Blood spots, double yolks and green rings in eggs: safe or not?

Most of what surprises you when you crack an egg does not mean it has gone bad. A small blood spot on the yolk, two yolks in one shell, a cloudy white, white strands clinging to the yolk and a green ring around a hard-boiled yolk are all, according to the US Department of Agriculture and the US Egg Safety Center, safe to eat. The signs that mean the bin are few and clear: a foul smell when the shell breaks, a white that is pink or shimmers like a rainbow, or an egg whose shell was already cracked when you bought it. This lesson explains the cause of each odd sight, and when to carry on cooking and when to throw the egg away without a second thought.

Before the details: looking odd and being spoiled are different things#

An egg forms inside the hen over roughly a day, and along the way things can happen that leave a visible mark: a tiny blood vessel bursts, two yolks are released together, or the white thins as the egg ages. These are traces of how the egg was formed or how old it is, and they do not mean bacteria are present. The bacteria that actually matter to your health, such as Salmonella, do the opposite: they change nothing about how an egg looks, its colour or its smell, so a contaminated egg looks perfectly normal. That is why the eye alone cannot judge safety. The real protection is refrigeration and thorough cooking; a visual check only catches spoilage that is already advanced.

Crack each egg into a small separate bowl before adding it to the mixture. If something about it puts you off, only that egg goes, instead of a whole batter or the six eggs already in the bowl.

Blood spots and meat spots#

A blood spot is a small red or dark dot, usually on the yolk. The USDA explains it as the rupture of one or more small blood vessels at the time of ovulation, when the yolk is released from the ovary, and states that it does not indicate the egg is unsafe. Egg Farmers of Canada put them at fewer than 1% of eggs. A meat spot, as the US egg-grading manual describes it, is a reddish-brown, brown or tan fleck: either a blood spot whose colour has changed over time, or a bit of tissue shed from the hen's reproductive tract. At packing centres eggs pass over candling lights or electronic scanners that look for these spots along with cracks and dirt, but not every spot is caught, so a few reach the shopper. If a spot bothers you, Egg Farmers of Canada suggest lifting it out with the tip of a clean knife before cooking.

Size matters in the same grading system: the US egg-grading manual drops an egg whose blood spots add up to no more than one-eighth of an inch, about 3 mm, to Grade B only, but if the spot is larger, or blood has spread from it into the surrounding white, the egg is classed as a loss, unfit to eat. The practical rule, then: a small, well-defined dot does no harm, while blood spread through the white or a large spot is something graders class as a loss, not to be sold as food, and the safer choice is not to use it. This lesson covers the food and health side only; anyone asking about the religious ruling on eating an egg with a blood spot should consult a qualified religious authority. In the Gulf, table eggs fall under the Gulf standard GSO 1002:2014 “Chicken eggs”; the figures quoted here come from the published US manual.

Two yolks in one shell#

According to the US egg-grading manual, a double-yolked egg happens when two yolks are released at about the same time, or when one yolk strays into the body cavity for a day and is picked up with the next, and both end up in one shell. The US Egg Safety Center notes they usually come from young hens whose reproductive systems have not fully matured, and can also come from older hens nearing the end of their laying period. In the Center's own words they are perfectly safe to eat, and they are handled like any other egg. The only thing worth watching is precise recipes: a double-yolker is bigger and carries more yolk, so if you are baking a cake that depends on a set weight of egg, weigh the eggs rather than counting them.

Cloudy whites, watery whites and the white strands#

A slightly cloudy white is a sign the egg is very fresh, not that it is off: the cloudiness comes from carbon dioxide that has not yet escaped through the pores of the shell. As the days pass the gas leaves, the white clears and thins, and the egg spreads out in the pan instead of standing tall. A watery white is therefore a sign of age and lower quality, not in itself a sign of spoilage: the USDA says that even an egg old enough to float in water may be of poor quality yet perfectly safe to use, and advises cracking it into a bowl and checking for an off-odour, which is the real test. The two twisted white cords on either side of the yolk are called chalazae, and they anchor the yolk in the centre of the egg. They are neither a flaw nor the start of an embryo; according to the US Egg Safety Center, the more prominent they are, the fresher the egg, and they need not be removed before cooking.

Yolk colour does not signal freshness either: it follows the hen's feed, and we covered it, together with shell colour, in the lesson on egg-carton claims, so we will not repeat it here.

The green ring around a boiled yolk#

The greyish-green ring around a hard-boiled yolk comes from overcooking: the USDA explains it as sulphur and iron compounds in the egg reacting on the yolk's surface, and notes that a high iron content in the cooking water can also cause it; the University of Nebraska–Lincoln Extension names the product of that reaction: ferrous sulphide. The USDA states the colour is safe to eat, but it usually tells you the egg was cooked longer than it needed. The fix needs no special equipment:

1. Boil the eggs for as long as your recipe needs and no longer; every extra minute brings the green ring closer.
2. As soon as they are done, run cold water over them or put them in a bowl of ice water until completely cool, as the University of Nebraska–Lincoln Extension advises; fast cooling stops the cooking.
3. If an egg cracks while boiling, it is still safe according to the US Egg Safety Center, so there is no need to throw it out.
4. Keep hard-boiled eggs in the shell in the fridge for up to one week; the time limits for cooked eggs are set out in the lesson on cooking eggs safely.

When to throw an egg away without hesitating#

Against everything above, there are a few signs that mean the egg really has spoiled, and no amount of cooking or reasoning rescues it. Some show before you crack it, most right after, which is why the separate-bowl habit pays off.

• A foul or strong sulphur smell the moment a raw egg is cracked: throw it out and do not taste it.
• A white that is pink or shimmers like a rainbow: the USDA states it indicates spoilage by Pseudomonas bacteria, some of which are harmful to people. A white with a bright, fluorescent green cast comes from the same bacterial family, and the US grading manual notes it may or may not smell sour, so do not wait for an odour before throwing it out.
• Muddy brown contents, or a yolk broken and mixed into a murky white with a foul smell: what the US grading manual calls black or mixed rots, always a loss. The same manual also classes an egg with blood spread through the white, or a spot larger than 3 mm, as a loss.
• A cracked or split shell at the shop: do not buy it at all, because a crack is a direct route for bacteria to the inside.
• A slimy, fuzzy or mouldy shell, or an egg glued to its carton by something that leaked from another egg.

If an egg cracks on the way home from the shop, do not bin it or leave it in the carton: break it into a clean container, cover it tightly, refrigerate it and use it within 2 days in a dish that is cooked through. That is the US Egg Safety Center's advice for eggs that crack after purchase; eggs already cracked on the shelf should not be bought at all.

A quick checklist when you crack an egg#

1. Check the shell before cracking: no cracks, no sliminess, no mould.
2. Crack it into a small separate bowl.
3. Smell it: any foul odour means straight into the bin.
4. Look at the white: cloudy or watery is fine, pink or shimmering goes.
5. A small blood or meat spot: lift it out if you like and use the egg.
6. Two yolks or prominent white strands: safe, and the strands are a sign of freshness.
7. Cook it until yolk and white are firm, especially for people at higher risk.