Salmonella, a type of bacteria, is transmitted to eggs through infected chickens, whose environment is easily contaminated by carriers like rodents, birds and flies. Once infected, a chicken shows no outward signs of illness, but some of her eggs may have a small amount of salmonella deposited in them. Though this amount is harmless, it can rapidly increase to dangerous levels if the eggs aren't cooled quickly and kept cool throughout their storage and shipment.
Eggs can also become infected from the outside in, picking up the bacteria from a contaminated environment or tainted equipment used in the cleaning and packing process.
Although there isn't much you can do about how eggs are handled before they arrive at the store, there are a few steps you can take to minimize your risk of salmonella infection.
• Be a careful buyer.
Buy eggs that are clean and free of cracks. The FDA recommends only buying eggs from refrigerated cases, but most eggs sold at markets by local farmers are kept at room temperature, and I would say part of being a careful buyer is knowing the conditions under which your eggs are farmed. The eggs I buy from a local farmer not only come from a small farm I know to be clean and well-maintained, they are also much fresher, which makes them preferable to buying refrigerated eggs at the supermarket which have traveled great distances from an unknown farm and processor. It's not a risk recommended by the government, but it's the choice I've made for myself. Regardless of where you buy your eggs, refrigerate them as soon as you get home, keeping them in their original carton in the coldest part of the fridge.
• Don't cross contaminate.
Before working with raw eggs, clean your work surface, utensils and hands thoroughly with soap and water. Do the same with any surfaces that come in contact with raw eggs during the cooking process.
• Cook eggs thoroughly or use pasteurized eggs.
To avoid salmonella infection, eggs should be cooked until both the whites and yolks are firm. Egg-containing dishes like casseroles should be cooked to at least 160°F. What to do if you like runny eggs or need a raw egg for a dish like Caesar salad? You can use pasteurized eggs, which have been heated to a temperature that kills the bacteria, a choice recommended by the FDA especially when cooking for the elderly, the very young, or anyone with a compromised immune system. If you cook non-pasteurized eggs just until the yolks are runny, they will not reach a high enough temperature to kill the bacteria, so you run the risk of infection. Whether or not this is an acceptable risk is your choice.
More About Eggs & Salmonella:
• Tips to Reduce Your Risk of Salmonella from Eggs at the Centers for Disease Control
• Eggs & Egg Safety at FoodSafety.gov
• Playing It Safe With Eggs at the U.S. Food and Drug Administration
• How Does Salmonella Get Into Eggs? at Discovery News