4. Conclusion
The signals from the gas sensor array described here showed a high
correlation toward ethanol concentration during cultivations of S.
cerevisiae growing on glucose . Similar to any other indirect
measurement method, a chemometric model is required for predicting the
ethanol concentration from the signals of the gas sensor array. This
approach normally requires off-line sampling for calibration purposes
which is expensive and time consuming. Alternatively, a simulation model
can be used, if the correct parameters for the model are known. In the
proposed method, the only requirement for calculating the parameters of
the simulation model is the response of the gas sensor array from a
single cultivation run. Then the parameters for the simulation model can
be calculated by minimizing the prediction error by optimizing the
kinetic parameter values of the simulation model as well as the
parameter values of the chemometric model.
The proposed model-based calibration method provided comparable results
to the reference ethanol concentration values obtained by HPLC.
Furthermore, compared to spectroscopy supported models for ethanol
prediction which have applied separate prediction models for each
diauxic growth phases of the cultivation, in this investigation only a
single model was applied for the two diauxic growth phases.
The mechanical stress on the sensor as well as the number of interfering
substances is smaller in the gas phase compared to the liquid phase and,
furthermore, the so-called sensor fouling by cell adhesion is not a
problem here, because a sterile barrier in the form of a mass filter can
be easily introduced into the gas stream, this gas sensor arrays have a
big potential. Compared to HPLC measurements the time delay is much
shorter. Besides these advantages of using this method for ethanol
detection, the proposed method is also inexpensive to implement and has
just a few maintenance requirements. Therefore the gas sensor array
seems to be a useful non-invasive tool for continues monitoring and
might also be used for closed loop controlling of processes involving
ethanol measurements.