Renewable lignocellulosic materials are attractive low-cost feedstocks for bioethanol production. Furfural and 5-hydroxymethylfurfural (HMF) are among the most potent inhibitory compounds generated from acid hydrolysis of lignocelluloses to simple sugars for fermentation. In Saccharomyces cerevisiae ATCC 211239 and NRRL Y-12632 and Pichia stipitis NRRL Y-7124, furfural and HMF inhibition were determined to be dose-dependent at concentrations from 10 to 120 mM. The yeast strains were more sensitive to inhibition by furfural than HMF at the same concentration, while combined treatment of furfural and HMF synergistically suppressed cell growth. A metabolite transformed from HMF by strain NRRL Y-12632 was isolated from the culture supernatant, and conclusively identified as 2,5-bis-hydroxymethylfuran, a previously postulated HMF alcohol, with a composition of C6H8O3 and a molecular weight of 128. It is proposed that, in the presence of HMF, the yeast reduces the aldehyde group on the furan ring of HMF into an alcohol, in a similar manner as for furfural. The accumulation of this biotransformed metabolite may be less toxic to yeast cultures than HMF, as evidenced by the rapid yeast fermentation and growth rates associated with HMF conversion. The ability of yeasts to adapt to and transform furfural and HMF offers the potential for in situ detoxification of these inhibitors and suggests a genetic basis for further development of highly tolerant strains for biofuel production.

Adaptive response of yeasts to furfural and 5-hydroxymethylfurfural and new chemical evidence for HMF conversion to 2,5-bis-hydroxymethylfuran

The sustainable production of biofuels will require the efficient utilization of lignocellulosic biomass. A key barrier involves the creation of growth-inhibitory compounds by chemical pretreatment steps, which ultimately reduce the efficiency of fermentative microbial biocatalysts. The primary toxins include organic acids, furan derivatives, and phenolic compounds. Weak acids enter the cell and dissociate, resulting in a drop in intracellular pH as well as various anion-specific effects on metabolism. Furan derivatives, dehydration products of hexose and pentose sugars, have been shown to hinder fermentative enzyme function. Phenolic compounds, formed from lignin, can disrupt membranes and are hypothesized to interfere with the function of intracellular hydrophobic targets. This review covers mechanisms of toxicity and tolerance for these compounds with a specific focus on the important industrial organism Escherichia coli. Recent efforts to engineer E. coli for improved tolerance to these toxins are also discussed.

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Cellulosic hydrolysate toxicity and tolerance mechanisms in Escherichia coli.

There is a growing awareness that lignocellulose will be a major raw material for production of both fuel and chemicals in the coming decades—most likely through various fermentation routes. Considerable attention has been given to the problem of finding efficient means of separating the major constituents in lignocellulose (i.e., lignin, hemicellulose, and cellulose) and to efficiently hydrolyze the carbohydrate parts into sugars. In these processes, by-products will inevitably form to some extent, and these will have to be dealt with in the ensuing microbial processes. One group of compounds in this category is the furaldehydes. 2-Furaldehyde (furfural) and substituted 2-furaldehydes—most importantly 5-hydroxymethyl-2-furaldehyde—are the dominant inhibitory compounds found in lignocellulosic hydrolyzates. The furaldehydes are known to have biological effects and act as inhibitors in fermentation processes. The effects of these compounds will therefore have to be considered in the design of biotechnological processes using lignocellulose. In this short review, we take a look at known metabolic effects, as well as strategies to overcome problems in biotechnological applications caused by furaldehydes.

Metabolic effects of furaldehydes and impacts on biotechnological processes

Furfural and 5-hydroxymethylfurfural (HMF) are representative inhibitors generated from biomass pretreatment using dilute acid hydrolysis that interfere with yeast growth and subsequent fermentation. Few yeast strains tolerant to inhibitors are available. In this study, we report a tolerant strain, Saccharomyces cerevisiae NRRL Y-50049, which has enhanced biotransformation ability to convert furfural to furan methanol (FM), HMF to furan di-methanol (FDM), and produce a normal yield of ethanol. Our recent identification of HMF and development of protocol to synthesize the HMF metabolic conversion product FDM allowed studies on fermentation metabolic kinetics in the presence of HMF and furfural. Individual gene-encoding enzymes possessing aldehyde reduction activities demonstrated cofactor preference for NADH or NADPH. However, protein extract from whole yeast cells showed equally strong aldehyde reduction activities coupled with either cofactor. Deletion of a single candidate gene did not affect yeast growth in the presence of the inhibitors. Our results suggest that detoxification of furfural and HMF by the ethanologenic yeast S. cerevisiae strain Y-50049 likely involves multiple gene mediated NAD(P)H-dependent aldehyde reduction. Conversion pathways of furfural and HMF relevant to glycolysis and ethanol production were refined based on our findings in this study.

