Purpose: The purpose of this evaluation was to perform inclusivity/exclusivity and an inter-laboratory comparison of the new method in comparison to the reference methods, VRB agar with BGLB confirmation and alternatively AOAC 989.1, in different dairy products.
Methods: Methods were evaluated at 32°C for inclusivity using 56 coliform strains and exclusivity study using 31 non-coliform strains. Heavy cream (40% fat) and whole (3.3% fat), chocolate (2% fat), and skim (1%) milks were inoculated with different heat stressed coliform and E. coli at 5 different contamination levels. Five laboratories received 5 replicates from each contamination level. All samples were tested in duplicate.
Results: Peel Plate EC detected 55 of 56 coliform strains, compared to 41 by AOAC 989.1, in inclusivity study. In exclusivity, the new method excluded 28 of 31 compared to 26 by AOAC 989.1. For the method comparison, results were evaluated for repeatability (Sr) and by paired t-test for statistical difference, > 0.5 log. In whole milk, chocolate milk, and cream the Peel Plate EC was not significantly different from the reference method; while, with whole milk and chocolate the AOAC 989.1 showed some statistically lower results. Peel Plate EC with skim at the 3 highest contamination levels were statistically lower from reference methods. Replicate pairs from new method had similar or better Sr than the reference. All E.coli tested were detected at 32°C but color differentiation was strain and dairy matrix dependent; thus E. coli in dairy matrices is only reliably scored as coliform.
Significance: Peel Plate EC demonstrates high selectivity with easy total coliform interpretation in dairy matrices at 24 h.