Automated Measurement and Monitoring of Bioprocesses: Key Elements of the M3C Strategy - Measurement, Monitoring, Modelling, and Control of Bioprocesses - page 6

The pH-FET also needs a reference electrode (with all the pitfalls discussed
above) but is constructed from unbreakable material. However, such pH electrodes
suffered from substantial drift during sterilization (up to 80 mV, corresponding to
[1 pH unit) and have, therefore, been partially taken off the market.
Another valuable alternative is the optical pH sensor, in which single or more
mixed and probably fluorescing pH indicator substances are immobilized in a thin
polymer film and mounted on a transparent carrier (e.g., a viewing glass or optical
window), which is exposed to the measuring solution. Optical fibers connected to
the transparent carrier allow excitation of the indicator(s) and reading of the
spectrum; depending on the indicator(s) used, the spectral data need to be con-
verted into pH units [ 14 - 16 ]. The indicator(s) must be biocompatible and show
sufficient long-term stability, especially against sterilization.
An indirect estimation of pH based on mid-infrared spectroscopy with a stan-
dard error of prediction [0.15 pH units was reported by Schenk et al. [ 17 ] for
microbial cultivations.
2.2.2 Redox Potential
The physical construction of redox electrodes is very similar to the above-
mentioned glass pH electrodes. However, the sensing element is a small ring or
wire made of noble metal such as Pt or Au instead of the proton-sensitive glass.
The noble metals sense the sum of all redox couples that are in solution, and the
signal represents an average of all those dissolved components. Most often it is
unclear what this means for aerobic cultivations, although sometimes one can
apply the interpretation that the signal mirrors ''the general availability of elec-
trons'' [ 18 ]. Although we normally do not know which compounds are captured by
this signal, the time trajectory of the redox potential may have a very typical shape.
This may allow discrimination between ''good'' and ''bad'' when comparing the
actual trajectory with historical ones. In some way, the redox potential depicts a
typical pattern (or signature) of qualitatively unknown bioreaction components. In
anaerobic cultivations, however, the redox signal gives a very good indication of
the degree of anaerobicity; strict anaerobes will not grow at redox potentials above
approximately -300 mV.
2.2.3 Oxygen Partial Pressure, pO 2
A substantially important variable in aerobic cultivations is the oxygen partial
pressure (pO 2 , or DOT for dissolved oxygen tension). The availability of oxygen to
the cells can be decisive for a range of effects from simple changes in metabolism,
to altering the spectrum of (by-)products, to loss of energy and even cell death.
Being aware of such effects and knowing the oxygen availability from a sensor
signal we can (in principle) steer the development of a bioprocess by applying
closed-loop control; the measured variable is the pO 2 (reflecting what the cells