3.1. Response of the gas sensor array
The gas sensor array response pattern from a cultivation of
microorganisms may have various origins. The sensors can, if sensitive
enough, respond to specific volatile compounds emitted by the
microorganisms in the culture or to the emission from the nutrient
components of the medium (Bachinger and Mandenius 2000).
In order to determine the sensitivity of the gas sensor array to the
nutrient components of the medium as well as to the volatile compounds
emitted by the cells, the bioreactor was filled with 1 L of the medium
and a glucose solution (to a final concentration in bioreactor of 7 g/L)
and five sampling cycles were performed from the headspace of the
bioreactor. Exactly before the 6th sampling cycle, the
suspended cell was added and the sampling cycles were performed every 5
minutes until the end of the cultivation process (Fig.5).
Fig.5 (a) indicates the off-line as well as the simulated evolution of
glucose, ethanol and biomass during the cultivation (BC2). The
parameters of the model were acquired by least squares fitting to the
off-line measurements. Fig.5 (b) illustrates the raw signal response of
one of the gas sensors during the same cultivation.
In Fig. 5 (b), the five peaks before \(T_{\text{start}}\) indicate the
response of the sensor to the components of the medium. At\(T_{\text{start}}\) the suspended cells are added to the bioreactor
(inoculation time), \(T_{\max}\) is the peak with the highest value and\(T_{\text{end}}\) is the last peak where the ethanol is depleted and
whose height is the same as before \(T_{\text{start}}\).
High sensitivity of the sensor to ethanol produced during the
cultivation process can be described by comparing the general pattern of
the peak heights (Fig.5 (b)) with the ethanol concentration (Fig. 5
(a)). In Fig. 5 (a), while the glucose is decreasing during the glucose
phase, the products (ethanol and biomass) are increasing. At around 2 h,
the glucose is depleted and the cultivation shifts to the ethanol
consumption phase. After around 7 h, the ethanol is also depleted and no
more ethanol remained in the culture broth. Similar behavior of the
sensor response can be observed from Fig. 5 (b). In Fig. 5 (b), the peak
height before \(T_{\text{start}}\) and after \(T_{\text{end}}\) is the
same. In addition, there are no changes in the peak heights after\(T_{\text{end}}\) (the region where all the ethanol is consumed). From
these illustrations it can be seen that the sensor is highly sensitive
to ethanol but not so sensitive to the other components of the
cultivation medium.