On‐Line Measurement of Culture Fluorescence for Process Monitoring and Control of Biotechnological Processes

In bioreactors, metabolically active cells are surrounded by a changing chemical and physical environment. To control such a complex system, the relationship between cells and their environment must be known. For this purpose, information on the regulation process within the cells and between the cells and their environment is important. The cell characteristics, the medium composition, and the physical environment (especially the fluid dynamics of the multiphase system) have to be monitored continuously. Unfortunately, there is still a lack in instrumentation and sensors for continuous monitoring of cell properties during cultivation. The only continuous monitoring technique providing information on the metabolic state of the cells is in situ fluorometry with a miniaturized fluorometer probe. In 1957, Duysens and Amesz first reported on the measurement of one of the key cell metabolites-the nucleotide NAD(P)H-in living microorganisms.’ For this purpose, cell cultures can be irradiated with UV-light, and the NAD(P)H fluorescence light can be detected. In 1970, a special fluorometer was used by Harrison and Chance for the first time to monitor biotechnological processes.’ During the following years, only a few authors reported about applications of this measuring technique in biotechn~logy.~’ In 198 1, Beyeler developed a miniaturized fluorosensor,8 and during the next few years, the interest in this technique increased more and The on-line measurement of NAD(P)H-dependent culture fluorescence gives a direct insight into the microorganisms during cultivation. By adding special tracers to the cultivation medium, fluorometry can also be used for studying the mixing-time behavior in bioreactors. In order to study the culture fluorescence during mixing-time experiments, a miniaturized fluorometer with a special fiber optics for the simultaneous detection of two different wavelengths was constructed and applied to process monitoring and mixing-time experiments. In addition, a commercially available microfluorometer was studied for the measurements of culture fluorescence (Zymomonas mobilis) .