Black Egg Yolks
If you crack an egg with a black yolk, the egg is spoiled and isn't safe to eat. However, a hard-boiled egg can have black rims and is perfectly edible.
Let's see what turns an egg yolk black and when to toss it away.
- Why does an egg yolk turn black?
- Is it safe to eat a black egg yolk?
- Why do hard-boiled eggs have a grey/black/green shade on the yolk?
- Can chickens lay eggs with black yolks?
- What are century eggs?
Why does an Egg Yolk turn Black?
Black egg yolks or spots indicate the egg is spoiled and decomposed by bacterial or fungal contamination or oxidation. Rotten black yolks can not be eaten, whereas black shades on the yolk of a hard-boiled egg are perfectly edible. Hard-cooked eggs turn black or dark green due to overcooking.
All chicken eggs naturally contain several strains of bacteria like Staphylococcus. These bacteria can enter during egg formation in the oviduct or via contamination after laying. However, contamination via the reproductive tract is far less common. Bacteria enter the egg after laying through tiny pores in the eggshell. The pores allow oxygen to enter the shell so the embryo can breathe.
Bacterial enzymes and fungi break down all the proteins and organic material in the egg. Similar breakdowns occur naturally due to an oxidation process during aging. The result of protein decomposition is bacterial waste and free radicals. Due to the yellow pigments breaking down, iron and sulfur reacting, and the organic carbon waste, the egg yolk gets its dark green or black color.
The rotting process generates a pungent rotten egg odor that's caused by hydrogen sulfide. The smell is apparent even before the yolk turns black. It's caused by the decomposing proteins releasing their sulfur, which binds with hydrogen to form hydrogen sulfide gas.
Is it safe to eat a Black Egg Yolk?
Hard-boiled egg yolks that get a black shade after cooking are edible, but when a raw yolk has black spots, it's not safe to eat. The egg is spoiled and can contain bacteria like Salmonella or E. Coli that can cause food poisoning. Even when the bacteria are not harmful, bacterial waste can be harmful with toxins causing illness.
Hard-boiled egg yolks can turn grey or green when cooked for too long or at high temperatures. That doesn't do any harm and is perfectly safe to eat.
However, the egg is spoiled if you crack an egg and the yolk is black or has large black spots. Rotten eggs have a pungent odor, so you will notice immediately.
If the spots are tiny and the egg doesn't smell tasty, little dark areas in the eggs can also be blood spots that turned brown due to oxidation. In that case, the egg is edible. If you are unsure, we've written an entire article on blood in the egg yolk.
Why do hard-boiled eggs have a grey/black/green shade on the yolk?
A black, grey, or green shade can occur on the surface of an egg yolk due to overcooking. The color results from iron in the yolk reacting with sulfur in the egg white to form iron sulfide.
The iron sulfide has a very dark grey color and can look greenish when combined with the yellow of the yolk. A yolk with a greyish or green ring is perfectly edible, although it might be less taste and can have a strange mouthfeel.
If you want to know more about how to avoid green decolorization or how the biochemical process works that underpin this transition, check out our article on green rings on the egg yolk of hard-cooked eggs.
Can chickens lay eggs with a black yolk?
No, no breed of chickens can lay eggs with black yolks. In 2021, some media reported about a regular grey goose that lays eggs with black yolks, which baffled scientists. However, such phenomena were never reported for chickens.
Images and videos of the eggs went viral on the Chinese social media Weibo. The man that posted the pictures shows that the eggs look like regular eggs, but when he cracks them, the yolks range from dark grey to almost entirely black. He claims the eggs taste just like other eggs.
Some experts reported in the media that it might have something to do with environmental pollution, but that seems very unlikely. Normal egg yolks are colored with yellow pigments (xanthophyll, zeaxanthin) and orange colors (canthaxanthin). Some experts think the eggs might be colored by eumelanin, the dark pigment responsible for the colors in the plumage.
The news website ETtoday, hosted by the Taiwanese nationwide cable television network EBC, reported that the Academy of Agricultural Sciences would investigate the yolks. There haven't been any follow-up reports since then.
What are Century Eggs?
Century eggs are a side dish in the Cantonese kitchen made by preserving chicken eggs in a mixture of clay, ash, salt, lime, and rice for several months. The natural decomposition and oxidation processes cause the yolk to turn dark green or dark grey, almost black. The egg whites become dark brown.
The century eggs have a strong flavor due to the hydrogen sulfide and ammonia that build up during the processing. The sulfur is released during the decomposition of the proteins. It's a similar process to the one that occurs when the egg yolk turns green when hard-boiling an egg.
The processing involves adding layers of clay around the egg so that there is no way for oxygen to enter the egg, but it opens the door for hydroxide and sodium ions. Modern processing doesn't use clay but soaks the eggs in a mineral solution. This prevents the egg from spoiling and creates a century egg.
The century eggs are generally safe to consume, but there have been several incidents with contaminated eggs that caused blood poisoning. In 2013 three factories in China were closed because they had heavy metals in their eggs as a result of using copper sulfate to shorten production times.
Black egg yolks in raw eggs are spoiled due to contamination by bacteria, fungi, or natural oxidation. These eggs are not safe to eat. When the rim of the egg yolks turns black after overcooking, the eggs are still edible. It's a chemical reaction of the iron and sulfur in the egg.
No breed of chickens lays black yolk eggs, but there are claims on social media that there is a grey goose that lays black yolk eggs. It's unclear how that would happen and if the information is correct.
Century eggs are a Chinese culinary dish with dark egg yolks from processing for several weeks. It's generally safe to eat century eggs, but there have been some cases of contaminated eggs.