Thermophilic ethanol fermentations.

Abstract

Thermophilic ethanol fermentations are of interest to industrial alcohol production because both the pentose and hexose fraction of biomass can be directly fermented in high yield (i.e., mol ethanol/mol substrate consumed), and because of potential novel process features associated with high temperature operation. As a net result, the co-culture cellulose fermentations described here may have the potential to convert more substrate to alcohol than some other bioconversion systems described [see Figure 11, (2)]. However, considerably more fundamental and applied research is required before realistic economic assessments can be made. Detailed analysis of the data presented above suggests key control parameters for thermophilic ethanol production (see Table IX). Understanding in detail the physiological and biochemical features that control rate limitation, yield limitation and concentration limitation appears to me as trends for future applied and fundamental studies on thermophilic ethanologenic bacteria. It is worth noting from the data reviewed here that understanding control of any one of these 3 major limitations is complex and multi-faceted. Indeed, improvement of ethanol tolerance (i.e. the ability to produce greater than 1% ethanol at high rates) in these bacteria appears to involve challenges by all three limitations. Furthermore, the biochemical basis for alcohol tolerance in thermophilic ethanologens appears to vary in different species. For example, the ethanol dehydrogenase of C. thermocellum is inhibited by physiological concentrations of alcohol (i.e. 1%) whereas, the reversible activity of T. brockii or C. thermohydrosulfuricum enzyme is increased by higher solvent concentration (greater than 5%).