A Review on Opportunities and Limitations of Membrane Bioreactor Configuration in Biofuel Production.

Biofuels are a clean and renewable source of energy that has gained more attention in recent years; however, high energy input and processing cost during the production and recovery process restricted its progress. Membrane technology offers a range of energy-saving separation for product recovery and purification in biorefining along with biofuel production processes. Membrane separation techniques in combination with different biological processes increase cell concentration in the bioreactor, reduce product inhibition, decrease chemical consumption, reduce energy requirements, and further increase product concentration and productivity. Certain membrane bioreactors have evolved with the ability to deal with different biological production and separation processes to make them cost-effective, but there are certain limitations. The present review describes the advantages and limitations of membrane bioreactors to produce different biofuels with the ability to simplify upstream and downstream processes in terms of sustainability and economics.

Xylose transport in yeast for lignocellulosic ethanol production: Current status.

Lignocellulosic ethanol has been considered as an alternative transportation fuel. Utilization of hemicellulosic fraction in lignocelluloses is crucial in economical production of lignocellulosic ethanol. However, this fraction has not efficiently been utilized by traditional yeast Saccharomyces cerevisiae. Genetically modified S. cerevisiae, which can utilize xylose, has several limitations including low ethanol yield, redox imbalance, and undesired metabolite formation similar to native xylose utilizing yeasts. Besides, xylose uptake is a major issue, where sugar transport system plays an important role. These genetically modified and wild-type yeast strains have further been engineered for improved xylose uptake. Various techniques have been employed to facilitate the xylose transportation in these strains. The present review is focused on the sugar transport machineries, mechanisms of xylose transport, limitations and how to deal with xylose transport for xylose assimilation in yeast cells. The recent advances in different techniques to facilitate the xylose transportation have also been discussed.

Effect of Evolutionary Adaption on Xylosidase Activity in Thermotolerant Yeast Isolates Kluyveromyces marxianus NIRE-K1 and NIRE-K3.

Efficient use of xylose along with glucose is necessary for the economic production of lignocellulosic based biofuels. Xylose transporters play an important role in the microorganisms for efficient utilization of xylose. In the present study, a novel method has been developed for a rapid assay of xylose transport activity in the xylose-utilizing isolates and other known yeasts. An assay was conducted to compare the activity of ß-xylosidase using p-nitrophenyl-ß-D-xylopyranoside (pNPX) in the intact, intracellular, and extracellular yeasts cells showing xylose transporter. Saccharomyces cerevisiae (MTCC 170) showed no xylosidase activity, while little growth was observed in the xylose-containing medium. Although other yeasts, i.e., Kluyveromyces marxianus NIRE-K1 (MTCC 5933), K. marxianus NIRE-K3 (MTCC 5934), and Candida tropicalis (MTCC 230), showed xylosidase activity in intact, intracellular, and extracellular culture. The xylosidase activity in intact cell was higher than that of extracellular and intracellular activity in all the yeast cells. The enzyme activity was higher in case of K. marxianus NIRE-K1 and K. marxianus NIRE-K3 rather than the C. tropicalis. Further, better xylosidase activity was observed in adapted K. marxianus cells which were 2.79-28.46 % higher than that of native (non-adapted) strains, which indicates the significant improvement in xylose transportation.

Enhancement in xylose utilization using Kluyveromyces marxianus NIRE-K1 through evolutionary adaptation approach.

The evolutionary adaptation was carried out on the thermotolerant yeast Kluyveromyces marxianus NIRE-K1 at 45 °C up to 60 batches to enhance its xylose utilization capability. The adapted strain showed higher specific growth rate and 3-fold xylose uptake rate and short lag phase as compared to the native strain. During aerobic growth adapted yeast showed 2.81-fold higher xylose utilization than that of native. In anaerobic batch fermentation, adapted yeast utilized about 91% of xylose in 72 h and produced 2.88 and 18.75 g l⁻¹ of ethanol and xylitol, respectively, which were 5.11 and 5.71-fold higher than that of native. Ethanol yield, xylitol yield and specific sugar consumption rate obtained by the adapted cells were found to be 1.57, 1.65 and 4.84-fold higher than that of native yeast, respectively. Aforesaid results suggested that the evolutionary adaptation will be a very effective strategy in the near future for economic lignocellulosic ethanol production.

A new search for thermotolerant yeasts, its characterization and optimization using response surface methodology for ethanol production.

