Simulation with yeast_fedbatch.fig

X: Biomass concentration; G: Glucose concentration; DOT: Dissolved oxygen tension; E: Ethanol concentration
my: Specific growth rate; F: Feed rate; RQ: Respiratory quotient.

Dynamic simulation of a fed-batch process with Saccharomyces cerevisiae. Also under fully aerobic conditions does this organism produce ethanol due to overflow metabolism, if the glucose concentration is too high. To avoid this, the process is usually started with a low (limiting) concentration of glucose which is kept approx. constant by an exponential feed. The specific growth rate (my) then becomes approx. equal to the exponent of the feed profile. Due to the exponential growth DOT declines and before it becomes too low the feed is switch to a constant flow.
The ethanol accumulation profile can be controlled via the glucose feed profile. If the initial specific growth rate is above a critical value (strain dependent but usually about 0.3 /h for S. cerevisiae) ethanol accumulates, more the higher the growth rate is. When the feed profile is switched to constant F the glucose concentration starts to decline, which reduces the specific growth rate, and when it passes below the critical value ethanol production is replaced by ethanol re-assimilation. As long as ethanol is consumed DOT declines, but when all ethanol is re-assimilated DOT rapidly increases to a level corresponding to the glucose feed rate.
The three phases of a typical yeast fed-batch process: ethanol production, ethanol re-assimilation, and growth on glucose without overflow metabolism are easily made visible by the respiratory quotient RQ = mmol CO2 produced per mmol O2 consumed.

For further illustration of the model see the SimuPlot simulation with yeast_model, which plots the specific rates against glucose concentration in Monod type plots.

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

For information about model equations and parameters: Open the SIMSPEC-file yeast_fedbatch.fig, available in the SimuPlot toolbox.

Updated 2015-02-01