Comparison Between Standard Methods Procedure and a Surface Plate Method for Estimating Psychrophilic Bacteria in Milk1

Summary Psychrophilic population levels were determined by Standard Methods procedure (1) and a surface plate method. Thirty-one samples of raw milk, 47 samples of commercial pasteurized milk, and 94 samples of milk containing pure cultures of selected psychrophiles were used in the comparisons. A statistical evaluation of the results indicated counts obtained by the surface plate method after incubation of the plates for five days at 6C were equivalent to counts obtained by the standard procedure after incubation for 7–8 days at 6C. Colonies on the surface plates were larger, more uniform in size, and consequently easier to count than those on poured plates. Also, enhanced pigmentation and more characteristic colonial morphology was observed with surface plates. Approximately 80% of the pinpoint colonies appearing on the poured plates after 8 days and on the surface plates after 7 days of incubation at 6C were Gram-positive cocci indicating the emergence of colonies of organisms not considered to be psychrophilic species important to the shelf life of milk.