The effect of lactic acid on fuel ethanol production byZymomonas |
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Authors: | Lawford Hugh G Rousseau Joyce D |
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Institution: | 1.Department of Biochemistry, University of Toronto, M5S 1A8, Toronto, Ontario, Canada ; |
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Abstract: | Efficient utilization of the pentosan fraction of hemicellulose from lignocellulosic feedstocks offers an opportunity to increase
the yield and to reduce the cost of producing fuel ethanol. During prehydrolysis (acid hydrolysis or autohydrolysis of hemicellulose),
acetic acid is formed as a consequence of the deacetylation of the acetylated moiety of hemicellulose. Recombinant Escherichia
coli B (ATCC 11303), carrying the plasmid pLO1297 with pyruvate decarboxylase and alcohol dehydrogenase II genes from Zymomonas
mobilis (CP4), converts xylose to ethanol with a product yield that approaches theoretical maximum. Although other pentose-utilizing
microorganisms are inhibited by acetic acid, the recombinant E. coli displays a high tolerance for acetic acid. In xylose
fermentations with a synthetic medium (Luria broth), where the pH was controlled at 7, neither yield nor productivity was
affected by the addition of 10.7 g/L acetic acid. Nutrient-supplemented, hardwood (aspen) hemicellulose hydrolysate (40.7
g/L xylose) was completely fermented to ethanol (16.3 g/L) in 98 h. When the acetic acid concentration was reduced from 5.6
to 0.8 g/L, the fermentation time decreased to 58 h. Overliming, with Ca(OH)2 to pH 10, followed by neutralization to pH 7 with sulfuric acid and removal of insolubles, resulted in a twofold increase
in volumetric productivity. The maximum productivity was 0.93 g/L/h. The xylose-to-ethanol conversion efficiency and productivity
in Ca(OH)2-treated hardwood prehydrolysate, fortified with only mineral salts, were 94% and 0.26 g/L/h, respectively. The recombinant
E. coli exhibits a xylose-to-ethanol conversion efficiency that is superior to that of other pentose-utilizing yeasts currently
being investigated for the production of fuel ethanol from lignocellulosic materials. |
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Keywords: | Index Entries" target="_blank">Index Entries Fuel ethanol xylose recombinantE coli yield prehydrolysate hemicellulose hydrolysate genetic engineering |
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