**2. Selected aspects of ethanol production from lignocellulosic biomass**

For the fermentative production of ethanol from biomass to be commercially successful, several key processes and organisms need to be considered [3, 4, 7–9]. These process requirements needed to simultaneously consider two viewpoints: the physiological properties of the ethanologen used and process requirements. Concerning organism requirements, an ideal strains should be homoethanologenic, with high productivity (> 1/g/L/h), have broad substrate spectra and high tolerance of ethanol, inhibitory compounds and high initial substrate concentrations. Other key factors include high cellulolytic activity, simple nutritional needs, low biomass production and ease of genetic manipulation. Ideally, a single organism that is both highly ethanologenic and cellulolytic would be ideal for consolidated bioprocessing (CBP) although co-cultures of organisms together fulfilling these requirements may also be considered in a simultaneous saccharification and fermentation (SSF) setup. To adequately meet the process requirements, ethanol yields should reach a minimum of 90% of theoretical, achieve high ethanol titers (> 5% v/v), have a minimum number of process steps, and require minimal or no process cooling. Additionally, cells should be robust enough to be recyclable and substrates co-fermented and pretreatment should be limited or excluded.

No single wild type organism possesses all these features. Although genetic manipulation has yielded only modest improvements for ethanologens although transformants are not always stable [9–11]. Many thermophilic clostridia have much broader substrate spectrum as compared to standard ethanologens such as *Saccharomyces cerevisiae* and *Zymomonas mobilis*. Additionally, cultivations of thermoanaerobes do not require extensive agitation or temperature control of the fermentation vessels and these often tolerate extremes of pH and salt concentrations during fermentation with minimal need for nutrient supplementation [4]. The mixing of reaction vessels and pumping of liquids are easier at elevated temperatures due to reduced viscosity as well increased substrate solubility [12].
