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pH. The most accurate and versatile among the many pH measuring methods are the glass electrode and the Ion Sensitive Field Effect Transistor (ISFET). Glass electrodes consist of a glass tube divided by a membrane (also made of glass) that is especially sensitive to hydrogen ions. Their fragility is their main limitation, as the membrane can be broken easily. On the other hand, the transistor electrode ISFET is practically unbreakable, very reliable, and responds quickly (Creus 1998d). The direct measurement of pH in porous solid substrates is not reliable due to poor contact between the solid and the sensitive part of the electrode. Although there are flat electrodes that adapt better to solid samples, their applicability is limited to static beds because agitation may damage the electrode. In this case the measurement cannot be extrapolated to the rest of the bed due to its heterogeneity (Mitchell et al. 1992).

Bed porosity. This effect can be assessed on-line by monitoring the pressure drop through the solid bed (Auria et al. 1993; Villegas et al. 1993; Bellon-Maurel et al. 2003). For example, in static bioreactors pressure drop measurements can be used to keep the inlet air flow rate under control, or in periodically agitated bioreactors, to establish the mixing intervals. Although Bourdon tubes are widely used pressure sensors since they are simple, inexpensive, and reliable, they are neither fast nor precise. On the other hand, piezoelectric sensors can be used to measure within a wider range, their response time is extremely short, and they are insensitive to temperature (Lipták 1995b).

Off-gas analysis. Off-gas analysis can be performed by Gas Chromatography (GC) (Saucedo-Castañeda et al. 1992; Saucedo-Castañeda et al. 1994) or by specific gas analyzers (Smits et al. 1996; Fernández et al. 1997). In both cases, in order to get meaningful results, care must be taken to keep the flow rate regulated and to dry the air sample before it enters the instrument. GC is sometimes preferred over specific analyzers since many compounds, in addition to CO2 and O2, can be monitored with the same instrument and over a wider range of values. However, each analysis takes several minutes. On the other hand, gas analyzers are more precise and have fast response times (of a few seconds). These devices make use of chemical or physical properties, like paramagnetism or infrared absorption, which characterize the measured gases. Paramagnetic analyzers, avail able for O2, are probably the most effective (Kaminski et al. 1995; Creus 1998e). Although expensive, they are very precise, do not require periodic calibration, present low interference with other gases (if water vapor is removed) and last long. These instruments exploit the property that some gases have of being magnetized when they are exposed to a magnetic field. Electrochemical analyzers are also commonly used to measure O2 concentrations, since they are low cost and provide good precision; however, the measuring cell must be changed periodically (one or two times per year). CO2 can be measured reliably with infrared instruments, which are precise and have a long lifetime, though they are expensive and need occasional calibration (Creus 1998e). These analyzers use the capacity that CO2 has to absorb infrared radiation within a characteristic spectrum.

Volatile metabolites. Traditional analytical methods use special resins to trap volatiles from a gas stream (Sunesson et al. 1995) that are then analyzed by gas chromatography coupled with mass spectrometry (GC/MS). For example, Gon-zález-Sepúlveda and Agosin (2000) used this technique to record the evolution of ent-kaurene in the outlet gas stream and relate this with the production of gibberel-lic acid GA3 in a pilot scale SSF reactor. There are also special devices known as "artificial noses" that are able to detect on-line particular chemical compounds in a mixture of gases. Although some applications of artificial noses in SSF processes have been reported (Wang 1993), their use has not spread yet, mainly due to a poor selectivity, slow response, and high sensitivity to environmental conditions (Bellon-Maurel et al. 2003).

Figure 26.1 shows a periodically agitated SSF bioreactor (Fernández 2001) that incorporates various of the monitoring devices discussed above.

Figure 26.1 shows a periodically agitated SSF bioreactor (Fernández 2001) that incorporates various of the monitoring devices discussed above.

Fig. 26.1. Instrumentation of the SSF bioreactor at Pontificia Universidad Católica de Chile. A: six thermocouples (bed temperature); B: thermocouple (inlet air temperature); C: thermocouple (outlet air temperature); D: Relative humidity transmitter (inlet air relative humidity); E: pressure drop transmitter; F: CO2 IR detector (CO2 concentration); G: O2 paramagnetic detector (O2 concentration); H: anemometer

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