On-line monitoring of dielectrical properties of yeast cells during a stress-model alcoholic fermentation

This paper reports the use of on-line dielectric spectroscopy to monitor the effect of a technological stress on Saccharomyces cerevisiae during an alcoholic fermentation. The dielectric spectrum (β dispersion) of a cell suspension depends on various variables related to cell state: cell concentration, cell size and cell viability as well as the states of the plasma membrane (membrane capacitance) and the cytoplasm (intracellular conductivity). Additional measurement methods (cell size, cell viability and cell counts) have been developed to identify the variables that describe the β dispersion and extract membrane capacitance and intracellular conductivity values. All measurements confidence intervals have been estimated in order to appraise the significance of the measured variations.In all the studied culture conditions, the amplitude of the β dispersion gave a reliable estimate of the viable cell volume fraction. The membrane capacitance value remained constant as long as the cells were viable (Cm = 0.6 μF cm−2) and dropped to zero at cell death, while the intracellular conductivity varied significantly depending on the growth phase and in response to stress (2 < σin < 3 mS cm−1). Hypotheses are proposed to explain the evolution of intracellular conductivity by intracellular pH changes and synthesis/consumption of reserve substances such as trehalose.