Factors Influencing Inactivation of Salmonella Enteritidis in Hard-Cooked Eggs

Abstract

The inside of a hen's egg, once considered sterile, is now known to occasionally harbor Salmonella Enteritidis. At least two recent outbreaks of salmonellosis in which Salmonella Enteritidis PT34 was involved have been associated with hard-cooked eggs. This study was undertaken to compare D56°C values of Salmonella Senftenberg 775W and six strains of Salmonella Enteritidis isolated from outbreaks associated with eggs. D56°C values for Salmonella Enteritidis in liquid egg yolk ranged from 5.14 to 7.39 min; the D56°C value for Salmonella Senftenberg was 19.96 min. The two PT34 strains from outbreaks associated with hard-cooked eggs did not exhibit significantly higher resistance to heat compared with two PT4 strains and one strain each of PT8 and PT13a. A PT4 strain and a PT34 strain of Salmonella Enteritidis were separately inoculated (107 to 108 CFU) into the yolk of medium and extra large shell eggs at 10 and 21°C, and survival was monitored using two cooking methods: (i) placing eggs in water at 23°C, heating to 100°C, removing from heat, and holding for 15 min (American Egg Board method) and (ii) placing eggs in water at 100°C, then holding for 15 min at this temperature. Within the 15-min holding periods, inactivation was more rapid using the method recommended by the American Egg Board compared with method 2. Within each cooking method, inactivation was most rapid in medium eggs initially at 21°C. The PT4 strain survived in yolk of extra large eggs initially at 10°C when eggs were held in boiling water 9 min using method 2. The final temperature of the yolk in these eggs was 62.3 ± 2°C. Of the two methods evaluated for hard cooking eggs, the American Egg Board method is clearly most effective in killing Salmonella Enteritidis in the yolk.