Note that these pH fluctuations are of rather small amplitudes,
much less than the amplitudes in experiments operated with
standard pH controllers. These data demonstrate the sensitivity of
the measurements during the cultivations. Normally, base has to
be supplied during cultivations exclusively. To keep the pH at the
set point in this type of cultivation it is necessary to supply acid as
well. This leads to a higher osmolality and, e.g., phosphate concentration,
which had no detectable influence on the growth or product
formation as can be seen in Fig. 6.
The corresponding specific power input at the combinations
of airflow rate QG and stirrer speed N, which is element of most
literature correlations, was determined during the running fermentation
experiments from the heat withdrawn from the culture via
the cooling jacket while the culture temperature was tightly controlled.
Several mechanisms contribute to the amount of heat generated
in the culture. However in the experiments described here, only the
heat generation by agitation and aeration and the heat metabolically
produced by the cells make significant contributions. All other
heat sources and sinks are negligible.The metabolic heat production Q˙ met can be assumed to be
proportional to the oxygen uptake rate which is measured
online during all of our cultivations