Analyst Variation in Doing the Standard Plate Count as Described in Standard Methods for the Examination of Dairy Products 1, 2.

Five analysts participated in a study to evaluate the following aspects of the 13th edition of Standard Methods for the Examination of Dairy Products (SMEDP): (a) analyst variation in overall Standard Plate Counts (SPC), and (b) analyst duplication of bacterial colony counts on agar places. Each analyst prepared 24 samples of pasteurized, homogenized milk during a successive 8-day period (i.e., 3 samples/day), and then the analysts estimated the numbers of bacterial colonies for these, as well as other analysts' plates, initially after 48 h of incubation, and then 1 h later and 24 h later. Statistically significant differences in colony enumerations were found between analysts in preparation of agar plates on 3 days. Significant differences were also noted between analysts for bacterial counts of agar plates. Mean bacterial estimates of certain analysts ranged between 565 and 948, and fluctuated greatly between the initial, 1-h, and 24-h determinations. These results indicate that the "standards of accuracy" currently specified in SMEDP are not realistic, i.e., (a) among-analyst variation of 18.2% compared to 10%, and (b) within-analyst variation of 7.7% compared to 5% in SMEDP.

A Collaborative Study to Determine the Feasibility of Using 0.40-, 0.20-, 0.14-, and 0.10-Inch-Diameter Discs to Measure Sediment in Fluid Milk.

This collaborative study was done to determine whether laboratory personnel could successfully grade raw milk for sediment content using 0.40-, 0.20-, 0.14-, and 0.10-inch-diameter sediment discs. The 0.40-inch disc presently is accepted for grading sediment in mixed bulk milk samples. Technicians in 17 separate laboratories made 1,360 determinations (80 per laboratory) or 20 determinations for each size of sediment disc. Each laboratory graded the same set of samples. Samples were graded using photoprint standards that were prepared for this study. The laboratories were evaluated on the ability of laboratory technicians to grade the various sizes of sediment discs. Technicians in 13 of the 17 laboratories graded samples showing no significant differences in their ability to grade the various sized sediment discs. Four laboratories were not consistent in their ability to grade discs and showed significant differences in agreements with the previously determined sediment level as the diameter of the disc was reduced. Based on these results, most laboratories evaluated were capable of grading sediment discs with diameters of 0.40-, 0.20-, and 0.14-inch with consistency. With properly trained personnel in the laboratories, the study indicates the 0.10-inch diameter disc also can be used effectively for grading sediment in milk.

Plate Count Accuracy: Analysts and Automatic Colony Counter Versus a True Count 1.

Colony counts obtained by (a) analysts and (b) an automatic colony counter (ACC) were compared to a true count obtained through use of photographs. Factors which caused counting dificulties with the ACC were identified. When plates were properly screened, ACC counts were as accurate as those obtained manually; therefore, it is recommended that further study of ACCs be conducted so that consideration may be given to their use in the forthcoming edition of Standard Methods for the Examination of Dairy Products.