2021 FDA Science Forum
Fate of Listeria Monocytogenes on Citric Acid-treated Hard-Cooked Eggs
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Contributing OfficeOffice of the Commissioner
Abstract
Commercially-prepared hard-cooked eggs are available for food service and to the public in retail grocers. Potential contamination with Listeria monocytogenes after the cooking and peeling steps is of concern since this pathogen can proliferate at refrigeration temperatures. This study evaluated the efficacy of citric acid treatment of hard-cooked peeled eggs to reduce the population levels of L. monocytogenes. Fresh eggs were boiled for 12 min, cooled to 4°C, peeled, and stored at 4°C for 24 h prior to experiments. Hard-cooked eggs were dip inoculated with a 4-strain cocktail of rifampicin-resistant L. monocytogenes resulting in either 4 (low) or 8 (high) log CFU/egg. Eggs were air dried 10 min, followed by treatment with pH 2.5 citric acid (2.6 g/L) at 5 or 25°C for 24 h. L. monocytogenes populations were enumerated at intervals up to 24 h by homogenization of eggs with BLEB and cultivation on BHI/rifampicin agar. Triplicate eggs were assessed for each timepoint and three independent trials were conducted. At a low inoculation level, the L. monocytogenes population on the eggs was 3.95±0.69 and 3.74±0.76 log CFU/egg prior to treatment with 5 or 25°C citric acid, respectively. After 1 h, L. monocytogenes was reduced by 0.56 and 0.18 log CFU/egg when treated at 5 or 25°C, respectively; after 24 h, reductions were 0.73 and 0.86 log CFU/egg. At a high inoculation level, the L. monocytogenes population on the eggs was 7.93±0.26 and 8.04±0.47 log CFU/egg prior to treatment with 5 or 25°C citric acid, respectively. After 1 h, L. monocytogenes was reduced by 0.48 and 0.67 log CFU/egg when treated at 5 or 25°C, respectively; after 24 h, reductions were 1.23 and 1.16 log CFU/egg. The data obtained in this study can be used to determine the efficacy of citric acid against L. monocytogenes during treatment of hard cooked eggs.
