Commercially-prepared hard-cooked eggs are available for foodservice and to the public in retail grocers. Potential contamination with... Show moreCommercially-prepared hard-cooked eggs are available for foodservice and to the public in retail grocers. Potential contamination with Listeria monocytogenes during or after the cooking and peeling steps is of concern since this pathogen can proliferate at refrigeration temperatures. Citric acid is a common preservative used in the food industry to treat hard-boiled eggs (HBEs). The purpose of this project was to evaluate the efficacy of citric acid treatment of HBEs to reduce the population levels of L. monocytogenes during 24 h (treatment trials) and 28 d storage (storage trials) at 5 or 25°C. Fresh eggs were boiled for 12 min, cooled to 4°C, peeled, and stored at 5°C for 24 h prior to experiments. In treatment trials, HBEs were dip inoculated with a 4-strain cocktail of rifampicin-resistant L. monocytogenes resulting in either 4 (low) or 7 (high) log CFU/egg. Eggs were air-dried 10 min, followed by treatment with pH 2.5 citric acid (PHCA) or 0.2 M citric acid (calculated as the molarity resulting in pH 2.5: MCA) at 5 or 25°C for 24 h. In treatment-storage trials, citric acid treatment of HBEs occurred before or after inoculation, followed by 28-d storage at 5 or 25°C. L. monocytogenes populations were enumerated by homogenization of eggs with BLEB and cultivation on BHI/rifampicin agar. Enrichment in BLEB was conducted if the pathogen was below the level of enumeration. Significant differences in the populations of L. monocytogenes due to temperature of the acid treatment (5 or 25°C) or the two citric acids (MCA and PHCA) were determined using Student’s T-test and ANOVA with Tukey’s post-test, p ≤ 0.05. Overall, the largest L. monocytogenes reduction occurred after 6 h treatment of HBEs with PHCA at 25°C (1.59 ± 0.00 log CFU/egg) and after 24 h with MCA at 5°C (1.23 ± 0.54 log CFU/egg) when the pathogen was inoculated at the low and high levels, respectively. In treatment-storage trials, citric acid treatment after HBE contamination resulted in a fewer number of samples where the pathogen was detected compared to when treatment occurred before contamination. Citric acid treatment for 24 h also resulted in a greater number of samples where L. monocytogenes was not detected than the 1 h treatment. The results of this study determined that L. monocytogenes could survive on HBEs treated with citric acid, regardless of treatment or storage temperature and acid concentration (PHCA or MCA). Show less
