Alfredo Martínez: Fuel ethanol production from corn stover with ethanologenic bacteria
Alfredo Martinez (UNAM, Mexico), presenting in Straubing
The next presentation was given by Alfredo Martinez from the Institute of Biotechnology at UNAM in Mexico (Department of Cellular Engineering and Biocatalyst), who spoke about fuel ethanol production from corn stover with ethanologenic bacteria.
Beginning, Mr. Martinez emphasized the relevance to use modern scientific tools, such as metabolic engineering and synthetic biology together with bioprocess design, to develop a wide array of biofuels and mono and dicarboxylic acids as precursors of biodegradable polymers.
Focusing in the challenge to produce second generation biofuels, Mr. Martinez described a general process to generate fuel-ethanol (agro-fuels) and, also, (agro-) chemicals from lignocellulose. The main objective of these projects in his laboratory is to design microorganisms and processes to transform all sugars contained in the lignocellulose (cellulose and hemicellulose: pentoses, hexoses, and disaccharides) into ethanol or other biochemicals, mainly organic acids. The whole chain process for the production of bioethanol from lignocellulosic biomass is relevant, and one of the key factors is the feedstock. In this regard, corn stover is produced worldwide, in many countries most of the stover is left in the field or, even worst, is burned before starting a new harvest. Several estimates indicate that 1.9 tons of corn stover are generated from 1 ton of produced corn, but to avoid nutrient depletion and decreased quality on the land used to produced corn, only 50% of the corn stover would be “harvested” to be used as feedstock into biorefineries. Hence, roughly 1 ton of corn stover per ton of produced corn could be harvested and used to feed the biorefineries. These numbers indicate that millions of tons of corn stover can be used worldwide to produce biofuels or other chemicals into the biorefineries.
Then Mr. Martinez described the concept of sequential thermochemical hydrolysis, enzymatic saccharification and fermentation with metabolic engineered bacteria that has developed in his laboratory to transforms pentoses (xylose and arabinose) and hexoses (glucose, manose and galactose, among others) to ethanol. The sequential process was studied and developed aiming to avoid washing the feedstock, avoid using intensive milling, reducing the generation of toxins, without a detoxification step, avoid the separation of pentose rich syrups from cellulose and lignin and washing these solids. Using this procedure, a simplified process can be developed aiming to reduce the equipment used to obtain sugars and ferment them to chemicals or biofuels: also, the one-pot process can be established.
In average and worldwide, corn stover contains (at least) 55% (w/w), in a dry weight basis, of polymerized sugars (roughly 30% glucan, 20% xylan, 5% arabinan), and if correctly processed (hydrolyzed) 550 kg of sugars per ton can be obtained (including maximum yields and loses). Furthermore, at laboratory level 270 liters of ethanol per ton of dry corn stover has been obtained; but, up to 320 liters of ethanol per ton of dry corn stover can be produced by process integration and optimization.
One of the proposals from this work is to use metabolic engineered bacteria, such as Escherichia coli, which can ferment all sugars in mixtures or from lignocellulosic hydrolysates (pentoses, hexoses and some disaccharides) to ethanol. The ethanologenic bacteria developed in his laboratory has been tested and reported (in scientific articles and patents) using several lignocellulosic biomasses (including corn stover and corn cobs), with conversions yields reaching 90% of the theoretical (the theoretical yield 0.640 liters of ethanol per kg of sugar), with a volumetric productivity close to or higher than 1 g of ethanol per liter per hour, and in the presence of some toxins (as acetic acid, furans and phenols) at a moderate level.
It is worth to mention that nowadays at industrial level other metabolic engineered strains of E. coli are used to produce at least 33% of the therapeutic proteins for human use (human growth hormones; interferons; interleukins; erythropoietin; among others) and several chemicals, including L-phenylalanine, polyhydroxybutyrate and Propanediol. Recently (2016), the technology developed to produce 1,4 butanodiol with metabolic engineered E. coli has been stablished at commercial scale in Europe. These facts shown the technological feasibility to use E. coli at industrial scale in biorefineries to produce fuel ethanol.
Furthermore, in the framework of biorefineries, in Martinez´ laboratory several E. coli strains has been developed to produce ethanol, acetate, D-lactate or L-Lactate. Specifically, for the SMIBIO project the strains to produce succinate or pyruvate has also been developed and tested at one-liter fermenter level using glucose, xylose or mixtures of this sugars. As the prices of these chemicals (lactate, pyruvate and succinate) are higher than the one for ethanol, the economic feasibility and the net present value at a 10-year period of the small scale biorefineries is higher than the biorefinery devoted to produce ethanol from lignocellulosic biomasses.