The development of automation in fermentation control has been hampered mainly by lack of suitable sensors to monitor and control the events in the fermentor. Normally, only a few parameters are registered and decisions are made based on empirical laws concerning what is really taking place in the fermentor. During the last ten years, development of specific sensors has taken place. Specific electrodes, based on a selective electrode in combination with a preparation of immobilized enzyme, have been developed.’ So far, however, such electrodes have been used mainly in clinical analysis,’ although some effort has been made to make them applicable to fermentation control.* A quite different detection principle is utilized in the enzyme thermi~tor .~ Here, a preparation of immobilized enzyme is placed in close contact with a thermistor. When substrate is fed to the enzyme, product is formed together with the liberation of a small amount of heat. This heat effect, measured as a temperature increase by the thermistor, is used to detect and register the concentration of the substrate. These new sensors, electrodes and thermistors, have been shown to work satisfactorily in the laboratory, but their applicability to real, large-scale fermentations has yet to be demonstrated. When analyzing the events in a fermentation, usually over several days, it is often sufficient to sample intermittently, although in some cases it might be preferable to register continuously. FIGURE 1 shows a schematic drawing of the type of enzyme thermistor now used in our laboratory. A thermostated aluminum cylinder contains the heat-exchanger tubing and provides a constant temperature environment to the enzyme column (0.2-1.0 ml). There are two parallel fluid lines, which could be used either independently or with one of them as a reference system.’ The sample/buffer is pumped through the enzyme thermistor unit with a peristaltic pump at a flow rate of 0.5-2.0 ml/min. The temperature at the outlet of the column is continuously monitored by a thermistor connected to a sensitive Wheatstone bridge. At the highest amplification, the recorder output is 100 mV for a temperature change of O C .