NATURE AND GROWTH RESPONSE OF THE MICROFLORA OF PASTEURIZED, PACKAGED MILK1

Abstract

Summary The microflora of freshly pasteurized, packaged milk is heterogeneous, and the numbers are generally low. While it is commonly assumed all bacteria are included in assays of numbers with plate count agar and standard methods, under normal conditions few are able to grow and contribute significantly to spoilage. Post-pasteurization contamination, which contributes insignificantly to the total count on freshly pasteurized, packaged milk, contributes most of the bacteria that are capable of growth to cause spoilage during subsequent storage. Though there is a delay after pasteurization before significant bacterial growth takes place, the same group of bacteria is responsible at either 5 C or 32 C. The growth response of these bacteria was measured with a selective medium of nutrient agar containing alkyl aryl sulfonate. Data obtained by the use of the selective medium indicated a potentially useful approach to quality control.