What Causes Rotten Eggs To Smell So Bad?

When an egg is already past its shelf life, it tends to produce a strong off-odor. Eggs that are 5 weeks old or more start to produce this unpleasant odor. But what causes rotten eggs to smell so bad though? Here’s the answer: sulfur-containing proteins in the egg white.

Proteins are the building blocks of life. They help build and maintain muscles, bones, and skin. Eggs are rich in proteins. More is found in the egg white (around 4 grams) than in the yolk (2.5 to 3 grams).

There are over 40 different types of proteins in egg white. Ovomucoid (12%), ovotransferrin (54%) and ovalbumin are the main protein (11 percent) in egg white. Each protein type has a unique shape.

So what do the proteins do?

When the egg is fresh, the sulfur-containing proteins in the egg white are able to keep their shape or form. Egg white proteins fold and twist into roughly spherical tangles. Another characteristic of fresh egg is the presence of contained acidic carbon dioxide (CO₂). This CO₂ gradually escapes through the thousands of pores of the shell as the egg ages.

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As a result, the inside of the egg shrinks slowly as the air cell gently expands by drawing in air (around 4 ml of air each day). The egg also becomes alkaline, lowering the acidity to around 7.6 to 9.6. This pH (acidity) range is favorable to allow the growth of bacteria.

Furthermore, the change in pH forces the proteins to untangle and release the sulfur atoms. These sulfur atoms are converted into hydrogen sulfide gas, the gas that causes rotten eggs to smell so bad. Hydrogen sulfide is also produced when an egg is cooked at a temperature of above 140ºF (60ºC).

Hydrogen sulfide (H₂S) is a colorless gas that can be detected even at low levels. According to Centers for Disease Control and Prevention (CDC), Exposure to H₂S may cause irritation to the eyes and respiratory system. Other discomfort may be experienced such as weakness, headache, dizziness, and convulsions.

HOW DO YOU TELL IF THE EGG IS ROTTEN?

Have you ever experienced cracking open an egg with that familiar sulfur smell? And then the egg white drops like water? You can save yourself from disappointment by checking an egg’s condition through several tests.

An egg can be visually checked if it is time to discard it through a crack test. By cracking an egg on a plate, you can simply check the condition of the yolk and the white.

A fresh egg has a yolk that sits in the middle, whereas the white is thick and cloudy. As the egg ages, the yolk flattens and the white loses its color. If the egg white has become runny or watery, it is time to discard it.

Another common way to tell the freshness is the float test. In this method, the egg is placed in a bowl of water.

The egg is fresh if it sinks horizontally to the bottom of the container. If it sinks to the bottom but slightly tilting, it is already past its peak freshness. But if the egg floats to the surface of the water, it is time to discard it. An egg will float if a large amount of CO₂ has already escaped, and the air cell has grown in size enough to make the egg no longer dense to float.