Monitoring lactobacillus bulgaricus growth in yoghurt by electrical impedance

Abstract

Characterization of different Lactobacillus bacterial strains has an increasing demand due to their potential health benefits. Classical cell counting methods are time-consuming and require well trained laboratory work, besides they are having higher subjectivity and error. The determination of electrical impedance spectrum is a promising method for the evaluation of changes in chemical structure and cell growth during the formation of yoghurt. Our aim is to use electrical impedance spectrum for the monitoring of cell growth of LABs. In this study different strains of Lactobacillus bulgaricus bacteria were used for the production of yoghurt from ultrahigh temperature processed (UHT) milk of 1.5% fat content. The cell count of the LABs was determined with plating method on MRS (De Man, Rogosa and Sharpe) agar in each hour during 12 h of cultivation at 37 °C. For impedance measurements, the real part (conductance) and the imaginary part (susceptance) of admittance (reciprocal value of electrical impedance) were recorded through 12 h at 37 °C in frequency range from 50 Hz up to 800 kHz with a HP 4284A precision LCR meter. Results showed a bacteria cell number increase from magnitude 103 to 107 (CFU/milliliters) as with conductance and the susceptance. Regression models could also accurately predict bacterial cell count. Impedance measurement during the formation of yoghurt can be useful in monitoring Lactobacillus growth and determination of cell number, when exact methodologies and setups are applied.