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Shell eggs are sterile at the time of oviposition or harbor very few microorganisms. Contamination of the shell occurs afterwards. Eggs possess several natural barriers that prevent bacterial invasion including the shell, membranes, and antibacterial factors present in the egg white. Bacterial contamination can occur through improper washing, e.g., using a wash solution at a temperature less than the temperature of the egg or a wash solution with a high iron content (excess iron overcomes the ability of the conalbumin to inhibit bacterial growth). Eggs are usually spoiled by gram-negative bacteria such us Pseudomonas, Serratia, Proteus, Alcaligenes, and Citrobacter. Good sanitation practices are essential.

Salmonella is the principal pathogen currently associated with eggs and egg products. Beginning in the late 1980′s, the incidence of human S. enteritidis infections began to increase dramatically in the United States. Outbreaks have been associated with clean, intact, Grade A shell eggs. This serotype of Salmonella can colonize the reproductive organs in laying hens leading to internal contamination of eggs before oviposition. Most egg-associated S. enteritidis outbreaks have also involved temperature abuse that allowed the pathogen to multiply to more dangerous levels.

Recommended tests:

Aerobic Plate Count (should be <10 per g).
Salmonella test.

Additional tests:

Total Coliforms.

Yeast and Mold Count.

Liquid, frozen, and dried egg products are usually pasteurized and, thus, the likelihood of finding Salmonella in a pasteurized product is very low. However, products can be recontaminated after pasteurization. This is also the case with Listeria monocytogenes. Good sanitation practices are very important. Indicators of lack of sanitation are the coliform group and E. coli.

Recommended tests:

Aerobic Plate Count (should
be <25,000
per g).
Salmonella test.
Listeria test.

Additional tests:

Total Coliforms and E. coli (should
be <10 per g).
Yeast and Mold Count (should be <10 per g).

Adapted from:

Ricke, S. C., Birkhold, S. G., and Gast, R. K. 2001. Eggs and Egg Products, p. 473. In F. P. Downes and K. Ito (eds.), Compendium of Methods for the Microbiological Examination of Foods. American Public Health Association, Washington, DC.