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
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.
further illustration of the model see the SimuPlot simulation
with yeast_model, which plots the specific rates against
glucose concentration in Monod type plots.
simulation of this type requires stoichiometric analysis.
that the model can not handle oxygen limitation.