A rapid, two-hour method for the enumeration of total viable bacteria in samples from commercial milk powder and whey protein concentrate powder manufacturing plants

A new protocol specifically designed to enumerate total viable bacteria in milk powder using flow cytometry was validated against aerobic plate counts using 178 samples of whole milk powder. Viable counts, as measured using flow cytometry, correlated well (r2>0.80r2>0.80) with plate counts across the range of powders with counts of 102–108 cfu g−1. The protocol was then used to enumerate total viable bacteria in samples of milk and whey collected at different stages during the manufacture of milk powder and whey protein concentrate. Flow cytometer results for samples taken during both processes were similar to those from the aerobic plate counts. However, the flow cytometer results were obtained within 2 h compared with 3 days for the plate counts. The strategic advantage of the close-to-real-time data obtained using flow cytometry is that results can be relayed back quickly to the management of the manufacturing plant for use in decision-making with respect to controlling the manufacturing process.