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Chapter 10 : The Development of Ethanologenic Bacteria for Fuel Ethanol Production
Category: Applied and Industrial Microbiology
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This chapter focuses on the development of two related bacteria that have proven to be effective in a variety of physical and chemical processes: Escherichia coli and Klebsiella oxytoca. Development of recombinant microbes that utilize a variety of sugars for ethanol production in laboratory media under optimum growth conditions has been repeated by several laboratories around the world. The use of dilute acid at temperatures above 140ºC is effective for the hydrolysis of hemicellulose in bagasse without significant loss of sugars or the production of degraded by-products. Dilute acid hydrolysis of hemicellulose has been used in order to produce high concentrations of hemicellulose sugars for fermentation by E. coli strain KO11-RD1. The yield of ethanol from acidic hydrolysis of cellulose is limited due to the poor recovery of glucose during the acid hydrolysis process. The degradation of glucose occurs very rapidly under conditions necessary for cellulose hydrolysis. Therefore, the use of cellulolytic enzymes has been pursued for several decades as a means of increasing the ethanol yield from cellulose. The simultaneous saccharification and fermentation (SSF) model has the following advantages over the sequential hydrolysis and fermentation process model: (i) lower enzyme dosages required for efficient conversion, (ii) compatibility with coproduction of enzymes during ethanol fermentation, and (iii) lower free-sugar concentrations during the SSF process.
(Left) Effects of growth temperature on the alcohol tolerance of Z. mobilis strain CP4. Cells were grown as described previously ( Carey et al., 1984 ). An overnight culture was diluted 1:200 into fresh medium containing a series of concentrations of ethanol and incubated for 48 h. Growth was measured as optical density at 550 nm. (Right) Effects of initial glucose concentration on the alcohol tolerance of Z. mobilis strain CP4. An overnight culture was diluted 1:200 into fresh medium containing 1, 10, 20, or 25% glucose and a series of concentrations of ethanol. Cultures were incubated at 30°C for 48 h, and growth was measured as optical density at 550 nm ( Ingram et al., 1984 ).
A 200-liter fed-batch fermentation of dilute acid hydrolysate of sugarcane bagasse by E. coli strain KO11-RD1. Final ethanol yield was 93% of theoretical.
SSF of sugarcane bagasse fiber after dilute acid hydrolysis of hemicellulose under low- and high-temperature conditions, using K. oxytoca strain SZ21.
Desirable microbial traits for conversion of biomass to ethanol a
Fermentation of hemicellulose sugars from various lignocellulosic biomasss sources by strains of ethanologenic E. coli and K. oxytoca a
Summary of cellulose conversion to ethanol using K. oxytoca strain P2 in SSF processes with added enzymes
Coproduction of ethanol plus enzymes by bacteria for the conversion of various polymers