Non-conventional yeast strains: Unexploited resources for effective commercialization of second generation bioethanol

Agricultural Waste Management by Production of Second-Generation Bioethanol from Sugarcane Bagasse Using Indigenous Yeast Strain

In the wake of rapid industrialization and burgeoning transportation networks, the escalating demand for fossil fuels has accelerated the depletion of finite energy reservoirs, necessitating urgent exploration of sustainable alternatives. To address this, current research is focusing on renewable fuels like second-generation bioethanol from agricultural waste such as sugarcane bagasse. This approach not only circumvents the contentious issue of food-fuel conflicts associated with biofuels but also tackles agricultural waste management. In the present study indigenous yeast strain, Clavispora lusitaniae QG1 (MN592676), was isolated from rotten grapes to ferment xylose sugars present in the hemicellulose content of sugarcane bagasse. To liberate the xylose sugars, dilute acid pretreatment was performed. The highest reducing sugars yield was 1.2% obtained at a temperature of 121 °C for 15 min, a solid-to-liquid ratio of 1:25 (% w/v), and an acid concentration of 1% dilute acid H2SO4 that was significantly higher (P < 0.001) yield obtained under similar conditions at 100 °C for 1 h. The isolated strain was statistically optimized for fermentation process by Plackett-Burman design to achieve the highest ethanol yield. Liberated xylose sugars were completely utilized by Clavispora lusitaniae QG1 (MN592676) and gave 100% ethanol yield. This study optimizes both fermentation process and pretreatment of sugarcane bagasse to maximize bioethanol yield and demonstrates the ability of isolated strain to effectively utilize xylose as a carbon source. The desirable characteristics depicted by strain Clavispora lusitaniae shows its promising utilization in management of industrial waste like sugarcane bagasse by its conversion into renewable biofuels like bioethanol.

Bioactive polysaccharides from Aegle marmelos fruit: Recent trends on extraction, bio-techno functionality, and food applications

Polysaccharides from non-conventional sources, such as fruits, have gained significant attention recently. Aegle marmelos (Bael), a non-conventional fruit, is an excellent source of biologically active components with potential indigenous therapeutic and food applications. Apart from polyphenolic components, this is an excellent source of mucilaginous polysaccharides. Polysaccharides are one the major components of bael fruit, having a high amount of galactose and glucuronic acid, which contributes to its potential therapeutic properties. Therefore, this review emphasizes the conventional and emerging techniques of polysaccharide extraction from bael fruit. Insight into the attributes of polysaccharide components, their techno-functional properties, characterization of bael fruit polysaccharide, emulsifying properties, binding properties, reduction of hazardous dyes, application of polysaccharides in film formation, application of polysaccharide as a nanocomposite, and biological activities of bael fruit polysaccharides are discussed. This review also systematically overviews the relationship between extraction techniques, structural characteristics, and biological activities. Additionally, recommendations, future perspectives, and new valuable insight towards better utilization of bael fruit polysaccharide have been given importance, which can be promoted in the long term.

Cellulose de-polymerization is selective for bioethanol refinery and multi-functional biochar assembly using brittle stalk of corn mutant

As lignocellulose recalcitrance principally restricts for a cost-effective conversion into biofuels and bioproducts, this study re-selected the brittle stalk of corn mutant by MuDR-transposon insertion, and detected much reduced cellulose polymerization and crystallinity. Using recyclable CaO chemical for biomass pretreatment, we determined a consistently enhanced enzymatic saccharification of pretreated corn brittle stalk for higher-yield bioethanol conversion. Furthermore, the enzyme-undigestible lignocellulose was treated with two-step thermal-chemical processes via FeCl2 catalysis and KOH activation to generate the biochar with significantly raised adsorption capacities with two industry dyes (methylene blue and Congo red). However, the desirable biochar was attained from one-step KOH treatment with the entire brittle stalk, which was characterized as the highly-porous nanocarbon that is of the largest specific surface area at 1697.34 m2/g and 2-fold higher dyes adsorption. Notably, this nanocarbon enabled to eliminate the most toxic compounds released from CaO pretreatment and enzymatic hydrolysis, and also showed much improved electrochemical performance with specific capacitance at 205 F/g. Hence, this work has raised a mechanism model to interpret how the recalcitrance-reduced lignocellulose is convertible for high-yield bioethanol and multiple-function biochar with high performance.

