Salmonella bacteria can be present on the surface of an eggshell or within the egg itself, with each type of contamination arising through distinct pathways. Understanding these different mechanisms helps explain why careful handling and cooking of eggs are important for food safety.
How Salmonella Gets on the Eggshell
External contamination of eggshells primarily occurs when an egg comes into contact with feces from an infected hen. As an egg is laid, it passes through the hen’s cloaca, a shared exit point for both eggs and waste, which can lead to bacteria being deposited onto the shell surface. Salmonella bacteria can also be found in the nesting environment, such as in bedding or dust, and transfer to the eggshell after it is laid.
How Salmonella Gets Inside the Egg
An egg can become internally contaminated through two main routes. One way is when Salmonella bacteria present on the shell surface penetrate the egg through its thousands of microscopic pores. This penetration is more likely if the egg is wet, or if there are temperature fluctuations that cause the egg’s contents to contract, potentially drawing bacteria inward. The other, and often less obvious, route is called transovarian transmission, where an infected hen passes the Salmonella bacteria directly into the egg’s yolk or white before the shell is even formed. This means that an egg can appear perfectly clean and uncracked but still harbor the bacteria internally.
Safe Egg Handling Practices
Thoroughly cooking eggs is a primary defense against Salmonella, as heat destroys the bacteria. For scrambled eggs, cooking until they are firm throughout is recommended, and egg dishes like casseroles or quiches should reach an internal temperature of 160°F (71°C). The yolk and white of fried or poached eggs should also be firm to ensure safety.
Maintaining cleanliness during food preparation is also important. Always wash hands thoroughly with soap and water after handling raw eggs, and ensure that all utensils, dishes, cutting boards, and countertops that have come into contact with raw eggs are cleaned properly. This practice helps prevent the spread of bacteria to other foods or surfaces.
Proper storage of eggs significantly reduces the risk of bacterial growth. Eggs should be stored in their original carton in the refrigerator at 40°F (4°C) or colder. This consistent cool temperature inhibits the growth of any potential Salmonella bacteria. Eggs should be refrigerated promptly after purchase and not left at room temperature for more than two hours.
Cross-contamination can be avoided by keeping raw eggs separate from other foods, using separate cutting boards and utensils. Additionally, consuming raw or undercooked eggs is not advised, especially for vulnerable populations like young children, older adults, pregnant women, and individuals with weakened immune systems, who face higher risks of severe illness.
Washing eggs at home is discouraged by food safety agencies because it can remove the egg’s natural protective coating, known as the bloom or cuticle. This coating acts as a barrier against bacteria, and its removal can make the egg more susceptible to bacterial penetration through its pores. Commercial eggs are washed and sanitized at processing plants using specific methods that minimize contamination risks, making home washing unnecessary and counterproductive.