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

Abstract

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): analyst variation in overall Standard Plate Counts (SPC), and analyst duplication of bacterial colony counts on agar plates. 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, and 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 565-948, and fluctuated greatly between the initial, 1 h and 24 h determinations. The standards of accuracy currently specified in SMEDP are apparently not realistic, i.e., among-analyst variation of 18.2% compared to 10%, and within-analyst variation of 7.7% compared to 5% in SMEDP.