
What you see: A raw egg in which the yolk appears to be leaking into the clear gloop of the egg white.
What it is: Something caused the membrane around the yolk to break; the USDA calls this a “mixed rot” egg.
Eat or toss: Toss. A mixed rot egg isn’t necessarily contaminated, but it could be.
When yolk seeps into the white it’s a “mixed rot” egg
Every egg yolk is encased in a sac known called the vitelline membrane. After the eggshell, shell membranes and egg white, this covering of protein fibers is the egg’s last line of defense between the outside world and the yolk’s nutritional treasure trove (an all-you-can eat buffet and spa for egg-ruining single-celled organisms).
Rough handling could rupture the vitelline membrane. Bacteria could also tear it open and invade. Or a vitelline membrane can simply collapse with age. Any of these vitelline-sabotaging events can be more likely if the membrane is weaker. What leads to a weaker membrane? Well if the chicken is older or stressed, the protective membrane might be weaker out of the gate. Or, if the egg is stored too warm or for too long, the vitelline membrane will lose strength. As an egg ages, water flows into the yolk, which stretches the membrane.
Once the yolk oozes into the clear gloop known as the albumen, any bacteria present can really get to work digesting and ruining your egg.
The U.S. The Department of Agriculture deems “mixed rot” eggs inedible. Here’s the definition from their Egg-Grading Manual:
“Mixed rot occurs when the vitelline membrane of the yolk breaks and the yolk mixes with the white, resulting in a murkiness throughout the interior of the egg when viewed before the candling light.”
(“Candling” refers to using light to get a sense of the quality of the egg and what might be going on inside it. Here’s an image of what that can look like.)
A breached yolk makes it easier for bacteria to dig in
When I shared this image with Deana R. Jones, director of the U.S. National Poultry Research Center, she quickly identified it as a mixed rot egg. It doesn’t, however, look as terrible as mixed rot eggs can look. When bacteria really get busy, Jones said, you’ll most likely see more than simply some yellow spilling into the white. Usually, she said, you’ll see other signs, like a change in color and texture (as in this crazy specimen) or a terrible smell (like, you know, rotten eggs).
“You’re going to notice some off colors or some unique odors that you would not expect when you’re cracking open an egg,” she said.
What if you’re not sure if you snagged the yolk on the shell?
Sometimes, it’s hard to tell if the yolk simply snagged when it was cracked or if the vitelline membrane was breached before it landed in the pan.
In a case like that Jones said she’d consider the history of the egg. If she knew it was very fresh, she’d assume she had snagged it. However, if she didn’t know the complete history of the egg, she’d toss it in.
So, ultimately if your egg looks like this, it doesn’t mean that bacteria is definitely in your egg. It does, however, mean that it’s possible bacteria could be kicking back and slurping up some egg yolk, making this an egg you don’t want to eat. Again, the USDA deems “mixed rot” eggs inedible.
SOURCES:
- Deana R. Jones. Acting Center Director, U.S. National Poultry Research Center. U.S. Department of Agriculture, Agricultural Research Service. Interview.
- Egg-Grading Manual. Agricultural Marketing Service. United States Department of Agriculture. July 2000
- The Hen’s Egg and its Formation. Ruth Bellairs, Mark Osmond, in Atlas of Chick Development (Third Edition), 2014.
- The Origin and Structure of the Vitelline Membrane of the Domestic Fowl’s Egg. Edmund H. McNally. Bureau of Animal Industry, United States Department of Agriculture, Beltsville, Maryland (Received for publication June 16, 1942).