Rapid detection of bacterial proliferation in food samples using microchannel impedance measurements at multiple frequencies

We present a novel method for detecting viable bacteria in suspensions such as milk and apple juice. Underlying the technique is the fact that bacteria in aqueous suspensions can store a large amount of charge, and thus act like (non-ideal) capacitors. Thus increased numbers of bacteria due to proliferation increases the capacitance of the bulk of the suspension. However, this increase cannot be directly measured since the capacitance of the solid-liquid interface ("double layer") in effect "screens" the latter. We present a method (derived from an earlier one) that is able to discern such changes with high sensitivity and robustness. We also demonstrate its ability to monitor food quality/safety by detecting bacterial proliferation in "real world" liquid food samples like milk and apple juice. We are able to detect ~ 1,10, 100, and 1000 CFU/mL of E. coli in milk in about 4.5, 3,2, and 0.5 h, respectively. For the same initial loads, the corresponding times to detection (TTDs) for Lactobacillus in apple juice are approximately 8,6,4, and 1 h. These represent a greater than 4-fold reduction in TTD when compared to automated systems on the market such as RABIT, Bactometer etc. We can achieve such low TTDs for low initial loads since, due to the much greater effective charge holding capacity of bacterial cells (compared to surrounding media), we are able to detect a change in the overall bulk capacitance of the suspension as the bacterial numbers cross a threshold of around 500 CFU/mL. © 2010 Springer Science+Business Media, LLC.