The design of fermentation processes is based on the knowledge of compounds that are consumed (substrate, O2, N-source, alkali) and are produced (biomass, product, CO2, heat, etc.). Their amounts can be found by knowing the process stoichiometry, reflected in the process reaction (see Section 18.5). The stoichiometry of the process reaction is only constant when the specific growth rate is kept constant, but changes when the growth rate changes. Therefore, it is most relevant to elaborate the relation between the process reaction stoichiometry and the specific growth rate. A Black Box model defines this relation. A Black Box model uses only input and output of the organisms which are directly coupled due to general (pseudo) steady-state properties of cellular metabolism. This coupling enormously diminishes the cellular kinetic freedom, which is further limited by element/charge conservation.