Hard boiled egg with a shifted yolk?

What you see: The yolk in your hard boiled egg is not centered, but on the side of the egg.
What it is: Likely an older egg whose insides (and defenses) have slumped with age.
Eat or toss: This egg should be fine for a healthy person if it’s cooked thoroughly, as this one is. However, out of an abundance of caution, don’t serve an egg like this to an immunocompromised person. And since an off-center yolk suggests the egg is older, any remaining carton mates could also be older and should also be cooked thoroughly. 

It’s generally OK to eat a hard boiled egg with an off-center yolk

Think of an egg as a fortress, set up to protect the treasure of the yolk at the center. The shell, the membranes and the white are all designed to ward off wound-be invaders (read: microbes, some of which can make us egg-eaters sick). 

But, like any fortress, the egg’s defenses wear out over time. Crack a just-laid egg and you’ll see a layered pyramid fall into your skillet. The yolk will be taller, resting on a thick saucer of egg white, surrounded by an outer ring of thinner egg white. Check out this old-school U.S. Department of Agriculture chart to see how the egg’s height, diameter and thickness can vary.

Yolk and egg white thin with time

As the egg ages, in an effort to create an equilibrium, water from the egg white flows into the yolk, leading the yolk to large and look flatter when cracked into a pan.

Meanwhile, even though the egg white is losing water, it becomes thinner and less like a gel. This is because it gets less acidic over time, which causes its proteins to arrange themselves in a less viscous way (Why does it get less acidic? Because the carbonic acid dissolved in a freshly laid egg gradually exits, as carbon dioxide, through the eggshell’s pores.) 

The protein “harness” holding the yolk in place also loosens

A fresh, healthy egg also has a set of protein strings, known as the chalaza, that help keep the yolk firmly in the middle of the egg so any invading bacteria have to travel the farthest possible distance to access the nutrient treasure trove of the yolk.

The chalaza also slackens with age.

So, in this egg, the thinner white, larger yolk, and sagging chalaza likely all enabled the yolk to stray from the egg’s center.

Is an egg with a yolk shifted to one side OK to eat?

In this egg, you can still see evidence of some egg white between the shell and the yolk, but not much. So, if any bacteria managed to make it through the shell, it had a shorter path through the white to travel before arriving at the bacterial party zone of the yolk. 

But the egg’s shell and outer membranes were still intact and since it was hard boiled, it was cooked hot enough to kill anything problematic, so we aren’t too worried. 

To be on the safe side though, you might not give an immunocompromised, very old or very young person an egg like this.

And, if you have any eggs left in the same carton, consider them to be slightly riskier. Skip any runny yolk preparations and use them in baking or hard boiling where you can be certain they’re heated thoroughly.