Effect of Various Combinations of Medium, Diluent and Incubation Conditions on Recovery of Bacteria from Manufacturing Grade and Grade A Raw Milk1

Abstract

Recovery of microorganisms from manufacturing grade and Grade A raw milk was determined using 18 plating combinations which consisted of three media, three diluents, and two incubation temperatures. Plating conditions specified in Standard Methods for doing the Standard Plate Count was one of the 18 combinations used. Combinations studied consisted of Standard Methods Agar, Schaedler Agar, and Eugonagar as plating medium; phosphate buffered distilled water, 0.1% peptone water, and Ringer solution as diluent; and 28 C for 72 h and 32 C for 48 h as incubation temperature. Forty manufacturing grade and 40 Grade A raw milk samples were plated using each of the 18 combinations. Highest mean counts were obtained for both grades of milk with the combination of Standard Methods Agar, phosphate buffered distilled water, and 28 C for 72 h. Samples, diluents, media, and samples × diluents interaction had a highly significant (P<.01) effect on counts of manufacturing grade milk samples; while samples, media, and temperatures had a highly significant (P<.01) effect on counts of the Grade A raw milk samples. Nonsignificant differences were obtained in counts of the manufacturing gradesamples with eight of 17 plating combinations when compared with counts obtained with Standard Methods. Counts for Grade A samples obtained with six of 17 combinations were similar to counts obtained with Standard Methods.