What Causes Blood Spots On Eggs, And When It's Safe To Eat Them

Blood spots do not seem like something you'd want anywhere near your eggs, and indeed, egg farmers do their best to ensure eggs with this imperfection never reach grocery stores. The occurrence of blood spots — basically, a small red discoloration in egg yolks — is quite rare. Fewer than 1% of eggs have these blood spots, and those that are found to have them during light sensor inspections are removed and not sold commercially.

Why do blood spots occur? These are caused by broken blood vessels that occur in hens during the egg-laying process. Typically, the spots develop in the ovaries or oviduct — the areas where the eggs originate and the passageway they travel through, respectively. Rest assured, these broken blood vessels don't hurt the hen, as they're very small, and can be repaired naturally. Furthermore, their presence has nothing to do with the egg being fertilized.

Blood spots are not a reason for concern in terms of safety, according to the Food Safety and Inspection Service (FSIS) of the U.S. Department of Agriculture. The FSIS does recommend cooking eggs properly, but that's not because of blood spots. It's because eggs are a potential carrier for salmonella, a bacterium that can infect a wide range of food items, from poultry to fruits and vegetables.

Blood spots are far more commonly found in brown eggs. In fact, although blood spots are very rare overall, they're about 36 times more likely in brown eggs than they are in white eggs. Of course, blood spots are also more difficult to identify in brown eggs, even using light sensors, which increases their odds of making it to market.

What determines the color of white and brown eggs? It all boils down to the breed of the chicken. Native American breeds, like Rhode Island Reds and Plymouth Rock chickens, only lay brown eggs. Meanwhile, White Leghorns, a species that was originally brought to the U.S. from Italy, lay white eggs, as do California Whites and other breeds.

Just as breed affects the likelihood of blood spots, the broken blood vessels that produce them tend to occur more frequently in hens of a certain age. Hens typically only produce eggs for two to three years — at least in a commercial setting — since they lay them less often as they get older. Generally speaking, blood spots happen to hens when they first start to lay eggs, and when they're older and nearly done as egg layers. The reasons can vary, but bad nutrition and stress have both been cited as potential causes.

There are several types of imperfections that can occur in eggs, but in terms of spots, the two to know are blood spots and meat spots. What's the difference between the two? Meat spots come from tissue, not blood, and typically have a brown or whitish color. But in both cases, the spots develop while the hen is laying the egg. Practically speaking, meat spots tend to be seen in egg whites, whereas blood spots are mostly observed on egg yolks (although, in rare cases, they too can be found in egg whites).

Either way, it might appear unusual to the eye. But it's not something that should cause you to throw away the egg, as eggs with meat spots may also be safely consumed. The FSIS suggests cooking all eggs until they reach an internal temperature of at least 160 degrees Fahrenheit. A thermometer is the best way to check this, obviously, but if one is not available, look for visual cues. Scrambled eggs, for example, are not done if any liquid can still be seen.