Recovery of microorganisms by Standard Methods Agar and Eugonagar was compared at 7, 21, 28, and 32C incubation temperatures for 10, 5, 4, and 2 days, respectively, from manufacturing-grade, grade-A, and pasteurized milk. The incubation temperature made a statistically significant difference in the mean logarithm of the count for all three grades of milk. The highest mean logarithm of the count with manufacturing-grade milk was obtained at 21 C for both agars. With grade-A raw milk, the greatest mean logarithm of the count was obtained at 21 C for Standard Methods Agar and at 28 C for Eugonagar. The highest mean logarithm of the count for pasteurized milk was obtained at 28 C with Standard Methods Agar and at 21 C with Eugonagar. There was a significant difference between means on Standard Methods Agar and Eugonagar on grade-A milk samples only; recovery was highest with Eugonagar.
Because of the presence of psychrophilic microorganisms, an incubation temperature lower than 32 C is needed for maximum recovery. Incubation at 28 C for 4 days was the optimum temperature-time combination in this study.
1Journal Paper No. J-6029 of the Iowa Agriculture and Home Economics Experiment Station, Ames, Iowa. Project No. 1050.