Genetically engineered Escherichia coli FBR5: part II. Ethanol production from xylose and simultaneous product recovery

Engineered bacteria for valorizing lignocellulosic biomass into bioethanol

Appropriate bioprocessing of lignocellulosic materials into ethanol could address the world's insatiable appetite for energy while mitigating greenhouse gases. Bioethanol is an ideal gasoline extender and is widely used in many countries in blended form with gasoline at specific ratios to improve fuel characteristics and engine performance. Although the bioethanol production industry has long been operational, finding a suitable microbial agent for the efficient conversion of lignocelluloses is still an active field of study. Among available microbial candidates, engineered bacteria may be promising ethanol producers while may show other desired traits such as thermophilic nature and high ethanol tolerance. This review provides the current knowledge on the introduction, overexpression, and deletion of the genes that have been performed in bacterial hosts to achieve higher ethanol yield, production rate and titer, and tolerance. The constraints and possible solutions and economic feasibility of the processes utilizing such engineered strains are also discussed.

A new approach for balancing the microbial synthesis of ethyl acetate and other volatile metabolites during aerobic bioreactor cultivations

Ethyl acetate is an organic solvent with many industrial applications, currently produced by energy-intensive chemical processes based on fossil carbon resources. Ethyl acetate can be synthesized from renewable sugars by yeasts like Kluyveromyces marxianus in aerobic processes. However, ethyl acetate is highly volatile and thus stripped from aerated cultivation systems which complicate the quantification of the produced ester. Synthesis of volatile metabolites is commonly monitored by repeated analysis of metabolite concentrations in both the gas and liquid phase. In this study, a model-based method for quantifying the synthesis and degradation of volatile metabolites was developed. This quantification of volatiles is solely based on repeatedly measured gas-phase concentrations and allows calculation of reaction rates and yields in high temporal resolution. Parameters required for these calculations were determined in abiotic stripping tests. The developed method was validated for ethyl acetate, ethanol and acetaldehyde which were synthesized by K. marxianus DSM 5422 during an iron-limited batch cultivation; it was shown that the presented method is more precise and less time-consuming than the conventional method. The biomass-specific synthesis rate and the yield of ethyl acetate varied over time and exhibited distinct momentary maxima of 0.50 g g‒1h‒1 and 0.38 g g‒1 at moderate iron limitation.

Ethanol production from olive stone hydrolysates by xylose fermenting microorganisms

Olive stones are the main solid byproducts obtained from olive oil production and from table olives production. As a lignocellulosic material, the use of olive stones for ethanol and other chemicals production has been proposed, particularly under the biorefinery concept. As part of such a process, this work deals with the fractionation of the lignocellulosic material by dilute acid autoclave pretreatment at 2% sulfuric acid, 130°C, 60 min and 1:1 liquid to solid ratio. Moreover, the work addresses the fermentation of the liquors obtained after pretreatment. The released sugars are composed mainly by xylose and other hemicellulosic sugars. The fermentation performance of three xylose-fermenting microorganisms, e.g. two Escherichia coli species and Scheffersomyces stipitis, are compared. The study analyzes in a first step the microorganism behavior on synthetic liquors, with a similar composition to that of the real liquors. Finally, and taken into account the results from the previous steps, the real liquor obtained from olive stones pretreatment is fermented. Results show that E. coli MM160 is the best ethanol producer out of the three microorganisms studied. Globally, the pretreatment produced a liquor containing 140 g hemicellulosic sugars/l and requiring firstly dilution by 50% and a detoxification step by overliming. The fermentation of this liquor by E. coli MM160 results in a 25 g ethanol/l solution equivalent to 50 g ethanol/kg olive stone, in spite of 20 g acetic acid/l also present. These results confirm both olive stones and E. coli MM160 as promising feedstock and microorganism for ethanol production.