Submitted to: Food Control
Publication Type: Peer reviewed journal
Publication Acceptance Date: 8/20/2013
Publication Date: 9/1/2013
Publication URL: http://handle.nal.usda.gov/10113/58525
Citation: Fang, T., Huang, L. 2013. Growth and survival kinetics of Listeria monocytogenes in cooked egg whites. Food Control. doi: 10.1016/j.foodcont.2013.08.034. Interpretive Summary: Hard-boiled eggs (HBE) are ready-to-eat products that are susceptible to surface contamination by Listeria monocytogenes. Recently, a major U.S.-based egg manufacturer recalled HBEs from the market due to discovery of L. monocytogenes, and products made from contaminated HBEs were also recalled across 34 states in America. This study was conducted to investigate the growth and survival of L. monocytogenes in cooked egg whites, and discovered that severe cooking at 100 degree Celsius allows the bacteria to grow without inhibition. However, if cooking occurs at lower temperatures (70 and 80 degree Celsius), the growth of L. monocytogenes can be inhibited. This study demonstrated that reduced heating during cooking can enhance the safety of HBEs.
Technical Abstract: 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 43C in egg whites cooked under different conditions (70C for 15 min, 80C for 20 min, and 100C for 10 min). Except at 43C, L. monocytogenes inoculated to samples cooked at 100C could grow uninhibitedly between 4 and 40C, exhibiting no lag phases. The growth process was described by a 3-parameter logistic primary model, with the specific growth rates fitted to the Ratkowsky square-root and Cardinal models. Using the Ratkowsky model, the estimated minimum and maximum growth temperatures were -0.3 and 47C, which were 1.6 and 44.3C, respectively, according to the Cardinal model. Images of scanning electron microscopy showed that L. monocytogenes was damaged in samples cooked at 70 and 80C. Although experiencing less than 2 log cfu/g initial growth, L. monocytogenes was inhibited in these samples at all storage temperatures, probably due to the antimicrobial activities of polymerized lysozyme formed during heating, which were absent in samples cooked at 100C. This study suggested that reduced heating during preparation of HBEs may enhance the safety of final products. Simulation by finite element analysis suggested that cooking medium-sized eggs at 80C for 15-17 min from room temperature (20C) is sufficient to inactivate Salmonella, the main foodborne pathogen associated with egg products. Actual cooking time must be adjusted according to the size and initial temperature of eggs.