Why Bad Eggs Float and Good Eggs Sink

Why Rotten or Bad Eggs Float
Fresh eggs sink because they are more dense than water. Bad eggs float because decomposition produces gases. Some gases escape the egg, making it lighter. When the egg loses enough mass, it floats.

You may have heard that rotten eggs float and good eggs sink in water. This is called the floating egg test and it’s a decent way to tell whether or not an egg is good to eat.

Performing the test is simple. Place an egg in a glass of cold water. A fresh egg sinks to the bottom of the glass. An egg that’s older but still fine to eat sinks but rests with the large end pointing up. Old and likely rotten eggs float in water.

Why Bad Eggs Float

Fresh eggs sink because they are more dense than water. Bad eggs float because they are less dense than water. As an egg ages, the yolk and white start to decompose. Some of the decomposition products are gases. But, bad eggs don’t float because they contain a bigger gas bubble! If you converted all the material inside an egg into a gas, the egg would still have the same amount of matter per unit volume. It would have the same density and the same buoyancy in water.

Instead, what happens is that some of the gases inside the egg escape through tiny pores in the shell. As the gas escapes, the egg loses mass. Its volume remains unchanged, so it becomes less dense. Once enough gas escapes, the egg’s density is less than that of water and it floats.

Does Egg Color Matter?

The floating egg test works the same regardless of the color of the egg shell. Chicken eggs may have white, brown, blue, or speckled eggs. The color results from genes for eggshell pigment, which depend on the hen laying the egg. Aside from color, eggs laid by different kinds of chickens have the same shell thickness and composition.

Water Temperature and Purity Matters

The floating egg test depends on buoyancy, so anything that affects the density of water changes the test. All eggs, good and bad, float in salt water. The test is more reliable using cold water instead of room temperature or hot water because temperature slightly affects water density.

How to Tell If an Egg Is Bad

The floating egg trick isn’t fool-proof. Sometimes old eggs don’t float. Other times, an egg might float, but still be safe to eat. Fortunately, there are other simple tests of egg freshness:

  1. Check the expiration date. Usually, this date is 30 days from the day of packaging. Eggs used before this date should be fine, unless they have cracks or have spent significant time at or above room temperature. Refrigerated eggs may still be good up to a month past the expiration date, but you need to check them.
  2. Perform the floating egg test.
  3. Shake the egg or swish it around. If it sounds sloshy, the egg is old. A fresh egg makes little to no sound when shaken.
  4. Examine the egg. If the shell is cracked, it’s best to discard the egg. Even if the egg appears undamaged, if the shell is slimy or has a powdery appearance, it may contain mold.
  5. Conduct a sniff test. Crack open the egg. If it smells rotten or like sulfur, discard it. Wash the dish with hot, soapy water before using it with food again. The rotten egg odor isn’t necessarily due to bacterial contamination. Eggs naturally become more alkaline over time, promoting hydrogen sulfide gas formation.
  6. Look at the egg. Discard it if there is any black, blue, green, or pink discoloration of the white or yolk. Cloudy whites are another sign of a bad egg. A small blood spot on the yolk may be unappealing to look at, but isn’t a health concern. If either the white or yolk is runny, it’s a sign the egg is old. However, it’s still safe to use providing it passed all the other tests.

What Happens If You Eat a Bad Egg?

Eggs go bad because bacteria and mold started to grow. The main risk comes from Salmonella, which is present in many eggs in low amounts, even if they are fresh. Cooking kills Salmonella, providing the white and yolk are fully solid. However, raw or soft eggs may still be contaminated. Bad eggs may contain other pathogens, including bacteria and mold that release toxins so that just killing them isn’t sufficient. Food poisoning symptoms typically appear between 6 and 28 hours after eating a contaminated product. Symptoms include:

  • Abdominal cramps
  • Diarrhea
  • Vomiting
  • Fever

Most cases of Salmonella poisoning resolve on their own, although you may feel queasy for four to seven days. Salmonella can result in a medical emergency for some people, though. Other types of food poisoning may be more serious. Even if you don’t get sick from bacteria or toxins in a bad egg, you’ll notice its (unpleasant) flavor!