Why Does a Spoiled Egg Smell Like Sulfur?

It is true that a spoiled egg smells like sulfur or rot; the reason is that as bacteria break down the proteins inside the egg, hydrogen sulfide gas is released. This gas gives off a sharp, distinctive odor similar to sulfur spring water. Fresh eggs have almost no smell because this breakdown has not happened yet; the sharper the odor, the further along the bacterial breakdown has progressed. How exactly does the hydrogen sulfide smell form? The sulfur-containing amino acids in egg white and yolk break down over time under the action of bacteria, and hydrogen sulfide gas is released in the process. This gas escapes easily through the porous shell. The same gas is behind the greenish-gray ring that forms around the yolks of hard-boiled eggs; that ring, however, comes from a harmless reaction caused by long cooking, not from spoilage. How does the water test show whether eggs are fresh? Eggs that sink and lie flat on the bottom of a glass are fresh, because the air pockets inside their shells are still small. If an egg tilts slightly or stands upright, it is a few weeks old, since the air pocket has grown over time. If the egg rises and floats fully on the surface, the air pocket has grown significantly from bacterial gas production; this egg is very likely spoiled and should be thrown away without being cracked open. How can you tell whether eggs are spoiled before cracking their shells? A spoiled egg usually gives off a clear sulfur smell the moment its shell is cracked; the odor is sharp enough to carry into whatever it is mixed with. The yolk may look paler than normal and the white may appear watery and cloudy. Cracks, staining or an unusual stickiness on the shell can also be a sign that bacteria have already worked their way in. Why do storage time and temperature matter so much? Eggs kept around 4°C / 40°F in the refrigerator spoil far more slowly than eggs left at room temperature; breakdown that happens in 1 day at room temperature can be stretched out to roughly a week in the refrigerator. Storing eggs with the blunt end facing up also helps the air pocket grow gradually rather than quickly. Eggs with cracked shells spoil much faster than intact ones, because bacteria can get in directly through the crack. What should you do if you suspect an egg has gone bad? An egg that seems suspicious based on smell or the water test should be cracked into a separate bowl on its own before it is mixed with anything else; this small precaution keeps 1 bad egg from spoiling an entire batch of batter. If there is even the slightest doubt about its color or smell, it should not be used. If a few eggs from a large pack spoil early, that can be a sign the rest of the pack needs to be used sooner than usual too; checking eggs regularly is worthwhile when buying in bulk. Checking the packing or expiration date printed on the carton before buying is also a simple habit that reduces the risk of spoilage from the start; using eggs closer to their date first lowers the chance of running into that sulfur smell after they have sat around too long. Putting eggs straight into the refrigerator as soon as you get home from shopping follows the same logic; time spent at room temperature on a store shelf or in a car cuts into how much benefit refrigeration can still provide afterward.