Fresh chicken eggs are great, except when you end up with rotten chicken eggs. Various factors can impact whether an egg is edible, with the occasional complaint from a chicken producer that a hen laid an egg that was rotten on day one. Though rare, laying a rotten or soon-to-be rotten egg can occur under the right circumstances. As we crack open the mystery behind hens that lay rotten eggs, prepare to learn what contributes to the presence of eggs that have run afoul of conventional standards.

What is So Rotten

If you’ve ever cracked open an egg with a putrid smell, you’re likely already quite familiar with the concept of rotten eggs. As a poultry farmer, you may have even developed various tricks to catch rotten eggs before you crack them open to avoid the stench.

Richard A. Blatchford is an associate professor of extension for small to large-scale poultry in the Department of Animal Science of the Center for Animal Welfare at the University of California, Davis. “A rotten egg is one in which there was a mold or bacterial invasion,” he says. “This typically happens after the egg is laid, and the microbes invade the egg from the environment. The appearance and smell of the rotten egg can vary based on the organism that invaded the egg.”

Blatchford specializes in the husbandry, behavior and welfare of poultry. He works with broilers, layers and backyard flocks as an extension specialist. One of his primary roles is to provide the latest scientific information to stakeholders in poultry and talk with producers about how to adopt housing or best manage behavioral issues in backyard flocks.

Is a Hen Laying Rotten Eggs?

Because this phenomenon is relatively rare, research isn’t plentiful. However, a study by the Institute for Agricultural and Fisheries Research, Animal Science Unit, found two potential routes of infection of chicken eggs.

One route is vertical transmission, which might explain why a hen could lay a rotten egg or one that went bad right away. Basically, a bacterial infection within the hen’s reproductive organs, namely the ovaries or oviduct tissue, directly contaminates the yolk, the albumen (egg white) and/or the membranes before the egg is covered by the shell during the laying process.

Alternately, the eggshell could become contaminated when microorganisms penetrate the shell as the egg passes through a highly contaminated cloaca area while being laid. Eggshells in these situations may have excessive fecal matter on them.

Once the shell has been contaminated and the egg laid, the contents can contract if the ambient temperature is around 68 degrees Fahrenheit cooler than the chicken’s normal body temperature of 105 to 107 degrees. When the egg content contracts, the negative pressure inside the egg moves the contaminant through the shell into the egg itself.

In the second scenario, the infection must exist as well as certain conditions also play a role. The result of either scenario could potentially be a rotten egg. However, depending on the contaminant, you could have a bigger problem. According to the same study, the most common containments in a poultry house are E. coli, salmonella and Staphylococcus aureus, which can all cause severe food poisoning.

“The bacteria most associated with eggs and food safety would be salmonella,” Blatchford says. “However, this bacteria doesn’t make the egg rot. Certain types of salmonella make a chicken sick but not people, and some types make people sick but not chickens. The type that makes people sick can be introduced to eggs in the formation process if the hen is infected (this type doesn’t make her sick) or through environmental contamination.”

So, while it’s possible for a hen to lay an egg that would rot quickly, Blatchford confirms that it’s uncommon.

“It’s very rare that a hen would lay a rotten egg,” he says. “For this to happen, the egg would have to be contaminated during egg formation. So, the hen would have to be infected for this to happen. Rotten eggs almost always happen after the egg is laid from environmental contamination.”

Why Chicken Eggs Go Bad

More commonly, eggs go bad due to environmental issues. For example, eggs laid in damp nests could lead to bacterial infections and crusted yolks. Crusted eggs typically have watery whites that appear yellowish and yolks with a light-colored crust. These eggs emit a putrid odor you can’t miss.

“Typically, mold or bacteria get through the shell, especially if the shell is cracked, but it doesn’t have to be,” Blatchford says.

Using good flock management practices, the incidence of rotten eggs of any kind is generally low but sometimes rises slightly in the summer when temperatures are higher. Abrupt temperature changes and cracked shells can allow bacteria into an egg. Other factors that can impact egg quality include improper storage and age.

Prevention Tips

Several things you can do throughout the egg-laying to consumption process can help prevent eggs from rotting before their time. Begin with the actual area you provide your hens for laying eggs.

Blatchford suggests that you “provide a clean place for her to lay eggs and provide some type of substrate in the nesting area such as wood shavings or straw so the egg has a soft surface to lie on and reduce the likelihood of cracking. Also, collect eggs on a regular basis to reduce the risk of breakage.”

Protect eggs during collection by placing them gently into a container instead of dropping them and allowing them to bang together. Allowing eggs to knock together could cause noticeable breaks or small hairline cracks you might miss.

Even the tiniest crack can allow bacteria to enter the egg and cause it to rot prematurely. Also, even if a crack leaves the membrane intact, you should cook the egg immediately after collection before it has a chance to go bad.