The progressive rise in energy crisis followed by green house gas (GHG) emissions is serving as the driving force for bioethanol production from renewable resources. Current bioethanol research focuses on lignocellulosic feedstocks as these are abundantly available, renewable, sustainable and exhibit no competition between the crops for food and fuel. However, the technologies in use have some drawbacks including incapability of pentose fermentation, reduced tolerance to products formed, costly processes, etc. Therefore, the present study was carried out with the objective of isolating hexose and pentose fermenting thermophilic/thermotolerant ethanologens with acceptable product yield. Two thermotolerant isolates, NIRE-K1 and NIRE-K3 were screened for fermenting both glucose and xylose and identified as Kluyveromyces marxianus NIRE-K1 and K. marxianus NIRE-K3. After optimization using Face-centered Central Composite Design (FCCD), the growth parameters like temperature and pH were found to be 45.17°C and 5.49, respectively for K. marxianus NIRE-K1 and 45.41°C and 5.24, respectively for K. marxianus NIRE-K3. Further, batch fermentations were carried out under optimized conditions, where K. marxianus NIRE-K3 was found to be superior over K. marxianus NIRE-K1. Ethanol yield (Y x∕s ), sugar to ethanol conversion rate (%), microbial biomass concentration (X) and volumetric product productivity (Q p ) obtained by K. marxianus NIRE-K3 were found to be 9.3, 9.55, 14.63, and 31.94% higher than that of K. marxianus NIRE-K1, respectively. This study revealed the promising potential of both the screened thermotolerant isolates for bioethanol production.

Scope of algae as third generation biofuels.

An initiative has been taken to develop different solid, liquid, and gaseous biofuels as the alternative energy resources. The current research and technology based on the third generation biofuels derived from algal biomass have been considered as the best alternative bioresource that avoids the disadvantages of first and second generation biofuels. Algal biomass has been investigated for the implementation of economic conversion processes producing different biofuels such as biodiesel, bioethanol, biogas, biohydrogen, and other valuable co-products. In the present review, the recent findings and advance developments in algal biomass for improved biofuel production have been explored. This review discusses about the importance of the algal cell contents, various strategies for product formation through various conversion technologies, and its future scope as an energy security.

Ethanol fermentation of sugarcane molasses by Zymomonas mobilis MTCC 92 immobilized in Luffa cylindrica L. sponge discs and Ca-alginate matrices

Bio-ethanol production from cane molasses (diluted to 15 % sugar w/v) was studied using the bacterium, Zymomonas mobilis MTCC 92 entrapped in luffa (Luffa cylindrica L.) sponge discs and Ca-alginate gel beads as the immobilizing matrices. At the end of 96 h fermentation, the final ethanol concentrations were 58.7 ± 0.09 and 59.1 ± 0.08 g/l molasses with luffa and Ca-alginate entrapped Z. mobilis cells, respectively exhibiting 83.25 ± 0.03 and 84.6 ± 0.02 % sugar conversion. There was no statistical significant difference (Fischer's LSD) in sugar utilization (t = 0.254, p <0.801) and ethanol production (t =-0.663, p <0.513) between the two immobilization matrices used. Further, the immobilized cells in both the matrices were physiologically active for three more cycles of operation with less than 15 % decrease in ethanol yield in the 4th cycle, which was due to some leakage of cells. In conclusion, luffa sponge was found to be equally good as Ca-alginate as a carrier material for bacterial (Z. mobilis. cell immobilization for ethanol production. Further, it has added advantages such as it is cheap, non-corrosive and has no environmental hazard.

Ethanol fermentation of sugarcane molasses by Zymomonas mobilis MTCC 92 immobilized in Luffa cylindrica L. sponge discs and Ca-alginate matrices.

Bio-ethanol production from cane molasses (diluted to 15 % sugar w/v) was studied using the bacterium, Zymomonas mobilis MTCC 92 entrapped in luffa (Luffa cylindrica L.) sponge discs and Ca-alginate gel beads as the immobilizing matrices. At the end of 96 h fermentation, the final ethanol concentrations were 58.7 ± 0.09 and 59.1 ± 0.08 g/l molasses with luffa and Ca-alginate entrapped Z. mobilis cells, respectively exhibiting 83.25 ± 0.03 and 84.6 ± 0.02 % sugar conversion. There was no statistical significant difference (Fischer's LSD) in sugar utilization (t = 0.254, p<0.801) and ethanol production (t =-0.663, p<0.513) between the two immobilization matrices used. Further, the immobilized cells in both the matrices were physiologically active for three more cycles of operation with less than 15 % decrease in ethanol yield in the 4(th) cycle, which was due to some leakage of cells. In conclusion, luffa sponge was found to be equally good as Ca-alginate as a carrier material for bacterial (Z. mobilis) cell immobilization for ethanol production. Further, it has added advantages such as it is cheap, non-corrosive and has no environmental hazard.