Efficacy of Organic Acids to Control Listeria monocytogenes on Hard-Cooked Eggs during Refrigerated Storage

Peeled hard-cooked eggs (HCEs) are ready-to-eat foods susceptible to Listeria monocytogenes contamination during post-processing. A 2019 multistate listeriosis outbreak linked to HCEs treated with 0.3% citric acid highlighted the need for effective antimicrobial treatment. This study evaluated the efficacy of organic acid treatments (0.3% and 2% citric, and 2% acetic, lactic, and malic acids) at 5°C and 25°C against low-level contamination (1 log CFU/HCE) on HCEs during 28-day refrigerated storage (5°C). Two contamination scenarios were assessed: inoculation prior to acid treatment (simulating processing contamination) and inoculation after acid treatment (simulating post-treatment cross-contamination). For pre-treatment contamination, 2% acetic acid at 5°C achieved complete elimination of detectable L. monocytogenes by 3 d, while 2% malic acid demonstrated 0-1/9 positive samples from 7 to 28 days. In contrast, 0.3% citric acid and 2% lactic acid were least effective, maintaining ≥3/9 positive samples throughout storage. For post-treatment contamination, only 2% citric acid achieved elimination of detectable L. monocytogenes (by 14 days at 5°C and 7 days at 25°C). The pH of HCE components decreased significantly during treatment with the 2% organic acids, with 2% citric acid achieving the lowest albumen pH (3.13-3.69). These results demonstrate that organic acid effectiveness against L. monocytogenes on HCEs is dependent on acid type, concentration, and whether the contamination occurs pre- or post-processing. Data from this study can be used to better understand the survival of L. monocytogenes on HCEs treated with organic acids to ensure the safety of this food product.