[Theoretical determination of the energy efficiency of microbial carbon substrate conversion and comparison with...

The phenomenological description of microbial substrate conversion is possible by connecting energy and material balance equations. The change of free enthalpy of microbial substrate conversion was determined on the basis of exact material balance equations. A simultaneous determination of heat production allowed to calculate the energetic efficiency of microbial growth processes. In addition to this the change of free enthalpy by microbial substrate conversion was standardized by the free standard formation enthalpy of biomass. Both quantities were used for comparing microbial combustion of several substrates into biomass. Comparable with the concept of substrate combustion enthalpy standardized by the combustion enthalpy of biomass an optimal strategy of mixing different substrates can be deduced from energy and material balance equations on the basis of theoretic consumption coefficients. Besides the energetic efficiency the binding state of the substrate is important in the search for optimal substrate mixing ratios. This could be shown by experimental work. Thus, the fermentation systems sucrose/yeast, paraffin/yeast, and sucrose/paraffin/yeast were studied using synchronous populations produced by the method of phased cultures. The investigations of real fermentation systems confirm the theoretical predictions on special conditions.