ERP Micro December 2019

Study Design The collaborative study consisted of the evaluation of one matrix, full-cream goat milk cottage cheese (8.4% fat). The matrix was obtained from a local market and screened for the presence of Listeria monocytogenes by the ISO 11290-1/Amd.1 reference method. No natural contamination was observed during the screening, so the cottage cheese was artificially contaminated with L. monocytogenes (1/2 b, raw milk), ISHA culture collection # LIS.4.67 at two levels of contamination: a high inoculation level of ~10 CFU/test portion and a low inoculation level of ~2 CFU/test portion. A set of non-inoculated control test portions (0 CFU/test portion) were also included. Eight replicate samples from each of the three inoculation levels were analyzed by each participant. A total of 48 unpaired samples, 24 for the candidate method and 24 for the ISO 11290-1/Amd.1 reference method, was sent to each participant. Collaborators were also sent a test portion for determining the total aerobic plate count (APC) using ISO 4833-1:2013 (12).

Preparation of Inoculum and Test Portions

The L. monocytogenes strain used in this evaluation was propagated onto Nutrient Agar (NA) from an ISHA (Massy, France) frozen stock culture collection. The isolate was incubated for 24 ± 3 h at 30 ± 1°C. Isolated colonies were picked to 10 mL of Half Fraser broth (HF) and incubated for 24 ± 3 h at 37 ± 1°C. Dilutions (1:10) were prepared in HF for both low and high inoculation levels. The matrix was inoculated, homogenized by hand and packaged for shipment.

Test Portion Distribution

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