Rapid enumeration of Bifidobacterium lactis in milk powders using impedance

An impedance method was evaluated to enumerate Bifidobacterium lactis, a probiotic, added to milk powder. Impedance changes were measured at 40 °C and recorded using the BacTrac™ 4100 microorganism growth analyser. Five different media were compared for the optimum impedance response. A raffinose-based medium (B. lactis medium) produced the fastest and most reproducible results. Good correlations were obtained between cell numbers from pure cultures of B. lactis (DR 10™) on reinforced clostridial agar plates and the impedance changes in the B. lactis medium. Enumeration of these bacteria in milk powder using the BacTrac™ 4100 impedance system showed no significant difference when compared with agar plate count results. The impedance counts estimated the cell counts of 106 cells g−1 within 15 h and was faster than the 3 days required to obtain a result using the agar plate count method.