Thermodynamic concepts have been used in the past to predict microbial cell yield under various growth conditions. Cell yield may be the key consideration in some industrial biotechnology applications. It is not the case, however, in the context of biofuel production. In this paper, we examine the thermodynamics of fermentation and concomitant growth of baker's yeast in continuous culture experiments under anaerobic, glucose-limited conditions, with emphasis on the yield and efficiency of ethanol production. We find that anaerobic metabolism of baker's yeast is very efficient; the process destroys less than 7% of the total chemical exergy supplied to the fermentation reactor. However, the exergy of ethanol secreted constitutes less than 60% of the in-flowing exergy, or 75% that of glucose fed to the continuous culture. Effects of varying the specific adenosine 5'-triphosphate (ATP) consumption rate, which is the fundamental parameter that quantifies the energetic requirements for cell growth and maintenance, are also examined.
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