Fate of Listeria monocytogenes on Chopped Citric Acid-Treated Hard-Cooked Eggs during Storage.

A multistate outbreak of Listeria monocytogenes linked to commercially prepared peeled hard-cooked eggs (HCEs) occurred in the U.S. in 2019. HCEs are commonly treated with citric acid to extend shelf life and inhibit post-contamination bacterial growth. However, no information is currently available on the efficacy of citric acid treatment to control L. monocytogenes when HCEs are chopped and stored. This study evaluated the survival and growth of L. monocytogenes on chopped HCEs treated previously with 2% citric acid or water during storage at 5, 10, or 25°C for up to 14 days. Whole HCEs were treated with citric acid or water for 24 h, inoculated with L. monocytogenes at approximately 4 log CFU/HCE, chopped, and stored. Citric acid treatment maintained chopped HCE pH at 4.11 to 4.44 throughout storage, while water-treated chopped HCEs had higher pHs (6.1 to 7.9). At 5°C, no significant increase in populations of L. monocytogenes was detected on citric acid-treated chopped HCEs. Water-treated eggs at 5°C supported slow growth (0.08 log CFU/g per day; a 1 log CFU/g increase in 12.5 days). At 10 and 25°C, citric acid treatment reduced L. monocytogenes growth rates on chopped HCEs, compared to the water controls (0.54 vs 1.03 log CFU/g per day at 10°C; 1.40 vs 4.38 log CFU/g per day at 25°C). These results demonstrate that 2% citric acid treatment prevented significant increases in populations of L. monocytogenes on chopped HCEs during refrigerated storage, and that temperature control remains critical for safety.