Energy balances play an increasingly important role in quantitative descriptions of growth processes, bath for biological research and for process design in biotechnology. This contribution contains a detailed description of how such balances can be set up and develops guidelines concerning the selection of accurate standard and reference states. Thermodynamic energy balances require a rigorous definition of the thermodynamic state of all components involved in the process, since the enthalpies of formation of any substance in different states may differ considerably. Biological processes generally take place in the aqueous state. This is particularly true for organisms living in an aqueous environment which is not in direct contact with the atmosphere. Thus, the aqueous state often defines the correct standard state for energy balances if the energy exchange is to be calculated for individual cells or for a specified unit of live biomass. If, however, a whole fermenter or growth vessel defines the system boundary, gaseous compounds such as O-2 and CO2 are often exchanged predominantly in the gaseous state, which then defines the standard state to be used for these compounds. It is not possible to assign a well-defined state for living biomass, which may be described as a solid in an aqueous environment. For biomass, the corrections for the aqueous environment have been ignored in the past. Experimental work reported in this paper shows that this is basically justified, the magnitude of the corrections being too small to make a noticeable difference. Sample calculations concerning real, measured growth stoichiometries show that the differences between results based on correct standard states and approximate balances simply based on pure compound data are quite negligible for at least moderately aerobic growth. In anaerobic and fermentative growth, however, neglect in selecting the proper standard states results in errors up to 70%. Equally important are corrections for side-effects and side-reactions, such as evaporation and neutralization, which can give rise to 50% errors in anaerobic growth. Either constituent elements or combustion products of the involved chemical species may be used as reference states for constructing energy balances. Choosing the latter often simplifies calculations. Tables containing standard enthalpies of combustion for most important chemicals not only in their pure, but also in the biologically important aqueous standard states are presented.
