3.1 TRADITIONAL ETHANOLOGENIC MICROBES

The fundamental challenge in selecting or tailoring a microorganism to produce eth­anol from the mixture of sugars resulting from the hydrolysis of lignocellulosic feed­stocks is easily articulated: the best ethanol producers are incompetent at utilizing pentose sugars (including those that are major components of hemicelluloses, that is, D-xylose and L-arabinose), whereas species that can efficiently utilize both pentoses and hexoses are less efficient at converting sugars to ethanol, exhibit poor tolerance of high ethanol concentrations, or coproduce high concentrations of metabolites such as acetic, lactic, pyruvic, and succinic acids in amounts to compromise the efficiency of substrate conversion to ethanol.1-4

Because bioprospecting microbial species in many natural habitats around the global ecosphere has failed to uncover an ideal ethanologen for fuel ethanol or other industrial uses, considerable ingenuity has been exhibited by molecular geneticists and fermentation specialists in providing at least partial solutions for the two most popular “combinatorial biology” strategies of

• Endowing traditional yeast ethanologens with novel traits, including the ability to utilize pentoses

• “Reforming” bacterial species and nonconventional yeasts to be more effi­cient at converting both pentoses and hexoses to ethanol

A third option, that is, devising conditions for mixed cultures to function synergisti — cally with mixtures of major carbon substrates, is discussed in chapter 4 (section 4.5).

Adding to the uncertainty is the attitude of the traditionally conservative alcohol fermentation industry toward the introduction of organisms that lack the accepted historic advantages of the yeast Saccharomyces cerevisiae in being generally regarded as safe (GRAS) and, by extrapolation, capable of being sold as an ingredi­ent in animal feed once the fermentation process is completed.5 At various times in the past 40-50 years, thermotolerant yeast strains have been developed to accelerate the fermentation process at elevated temperature.6 Bioprocesses have been advocated and, to varying degrees, developed, in which polymeric carbohydrate inputs are both hydrolyzed with secreted enzymes and the resulting sugars and oligosaccharides are taken up and metabolized to ethanol by the cell population, the so-called simultane­ous saccharification and fermentation (SSF) strategy.[18] [19] [20] Because the potential advan­tages of SSF are best understood in the light of the differing fermentation hardware requirements of multistage and single-stage fermentations, consideration of SSF technologies is postponed until the next chapter (section 4.5).