Purpose: To increase Lm product test portion size for certain sampling projects in order to enhance public health protection and achieve consistency with Codex Alimentarius Lmtesting guidelines. This also will permit sample compositing.
Methods: The Midwestern Laboratory spiked samples of cooked turkey with Lm at fractional recovery levels. Five sample sets were used, each containing equal numbers of 25 g and 125 g samples spiked randomly. Ninety samples were analyzed at each weight. Each sample was analyzed individually after 20, 22, and 24 hours of primary enrichment incubation using Microbiological Lab Guidebook methodology. All samples were analyzed both culturally and by PCR screen. Reference method samples with fractional recovery between 20% and 80% constituted a valid set.
Results: Of the ninety 125 g samples, 27, 37 and 42 were positive at 20, 22 and 24 hours, respectively, whereas 36 of the 25g samples were positive at all 3 time points. Differences in the proportions of positive samples for 25 g vs. 125 g at 24 h were not significant (Pearson Chi-Square, 2-sided, P = 0.37). At 22 hours, a statistically significant difference in the proportions of detected positive samples for 25 g (36/36) vs. that for 125 g (37/42) was observed (Fisher’s exact test, 1–sided, P = 0.04).
Significance: Five 25 g RTE samples can be composited into a single 125 g sample with little loss of recovery, using a slightly longer incubation time. This allows FSIS to demonstrate method equivalency to its trading partners. Sample compositing allows for significant savings of laboratory resources without compromising method sensitivity. This provides an opportunity to detect more Lm-contaminated product.