A Comparison of Roll‐tube and Petri Dish Colony Counts on Raw Milk

SUMMARY: A co‐ordinated experiment was carried out at two centres, in which colony counts from raw milk samples determined by the roll‐tube and Petri dish methods were compared. A bulk medium was standardized for both laboratories, as also were details of technique. Statistical analyses showed that roll‐tube counts were generally lower than the corresponding Petri dish counts, but the difference varied considerably from milk to milk. The variation between replicate sub‐samples was about the same for both methods. Experience and the results both indicate that roll‐tubes were slightly more difficult to count than Petri dishes.