How can you tell if eggs have gone bad?

Eggshells are porous, and as an egg ages moisture and carbon dioxide escape while air enters, enlarging the air cell inside. That is the basis of the float test: place the egg in a bowl of water, and a fresh one sinks and lies on its side, a somewhat older one stands on end while still submerged and is fine to use, and one that floats has a large enough air cell that it should be discarded. Floating indicates age rather than proving spoilage, but it is the right threshold for caution.

The definitive check is cracking the egg into a separate bowl rather than directly into your mixture. A spoiled egg has an unmistakable sulfurous smell that leaves no ambiguity. Visually, a fresh egg has a thick white that holds close to the yolk and a rounded, firm yolk, while an older egg has a runny, spreading white and a flatter yolk. Discoloration, particularly pink, green, or iridescent white, indicates bacterial spoilage. Cloudy white is a sign of freshness rather than a problem, and blood spots or the white cords called chalazae are normal.

Storage practice differs internationally for a reason worth knowing. In the United States, eggs are washed and sanitized commercially, which removes the natural protective cuticle and requires refrigeration thereafter, so American eggs must stay cold and should be kept in the main body of the fridge rather than the door where temperature fluctuates. In much of Europe, eggs are not washed, the cuticle remains intact, and room-temperature storage is standard, which is why the same product is handled differently in different countries. Refrigerated eggs typically remain good for three to five weeks past purchase, and the carton date is a quality indicator rather than a safety deadline. Cook eggs until whites and yolks are firm, or to 160 degrees Fahrenheit in dishes containing them, to address salmonella risk.