Oxygenation influences xylose fermentation and gene expression in the yeast genera Spathaspora and Scheffersomyces

BACKGROUND

Cost-effective production of biofuels from lignocellulose requires the fermentation of D-xylose. Many yeast species within and closely related to the genera Spathaspora and Scheffersomyces (both of the order Serinales) natively assimilate and ferment xylose. Other species consume xylose inefficiently, leading to extracellular accumulation of xylitol. Xylitol excretion is thought to be due to the different cofactor requirements of the first two steps of xylose metabolism. Xylose reductase (XR) generally uses NADPH to reduce xylose to xylitol, while xylitol dehydrogenase (XDH) generally uses NAD+ to oxidize xylitol to xylulose, creating an imbalanced redox pathway. This imbalance is thought to be particularly consequential in hypoxic or anoxic environments.

RESULTS

We screened the growth of xylose-fermenting yeast species in high and moderate aeration and identified both ethanol producers and xylitol producers. Selected species were further characterized for their XR and XDH cofactor preferences by enzyme assays and gene expression patterns by RNA-Seq. Our data revealed that xylose metabolism is more redox balanced in some species, but it is strongly affected by oxygen levels. Under high aeration, most species switched from ethanol production to xylitol accumulation, despite the availability of ample oxygen to accept electrons from NADH. This switch was followed by decreases in enzyme activity and the expression of genes related to xylose metabolism, suggesting that bottlenecks in xylose fermentation are not always due to cofactor preferences. Finally, we expressed XYL genes from multiple Scheffersomyces species in a strain of Saccharomyces cerevisiae. Recombinant S. cerevisiae expressing XYL1 from Scheffersomyces xylosifermentans, which encodes an XR without a cofactor preference, showed improved anaerobic growth on xylose as the primary carbon source compared to S. cerevisiae strain expressing XYL genes from Scheffersomyces stipitis.

CONCLUSION

Collectively, our data do not support the hypothesis that xylitol accumulation occurs primarily due to differences in cofactor preferences between xylose reductase and xylitol dehydrogenase; instead, gene expression plays a major role in response to oxygen levels. We have also identified the yeast Sc. xylosifermentans as a potential source for genes that can be engineered into S. cerevisiae to improve xylose fermentation and biofuel production.

General mechanisms of weak acid-tolerance and current strategies for the development of tolerant yeasts

Yeast cells are often subjected to various types of weak acid stress in the process of industrial production, food processing, and preservation, resulting in growth inhibition and reduced fermentation performance. Under acidic conditions, weak acids enter the near-neutral yeast cytoplasm and dissociate into protons and anions, leading to cytoplasmic acidification and cell damage. Although some yeast strains have developed the ability to survive weak acids, the complexity and diversity of stresses during industrial production still require the application of appropriate strategies for phenotypes improvement. In this review, we summarized current knowledge concerning weak acid stress response and resistance, which may suggest important targets for further construction of more robust strains. We also highlight current feasible strategies for improving the weak acid resistance of yeasts, such as adaptive laboratory evolution, transcription factors engineering, and cell membrane/wall engineering. Moreover, the challenges and perspectives associated with improving the competitiveness of industrial strains are also discussed. This review provides effective strategies for improving the industrial phenotypes of yeast from multiple dimensions in future studies.

Microbial co-cultures for biochemicals production from lignocellulosic biomass: A review

Global reliance on fossil oil should shift to cleaner alternatives to get a decarbonized society. One option to achieve this ambitious goal is the use of biochemicals produced from lignocellulosic biomass (LCB). The inherent low biodegradability of LCB and the inhibitory compounds that might be released during pretreatment are two main challenges for LCB valorization. At microbiological level, constraints are mostly linked to the need for axenic cultures and the preference for certain carbon sources (i.e., glucose). To cope with these issues, this review focuses on efficient LCB conversion via the sugar platform as well as an innovative carboxylate platform taking advantage of the co-cultivation of microorganisms. This review discusses novel trends in the use of microbial communities and co-cultures aiming at different bioproducts co-generation in single reactors as well as in sequential bioprocess combination. The outlook and further perspectives of these alternatives have been outlined for future successful development.