“Then, refrigerate the eggs,” Blatchford says. “If the egg is dirty, you can wash it right before use. Refrigeration will keep any microbes from growing enough to contaminate an egg before it’s consumed.”

Place fresh eggs in soft cardboard or foam containers if you’re transporting them. Avoid plastic trays that could have sharp edges and cause unnoticed hairline fractures on an egg’s surface, opening it to a bacterial infection leading to rot.

To Wash or Not to Wash

When and how to wash eggs seems to be a highly debated topic with differing opinions. While many producers and an article from PennState Extension recommend washing eggs immediately after collection, many chicken producers disagree — as does Blatchford. He recommends not washing eggs. “They can be dry cleaned with a paper towel if needed,” he says.

You can also try using a dry toothbrush or other small hard-bristled brush dedicated to just egg cleaning. If eggs are extremely dirty, especially with fecal matter, or cracked, they’re more likely to be contaminated and should be discarded.

Many people might hesitate to place an even slightly soiled egg in the refrigerator, thinking the dirt or other debris might cause the egg to go bad. However, washing an egg too soon could cause more damage, including premature rotting.

Eggshells have a natural coating called a cuticle. When you wash a chicken egg, you remove the protective cuticle, its first line of defense against microbes. Because shells are porous, without the cuticle, bacteria can enter the inside of the egg.

If you do decide to wash them, use warm running water. You don’t want to use cold water, causing the content to contract and potentially pull in any microbes that might be present. You also don’t want to place them in a pan of water to wash because the remaining eggs sit in the dirty water until it’s their turn.

Once washed and dried, you must refrigerate the clean eggs immediately. Unwashed, never refrigerated eggs may sit on the counter at room temperature for two to four weeks. Then, they can go to the fridge and remain there for two to three months more.

Catching Before Cracking

Rotten eggs often have a putrid smell that permeates the area in record time. No matter how quickly you dispose of the egg, the nasty smell often lingers much longer than you’d like. Luckily, a few tricks can help you catch at least some rotten eggs before cracking them open and allowing the scent to invade your space.

“Most rotten eggs have a pungent odor you can smell without breaking the egg open, but not always,” Blatchford says. “Best practice is to always crack eggs in a separate container, one at a time, so that if an egg is rotten, you can throw it out and not have contaminated whatever you were using the eggs for.”

Depending on the rot, the smell can vary. For example, white rot causes a fruity odor, green rot has more of a musty odor, while black rot results in the repulsive odor you expect. An egg can also experience mixed rot, with varying degrees of rot and odor occurring.

Consider candling if you want to check questionable eggs without cracking them first. Blatchford explained the process of candling can use any bright, focused light. It’s simply called candling because candles were the first things used for the process.

“If you shine a bright, focused light, you see the internal contents of the eggs,” he says. “If the egg is very dark and/or has an enlarged and moving yolk, the egg is likely rotten.”

While candling, look for any cracks in the shell. Also, if the egg appears solid without any defined shapes or all you can see is the air sac, it’s probably gone bad. You should clearly see the yolk and the egg white should look translucent.

You can also do a float test, where you place an egg in a bucket of water. If the egg sinks to the bottom, it’s likely fine, but its freshness is in doubt if it floats. However, this test isn’t 100% reliable.

Funny-Tasting Eggs

The way a rotten egg tastes should deter most people from eating it. Assuming you can get past the rancid taste, consuming a rotten egg causes various digestive issues, some more serious than others. However, chicken eggs can taste odd for various reasons that don’t necessarily mean they’re rotten or unsafe to eat.

Issues with egg flavor often stem from a hen’s diet. While you may have read somewhere that diet may cause rotten eggs, it’s not generally likely. Poor nutrition can cause egg deformities and impact quality, but Blatchford says it wouldn’t cause the eggs to rot since microbes cause this. However, some dietary issues could potentially produce eggs that taste off. For example, some producers have noticed that when hens consume too much of certain types of food, it can eventually cause unpleasant flavors in the eggs.

While debatable, onions and garlic may transfer their flavors to your eggs, but neither is good for chickens in large quantities anyway. Asparagus is noted for its strong taste and odor and might lead to stinky eggs if feeding excessive amounts to your chickens. Broccoli and Brussels sprouts could also potentially cause off-tasting eggs. Many producers agree that fish and fish meal-based commercial chicken feed can cause fishy flavors in your eggs, as can too much flax in some chickens.

Even if they’re safe, some funny-tasting chicken eggs may be unpalpable, and you might want to avoid whatever caused the unwanted flavor.

The investigation into chickens laying rotten eggs underscores the importance of vigilant poultry management. By understanding the health, dietary and environmental factors that can contribute to the production of subpar eggs, chicken farmers can implement proactive measures to ensure a healthier and more consistent egg supply.

This article about rotten chicken eggs was written for Chickens magazine online. Click here to subscribe.