Methods of Assessing the Psychrotrophic Bacterial Content of Milk

Summary The various methods used for the determination of the psychrotrophic bacterial content of refrigerated milk are reviewed and assessed. A standard reference method, involving incubation of poured plates at 7° for 10 days, is recommended for the determination of psychrotrophic colony counts for investigational purposes. Many ingenious techniques, based on media selective for Gram negative bacteria, with incubation at higher temperatures and shorter times are described and appraised. A simple procedure which would provide a more rapid assessment of the number of active milk spoilage types of psychrotrophs is needed for routine purposes, and it is suggested that the enumeration of microcolonies by the drop plate or membrane filter methods with incubation at 7° for 72 h would provide a suitable technique for this purpose. Incubation of plates at 25° for 5 days is recommended as a standard reference method for determining the total colony count of bacteria in refrigerated milk, but for routine purposes where large numbers of milk samples are being examined incubation at 28–30° for 3–4 days would provide a sound enough estimate.