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*Reliable sensors are not available.

*Reliable sensors are not available.

Desschema Probleml Sning
Figure 6. Estimation of metabolic parameters using gateway sensor.

The data analysis scheme of Fig. 6 includes the steady-state oxygen balance method and the carbon balancing method. In addition, the system can provide the oxygen supply conditions that relate to volumetric oxygen transfer coefficient (kLa), oxidation-reduction potential (ORP) and degree of oxygen saturation Qo2X/(Qo2X)max. For the data analysis scheme of Fig. 6, the most significant advance in the fermentation field has been the development of steam sterilization, dissolved oxygen electrodes and the application of mass spectrometry to the exhaust gas analysis. Dissolved oxygen probes can be classified as either potentiometric (galvanic) or amperometric (polarographic). These electrodes are covered with a gas-permeable membrane; an electrolyte is included between the membrane and the cathode. It should be noted that these probes can measure the oxygen tension but not the concentration. The signal from both models of electrodes often drifts with time for long continuous measurements. Calibration then becomes difficult because of possible contamination. Most commercial probes have a vent to balance the pressure between the inside and outside of the probe. Often, the broth and electrolyte mix through the vent causing signal drift and rapid reduction in probe life. Therefore, fiber-optic chemical sensors such as pH, dissolved oxygen and carbon dioxide electrodes which need pressure compensation interference by medium components, drift and so on. This type of sensor is based on the interaction of light with a selective indicator at the waveguide surface of optical fiber. Fiber-optic sensors do not suffer from electromagnetic interferences. Also, these can be miniaturized and multiplexed, internally calibrated, steam-sterilized and can transmit light over long distances with actually no signal loss as well as no delayed time of the response. At present, a key factor for these sensors is to avoid the photodecomposition of the dyes during longtime measurements. Generally, the majority of measurements on oxygen uptake (Qo2X) have been made with a paramagnetic oxygen analyzer while those on carbon dioxide evolution rate (Qco2X) have been made with an infrared carbon dioxide analyzer.

Gateway sensors have become quite widespread in use in fermentation processes at both the pilot and plant levels. The sample's gas has to be dried by passing through a condenser prior to the exhaust gas analysis to avoid the influence of water vapor on the analyzers. Except for bakers' yeast production, few studies have been reported documenting the application of the steady-state oxygen balance method to the process control of fermentation processes in pilot and production plants. Recently the industrial use of this method has been published for the fed-batch process of glutathione fermentation. Based on the overall oxygen uptake rate Qo2XV and the exit ethanol concentration, the feed-forward/feedback control system of sugar feed rate has been developed to successfully attain the maximum accumulation of glutathione in the broth on the production scale (Fig. 7). In the figure, the feed-forward control of sugar cane molasses feeding was made with total oxygen uptake rate Qo2XV and the sugar supply model which is based on the oxygen balance for both sugar and ethanol consumptions. In this system, oxygen, carbon dioxide and ethanol in outlet gas were measured on-line with a paramagnetic oxygen analyzer and two infrared gas analyzers as "gate way" sensors for a 120-kl production fermenter. Oxygen and ethanol concentration in outlet gas at the pilot level was continuously monitored with the sensor system consisting of two semiconductors. For the feedback control, a PID controller was used to compensate for a deviation, e, from a present ethanol concentration, Eset, calculated by the ethanol consumption rate model. Based on the deviation e, a deviation AFfrom the set-point feed rate Fcan be calculated as shown in Fig. 7. The performance of this system was found to be very good using a YEWPACK Package Instrumentation System (Yokogawa Electric Corporation, Tokyo) and a 120-kl production fermenter (Fig. 8). The results, an average of 40% improvement of glutathione accumulation in the broth was attained, were compared with a conventionally exponential feeding of sugar cane molasses.

Figure 7. Configuration of process control system for glutathione fermentation. ♦The feed rate F can be calculated from the oxygen balance for sugar and ethanol consumption in the broth. **The optimal ethanol consumption profile is obtained for a constant consumption rate.

Figure 7. Configuration of process control system for glutathione fermentation. ♦The feed rate F can be calculated from the oxygen balance for sugar and ethanol consumption in the broth. **The optimal ethanol consumption profile is obtained for a constant consumption rate.

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