Multiple gene-mediated NAD(P)H-dependent aldehyde reduction is a mechanism of in situ detoxification of furfural and 5-hydroxymethylfurfural by Saccharomyces cerevisiae

Development of yeast cell factories for consolidated bioprocessing of lignocellulose to bioethanol through cell surface engineering

Power utilities are struggling to reduce power failure incidents in substations and their components to operate more reliably and economically [1]. Many power failures are produced directly or indirectly because of the insulation system of utility components [2], [3]. The selection of the insulation should ensure power plant operational continuity along with completely resolving or significantly limiting the actual power system's failures [4]. Gas insulated substations (GIS) have the best insulation performance which ensures achieving minimum failure incidents, although at high installation cost. The most common insulating gas used in GIS is Sulfur hexafluoride (SF 6 ) gas, which is widely used as an effective electrical insulation as well as an arc-quenching medium [5]. Basic GIS and gas insulated transmission lines (GITL or GIL) consist of a conductor supported by solid insulators inside an enclosure filled with SF 6 gas or its mixture [6].

Partial discharge detection and diagnosis in gas insulated switchgear: State of the art

Furfural is a dietary mutagen and is present in various frequently consumed food products. In earlier work we have shown that it induces single strand breaks in duplex DNA which occur preferentially in AT base pairs. Experiments on the conservation of EcoRI cleavage site in mutant plasmids further established that repair of furfural damaged plasmid DNA occurs on propagation in the host cells. In this paper it is shown that the strand scissions induced by furfural in DNA account for its biological activity as assayed by inactivation of bacteriophage lambda. Using various repair defective mutants of E. coli we have shown that DNA repair predominantly involves the polA pathway.

Inactivation and repair of bacteriophage lambda by furfural.

Mutagenicity of furfural in plasmid DNA

Furfural is a dietary mutagen and is present in various food products and beverages. We have previously shown that it induces single strand breaks in double stranded DNA that occur mainly in AT sequences. In this paper, we have examined the mutagenic effect of furfural induced lesions in the plasmid pBR322. Plasmid DNA was treated with furfural before transformation in E. coli. There was a progressive decrease in the transformation capacity of the plasmid both as a function of furfural concentration and time of reaction. Several mutants were isolated from plasmid preparations whose transforming capacity had been decreased by treatment with furfural. Depending on the concentration of furfural used, these were found to have undergone DNA alterations including deletions. Experiments on the conservation of EcoRI cleavage site in mutant plasmids indicated that repair of furfural damaged plasmid DNA takes place on propagation in host cells.

Effect of furfural on plasmid DNA.

Dielectric fluids or commonly oils serve as the protection of any power utility components, which make it selection crucial for utility. Due to its significance, consistent research has been performed and new insulants have been introduced for power utilities to attain better, reliable, and a longer lifespan than the former one. Nanotechnology-based insulating liquids are still under research but show the potential to achieve the objective of smart and futuristic insulation. This paper provides the critical review for the development of insulating liquid and promising results shown by the nanoparticles dispersed fluid termed nanofluids. The state-of-the-art research illustration includes synthesis, experimental investigation, and dielectric and physio-mechanical characterization of nanofluids. The enhancements in the characteristics of nanofluids are elucidated along with its possible mechanisms and shortcomings.

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Qasim Khan

Papers

Partial discharge detection and diagnosis in gas insulated switchgear: State of the art

Inactivation and repair of bacteriophage lambda by furfural.

Mutagenicity of furfural in plasmid DNA

Effect of furfural on plasmid DNA.

State-of-Art in Nano-Based Dielectric Oil: A Review