On-line measurement of biomass using colloid dielectric probe and open-ended cell. Determination of the aggregation threshold

This paper presents a new approach to assess the biomass concentration of bacteria in a culture medium. It is based on the use of the colloid inductive permittivity probe, which is not affected by the electrode polarization, complemented by the traditional open-ended coaxial line measuring method. The method probes the interactions between the membranes of the cells and the applied electric fields in the RF and the Microwave frequency ranges. It allows the characterization of each type of cells with a specific relaxation process. The specific relaxation frequency is used as a fingerprint of the cell strain, while the relaxation amplitude is related to its concentration. By using appropriate and complementary de-convolution techniques, it is possible to identify reliably the elementary relaxation processes. Specifically, the frequency-dependent dielectric responses of Lactobacilli in DMEM are investigated in the [75 KHz–30 MHz] and in the [130 MHz–20 GHz] ranges. The method is tested against biological standard methods, and used successfully to predict concentration of Lactobacilli in adverse conditions. Moreover, because of the direct measurement over large concentration ranges, the concentration threshold of the bacteria aggregation can be precisely identified from the behavior of the amplitude of the specific medium-frequency relaxation process, characterized by a transition from a linear evolution to a second straight line with a different slope. The method can be easily adapted to the on-line monitoring of the growth of biological cells and to the control of the fermentation processes.