable renewable energy for the future generations. Biofuels emerged as a promising viable alternative to replace the existing fossil fuels. Among these, bioethanol outstands due to its ability to substitute gasoline. However, the major challenge in bioethanol industry is the need to discover a suitable feedstock together with an environmentally friendly approach and an economically feasible process of production. The first generation and second generation bioethanol appeared unsustainable due to its impact on food security as well as inflated production process. These problems and concerns have directed the search for the third generation bioethanol (TGB) feedstock from marine algae. The integration of algae (microalgae and macroalgae) as a sustainable feedstock for bioethanol has gained worldwide attention in terms of food security and environmental impact. The research on algal utilization in bioethanol has increased in recent years and is expected to become the major drives in bioethanol industry. Therefore, the potential and prospects of the third generation bioethanol feedstock are being highlighted in this review. An insight into the current hydrolysis and fermentation technologies on algal conversion together with the economics and viability of the process are also accounted. This review can be crucial in providing ideas for the future studies that can be implemented in the commercialization of bioethanol from the third generation feedstock.

A review on third generation bioethanol feedstock

Lignocellulosic biomass: Acid and alkaline pretreatments and their effects on biomass recalcitrance - Conventional processing and recent advances.

Proteases have a broad range of applications in industrial processes and products and are representative of most worldwide enzyme sales. The genus Bacillus is probably the most important bacterial source of proteases and is capable of producing high yields of neutral and alkaline proteolytic enzymes with remarkable properties, such as high stability towards extreme temperatures, pH, organic solvents, detergents and oxidizing compounds. Therefore, several strategies have been developed for the cost-effective production of Bacillus proteases, including optimization of the fermentation parameters. Moreover, there are many studies on the use of low-cost substrates for submerged and solid state fermentation. Other alternatives include genetic tools such as protein engineering in order to obtain more active and stable proteases and strain engineering to better secrete recombinant proteases from Bacillus through homologous and heterologous protein expression. There has been extensive research on proteases because of the broad number of applications for these enzymes, such as in detergent formulations for the removal of blood stains from fabrics, production of bioactive peptides, food processing, enantioselective reactions, and dehairing of skins. Moreover, many commercial proteases have been characterized and purified from different Bacillus species. Therefore, this review highlights the production, purification, characterization, and application of proteases from a number of Bacillus species.

An overview of Bacillus proteases: from production to application.

Bioconversion of lignocellulosic biomass to xylitol: An overview.

Evaluation of pretreatment methods for enzymatic saccharification of wheat straw for bioethanol production

Bioconversion of alkali delignified cotton stalk using two-stage dilute acid hydrolysis and fermentation of detoxified hydrolysate into ethanol

The present study demonstrated the simultaneous production and optimization of pectinolytic enzymes (pectate lyase and polygalacturonase) under SSF from Bacillus tequilensis SV11-UV37 using wheat bran as a substrate, which is commercially viable and cost-effective. Optimization by one variable-at-a-time-approach showed a maximum yield of pectate lyase (1371.25 U/gds) and polygalacturonase (85.45 U/gds) with wheat bran using 80 % (v/w) moisture, 0.7 mm particle size, 20 % (v/w) inoculum, 1 % (w/w) pectin at 37 °C, pH 6 and 72 h of incubation. In addition, optimization using central composite design achieved 1.6-fold improvement in both pectate lyase (1828.13 U/gds) and polygalacturonase (105.55 U/gds) yield at optimum levels of pectin (3 %, w/w), inoculum size (20 %, v/w) and moisture level (80 %, v/w). Further, Retting studies concluded that the enzyme mixture was efficient in separating the whole fiber from kenaf and part (>75 %) from sunn hemp. In degumming of sunn hemp fibers, amount of galacturonic acid released and percentage weight loss was higher in successive alkali and enzymatic treatment than their independent treatments. The scanning electron microscopic analysis also confirmed that alkali followed by enzymatic treatment effectively removed non-cellulosic gummy material from the fiber; hence, this enzyme mixture may find feasible applications in the fiber and textile industry.

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Retting and degumming of natural fibers by pectinolytic enzymes produced from Bacillus tequilensis SV11-UV37 using solid state fermentation

The main aim of the present study was to mutate yeast strains, Pichia stipitis NCIM 3498 and Candida shehatae NCIM 3501 and assess the mutant’s ability to utilize, ferment wheat straw hemicellulose with enhanced ethanol yield. The organisms were subjected to random mutagenesis using physical (ultraviolet radiation) and chemical (ethidium bromide) mutagens. The mutant and wild strains were used to ferment the hemicellulosic hydrolysates of wheat straw obtained by 2 % dilute sulphuric acid and enzymatic hydrolysis by crude xylanase separately. Among all the mutant strains, PSUV9 and CSEB7 showed enhanced ethanol production (12.15 ± 0.57, 9.55 ± 0.47 g/L and yield 0.450 ± 0.009, 0.440 ± 0.001 g/g) as compared to the wild strains (8.28 ± 0.54, 7.92 ± 0.89 g/L and yield 0.380 ± 0.006 and 0.370 ± 0.002 g/g) in both the hydrolysates. The mutant strains were also checked for their consistency in ethanol production and found stable for 19 cycles in hemicellulosic hydrolysates of wheat straw. A novel element in the present study was introduction of chemical mutagenesis in wild type as well as UV induced mutants. This combination of treatments i.e., UV followed by chemical mutagenesis was practically successful.

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Sravanthi Koti

Papers

Bioconversion of lignocellulosic biomass to xylitol: An overview.

Evaluation of pretreatment methods for enzymatic saccharification of wheat straw for bioethanol production

Bioconversion of alkali delignified cotton stalk using two-stage dilute acid hydrolysis and fermentation of detoxified hydrolysate into ethanol

Retting and degumming of natural fibers by pectinolytic enzymes produced from Bacillus tequilensis SV11-UV37 using solid state fermentation

Enhanced bioethanol production from wheat straw hemicellulose by mutant strains of pentose fermenting organisms Pichia stipitis and Candida shehatae.