Growth and survival kinetics of Listeria monocytogenes in cooked egg whites

- Growth and survival of Listeria monocytogenes in cooked egg whites were investigated.
- L. monocytogenes grew very well in samples cooked at 100 °C, but not at 70 and 80 °C.
- Growth and survival models were developed for risk assessments.
- The results of this study may be used to design new thermal processes for cooking ready-to-eat hard-boiled eggs.

Peeled hard-boiled eggs (HBE) are ready-to-eat products susceptible to surface contamination by Listeria monocytogenes. This study investigated the growth and survival of L. monocytogenes between 4 and 43 °C in egg whites cooked under different conditions (70 °C for 15 min, 80 °C for 20 min, and 100 °C for 10 min). L. monocytogenes inoculated to samples cooked at 100 °C could grow uninhibitedly between 4 and 40 °C, exhibiting no lag phases, but failed to grow at 43 °C. The growth process was described by a 3-parameter logistic primary model, with the specific growth rates fitted equally well to the Ratkowsky square-root and Cardinal models. According to the Ratkowsky square-root model, the estimated minimum (nominal) and maximum growth temperatures were −0.3 and 47 °C, which were 1.6 and 44.3 °C, respectively, according to the Cardinal model.L. monocytogenes did not grow well when inoculated to egg white samples cooked at 70 and 80 °C. Images of scanning electron microscopy showed that L. monocytogenes was damaged in samples cooked at these temperatures. Although experiencing a <2 log cfu/g initial growth, L. monocytogenes was inhibited in these samples at all storage temperatures, probably due to the antimicrobial activities of heat-denatured and polymerized lysozyme formed at 70 and 80 °C, which were absent in samples cooked at 100 °C.The results of this study showed that cooking temperature affected the survival and growth of L. monocytogenes in cooked egg whites, suggesting that HBEs may be cooked at a lower temperature in order to retain the antilisterial activities. The mathematical models developed in this study can be used to predict the growth and survival of L. monocytogenes in BHEs and for conducting risk analysis of this type of products.