Purpose: Selectivity and growth promotion of Listeria monocytogenes in autoclaved Granucult® and non-autoclaved, gamma-irradiated, Readybag® Half Fraser broths were tested to demonstrate that irradiation has no influence, including cold room storage of primary enriched food samples for up to three days, as permitted by the new EN ISO/DIS 11290-1:2014.
Methods: For growth promotion four L. monocytogenes strains were incubated in both media preparations for 22h and 26h and two food matrices (prawns, cheese) were spiked with two L. monocytogenes food isolates. A 72h cold room storage step was included. Results were confirmed by Singlepath®L’mono immunoassay. Prepared media were stored up to 96 hours at 5°C and 25°C. Productivity and selectivity were tested at different time points.
Results: Both media preparations provided similar growth rates with all four L. monocytogenes strains after 22h and 26 h incubation (inoculation 10 cfu/250ml, total 40 samples). The cfu/ml slightly increased during cold room storage as Listeria are able to grow at low temperatures. Low level spiking (1 cfu/25g, 40 samples) of cheese and prawns showed comparable results, as calculated by Fisher's extract test. Productivity and selectivity were acceptable up to 96 h at 5°C and 25°C, during stability studies.
Significance: Irradiation of Readybag® medium has no significant influence on the growth promotion of Listeria, as verified in food trials and after cold room storage, which is in accordance with EN ISO/DIS 11290-1:2014. The avoidance of autoclaving and a media storage option reduces the food testing process and test costs.