Ethanologenic microorganismsMicroor

Ethanologenic microorganisms
Microorganisms for bioethanol fermentation can best be described in terms of their performance parameters and other requirements such as compatibility with existing products, processes and equipment. The performance parameters of fermentation are temperature, pH, alcohol tolerance, growth rate, productivity, osmotic tolerance, specificity, yield, genetic stability, and inhibitor tolerance [46]. Thermophilic microorganisms can be categorized into several groups: moderate thermophiles, or simple thermophiles, are those that grow optimally between 50 and 64 °C, extreme thermophiles are those with optimal growth temperatures between 65 and 79 °C and the organisms that can grow optimally above 80 °C are called hyperthermophiles [47]. Contamination is a major factor in industrial fermentation which results in the reduction of the ethanol yield and formation of inhibitory compounds. The use of low pH, high temperature and high ethanol tolerant ethanologen can reduce the possibility of contaminations in large scale ethanol production processes [48], [49] and [50].

Ethanologenic thermotolerant and thermophilic bacteria are of current interest due to their ability to ferment various components of cellulosic biomass into chemicals and fuels. Thermoethanologenic bacteria include Clostridium acetobutylicum, C. thermohydrosulfurium, C. thermosaccharolyticum, C. thermosulfurogenes, C. tetani, Thermoanaerobacterium saccharolyticum, Thermoanaerobacter ethanolicus, Kluyveromyces marxianus, Pichia sp., Geobacillus sp., Bacillus macerans, B. polymyxa, Klebsiella pneumoniae, Aeromonas hydrophila, Aerobacter sp., Erwinia sp., Escherichia sp., Leuconostoc sp., Lactobacillus sp., and Zymomonas mobilis [51] and [52]. Table 3 describes the physiological characteristics and limitations of thermoethanologens, used in recent years on a broad substrate range.