Kinetic model of overflow metabolism in Saccharomyces cerevisiae.
The specific rate for glucose uptake (qG) is assumed to follow Monod kinetics.
At glucose concentrations below a critical value (typically about 30 g/L glucose) corresponding to a critical metabolic rate for glucose uptake, oxygen consumption (qO) and growth rate (my) are proportional to qG and no ethanol is produced. At higher glucose concentrations glucose uptake rate increases but no further increase in oxygen consumption is seen and the additional glucose uptake is metabolized fermentatively, which results in ethanol production and additional growth but at lower yield. The simulation program also calculates the respiratory quotient RQ, which is about 1 when no ethanol is produced but increases towards 3 at high glucose concentrations.

In a process where both ethanol and glucose are present at glucose concentrations below the critical value, ethanol is re-assimilated at rate (qEcmax*E/(E+Ke)) that is proportional to the “free capacity”, i.e. the difference between qOmax and the actual qO. Therefore qO is equal to qOmax as long as ethanol is present in the process.

For further illustration of the model check the simulation with yeast_fb.fig, which uses this model for simulation of a fed-batch process where ethanol is first produced but later on in the process consumed.

A simulation of this type requires stoichiometric analysis.
Note that the model can not handle oxygen limitation.