Contactless Electrical Sensor Based on Resonance Frequency for Real-Time Monitoring of Bacterial Growth

An electrical sensor based on a capacitively coupled contactless resonance frequency detector (RFD) was developed for real-time monitoring of bacterial growth. The electrical bacterial growth sensor (EBGS) uses capacitively coupled electrodes to a glass tube containing the sample. This configuration makes it possible to detect variations in the resonance frequency caused by biochemical changes in the sample. The circuit induces an oscillation at the resonant frequency of an inductor-capacitor circuit coupled to the sample. The frequency is measured using a simple digital frequency meter without requiring an analog-to-digital converter. The proposed EBGS-RFD sensor presented a high gain in signal magnitude by operating at a resonance frequency compared to the EBGS sensor, which used a fixed frequency. Thus, a better signal-to-noise ratio was obtained and allowed the acquisition of high-quality electrical data. Furthermore, the proposed EBGS-RFD presented a straightforward electronic design compared to other EBGS. The proposed sensor requires fewer electronic components and eliminates the use of high-bandwidth components, such as operational amplifiers. The EBGS-RFD was evaluated for total bacterial count (TBC) using fifty-six raw milk samples. The β value representing the inflection points on the characteristic bacterial growth curves was obtained by the EBGS-RFD. From the significant correlation between β and TBC, obtained by the standard plate count (SPC), it was possible to cheaply and simply determine the TBC at various contamination levels in only 8 hours. Therefore, EBGS-RFD is suitable for TBC and real-time monitoring of bacterial growth in raw milk samples.