Colorado State University Extension
SafeFood Rapid Response Network


SAFEFOOD NEWS - Spring 2000 - Vol 4 / No. 3

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Hard-Cooking Eggs to Minimize Salmonella Enteritidis

Which is the best way to hard cook eggs? Boil them for 15 minutes or bring them to a boil and let them set 15 minutes? According to a recent study, either method can work, but the second one actually does a somewhat better job of controlling Salmonella enteritidis (SE).

The inside of an egg, once considered sterile, is now known to occasionally harbor Salmonella enteritidis. The contamination occurs as a result of deposition in the oviduct before shell formation. While less than 1 percent of all eggs on the retail market contain the bacteria, up to 8 percent of eggs from infected flocks may contain SE. Therefore, it's important to properly handle and cook eggs.

In a recent study, Chantarapanont and co-workers found that the size and starting temperature of the egg, the starting temperature of the water, and method of cooking all affected the inactivation rate in eggs inoculated with SE. The authors studied two methods of hard-cooking eggs: 1) placing cold or room-temperature eggs in slightly cool (73 F) water and bringing to a boil, removing from heat, letting set 15 minutes, then cooling in cold water (Egg Board method), and 2) placing cold or room-temperature eggs in boiling water, heating for 15 minutes, then cooling in cold water (common method). Because of the come-up time required for the eggs started in the cooler water to reach a boiling temperature, eggs cooked this way actually showed inactivation of SE sooner following boiling than did those placed directly into boiling water. The Egg Board method also is less likely to result in cracking than the common method.

One problem with translating the results of this study to recommendations for hard-cooking eggs in Colorado is the fact that boiling water never reaches 212 F here, the boiling temperature reported in the study. The authors did note, however, that the key factor in inactivating SE was keeping the egg in the boiling water long enough to solidify the yolk. When we repeated the Egg Board method at 5,000 feet, it took 24 minutes to bring the water to a boil and the highest temperature reached was only 198 F. However, the eggs' yolks were well solidified after setting for 15 minutes and cooling in cold water.

The Egg Board recommends that the following method for hard-cooking eggs: place eggs in a single layer in a saucepan; add tap water to cover at least 1 inch above eggs; cover pan, place on burner and bring to a boil. Upon boiling, remove the pan from the burner, allow eggs to set for 15 minutes (12 minutes for medium eggs, 18 minutes for extra large eggs), then place in ice cold water until completely cooled. This last step prevents the yolks from turning green by keeping the sulfur produced during the cooking process from adhering to the iron in the yolk and forming a green deposit.

Hard-cooked eggs can also be contaminated after cooking. The following steps can help keep hard-cooked eggs safe when coloring and hiding them. Don't color or hide cracked eggs. If you plan to eat colored eggs, be sure to use food coloring or specially made food-grade egg dyes. Once eggs are colored, remember to keep them refrigerated in their cartons. And when hiding eggs, carefully place them in areas safe from contact with pets, wild animals, birds, reptiles, insects or lawn chemicals.

Sources: 1) Chantarapanont, W., et al. 2000. Factors influencing inactivation of Salmonella enteritidis in hard-cooked eggs. J Food Prot. 63:36-43; 2) American Egg Board. Eggcyclopedia, America Egg Board, Park Ridge, IL, 1999. www.aeb.org

Updated Tuesday, March 26, 2013