Challenge studies with Listeria monocytogenes and proteolytic Clostridium botulinum in hard-boiled eggs packaged under modified atmospheres

Studies were conducted to determine the sensory shelf-life and microbiological safety of hard-boiled eggs packaged under various gas atmospheres, challenged with Listeria monocytogenes and proteolytic strains of Clostridium botulinum and stored at various temperatures. Growth of L. monocytogenes occurred in all inoculated egg samples stored at 4°C, 8°C, and 12°C with counts increasing from ∼10 2 to >10 6 cfu g −1 after 3–20 days, depending on the packaging atmosphere and storage temperature. Growth of L. monocytogenes to these levels either preceeded, or occurred simultaneously with, spoilage. These results showed that packaging eggs, even under elevated CO 2 concentrations (80%), had a limited inhibitory effect on the growth of this psychrotrophic pathogen. In challenge studies with proteolytic strains of C. botulinum, botulinum neurotoxin was not detected in any samples after 21 days at 12°C. However, neurotoxin was detected in all inoculated eggs that had been initially stored at 12°C, then transferred to 25°C for a further 7 days, regardless of the packaging atmosphere. Spoilage preceded toxigenesis in all cases and toxin was not detected in any uninoculated samples. This study has shown that modified atmosphere packaging cannot be regarded as an adequate barrier to control the growth of L. monocytogenes in hard-boiled eggs, even at refrigerated storage temperatures. However, refrigerated temperatures were essential to extend the sensory shelf-life and prevent the growth of proteolytic strains of C. botulinum in these packaged products.