The use of capacitance measurement in fermentation monitoring

On-line biomass measurement of fermentations has the potential to facilitate the use of real-time control strategies that can improve fermentation performance. This work investigates the utility of capacitance measurement as a microbial biomass measurement technique, used on-line in fermentation vessels and off-line. The capacitance of a cell suspension increases with the introduction and proliferation of microbial cells by an amount which is directly proportional to the volume bound within the cell membranes of the cells. This means that cell size and viability has a profound effect on the capacitance measurements. Four representative organisms were used in the experimental work: Saccharomyces cerevisiae; Pseudomonas putida: Penicillium chrysogenum and Streptomyces sp. Experiments were carried out monitoring on-line changes in capacitance during a variety of fermentations of these organisms and correlating these with conventional biomass measurements. The variety of scales, modes of fermentation and media composition were used to examine the effects of these on capacitance and its correlations with other measurements. Other experiments were carried out off-line looking at changes in capacitance resulting from homogenisation of cell suspensions. In all fermentations monitored on-line, capacitance was found to increase with increasing biomass. Correlations were found with most others measures of growth, particularly metabolic measures such as Carbon Dioxide Evolution rate. Correlations were also found in the off-line experiments where capacitance accurately detected decreases in cell viability due to homogenisation. Specific capacitance measurements were calculated from the data and found to be consistent for the experiments carried out in this work and very similar to those found by other researchers using similar organisms. In addition, the values found in this work were compared with theoretical values derived from the equations describing capacitance and its relationship to cell size. The values from this work were found to be comparable to the theoretical equivalents calculated. This work shows that on-line capacitance measurement is a good tool for the on-line measurement of biomass and for the off-line measurement of cell viability