Radio-frequency and microwave spectroscopy investigation of bacteria solutions: determination of the aggregation threshold

Abstract

This paper presents novel approaches to assess and characterise the biomass concentration of bacteria in a broth culture medium. It is based on the use of the colloid inductive permittivity probe, which is not affected by the electrode polarisation, and on the analysis of the distribution of 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 to 20 GHz] ranges. Moreover, due to the direct measurement over wide concentration ranges, the concentration threshold of the bacteria aggregation can be precisely identified. As a matter of fact, the behaviour of the amplitude of the medium-frequency relaxation process shows two distinct linear evolutions with an intersection corresponding to the aggregation threshold. The dielectric method can be easily adapted to the online monitoring of the growth of biological cells and to the control of the fermentation processes.