Survival of Listeria monocytogenes on the Surface of Egg Shells and During Frying of Whole and Scrambled Eggs

Abstract

Studies were done to determine the survival characteristics of Listeria monocytogenes on shell eggs and after cooking raw whole and scrambled eggs by frying. Samples were inoculated with low or high populations of a five-strain mixture of L. monocytogenes. Survival of the organism on shells of unbroken eggs was monitored over a 6-week storage period at 5 and 20°C. The presence of L. monocytogenes was determined by subjecting egg samples to primary enrichment in tryptose phosphate broth followed by plating the broth on Lee Modified Oxford agar. Enumeration was done by directly plating serially diluted wash buffer from shell eggs and diluted fried eggs directly on Lee Modified Oxford agar. Both low (102 CFU per egg) and high (104 CFU per egg) populations of L. monocytogenes on the surface of egg shells decreased to <10 CFU per egg after 6 d of storage at 5 and 20°C. Frying whole eggs “sunnyside up” until albumen was partially coagulated reduced both low (102 CFU/g) and high (105 CFU/g) populations of L. monocytogenes by only 0.4 log10 CFU/g. In contrast, frying one or three scrambled eggs to an internal temperature of 70–73°C reduced low (102 CFU/g) populations of L. monocytogenes to undetectable and <102 CFU/g, respectively. Frying three scrambled eggs containing high (105 CFU/g) populations caused a 3 log10 reduction. Frying one scrambled egg containing a high population resulted in <102 CFU/g. Both low (104 CFU/g) and high (107 CFU/g) populations of L. monocytogenes remained unchanged or decreased slightly when raw slightly beaten whole eggs were allowed to stand for up to 3 h at 20°C.