Are raw eggs more nutritious? Can eggs be eaten if the yolk turns green when boiled? Rumors about eggs are cleared up!

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Eggs are the best choice for protein supplementation. They have entered thousands of households, and most people eat one egg a day. However, there are many opinions on eating eggs. Some say that raw eggs are more nutritious , some say that eating egg yolks can supplement iron , and some say that egg yolks should not be eaten if they turn green ...

Are these claims really reliable? Let’s find out .

Are raw eggs more nutritious?

This is a misunderstanding of raw eggs. Raw eggs do not have any outstanding nutritional value and are difficult for the human body to digest and absorb. Eating raw ordinary eggs also increases the risk of food poisoning.

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Eating raw eggs can be said to be a "net celebrity way of eating eggs". Its appearance has also led many people to follow suit and eat raw eggs, such as mixing raw eggs with rice, mixing raw eggs with noodles... or even drinking them raw.

As for why we eat raw eggs, some people say that they are delicious, while others say that raw eggs retain more nutrients. In fact, it is not recommended to eat raw eggs from the perspective of safety or nutritional absorption.
▶ Food safety: Raw eggs may carry salmonella and other bacteria , and eating them raw increases the risk of food poisoning.

Salmonella is a bacteria that can be found in the intestines of humans and animals and is commonly found in animal-derived foods, including eggs, meat, and raw milk. There are two ways that Salmonella can infect eggs: first , it can infect eggs through infected hen reproductive tissue before the eggshell is formed; second , it can contaminate eggs through feces on the eggshell. Eggs are not completely "sealed" and are covered with small holes on the surface, so bacteria can easily penetrate the egg shell and enter the egg. [1]

Don’t underestimate salmonella. Infection with it can cause abdominal pain, diarrhea, and vomiting at the mildest, and even death at the worst. However, it is not heat-resistant and will die immediately when heated to 100°C, but it can be killed in 15 minutes at 60°C. [2]
There are some sterilized eggs on the market, which are advertised as "edible eggs", but they are more expensive. As long as these eggs pass the test, they do have relatively few bacteria when they leave the factory, but if they are not stored properly, they may be contaminated by bacteria.

▶ Nutrient absorption: The protein in raw eggs is in a gelatinous form, which is difficult for the human body to digest and absorb. It also contains avidin and anti-trypsin substances. The former will affect the absorption of biotin, and the latter will inhibit the activity of trypsin, hindering the digestion of protein. [3]

Therefore, for safety and nutrition, it is best not to eat raw eggs. If you really like the taste and flavor of raw eggs, choose sterilized eggs and pay attention to the storage conditions.

Can eating egg yolks supplement iron?

The egg yolk contains some iron, but due to the size of the yolk, its contribution to iron supplementation is not significant.

Many mothers will use egg yolk as the first complementary food for their baby, thinking that eating egg yolk can supplement iron. Although egg yolk contains some iron, it is not the best choice for iron supplementation.

This is because although the iron content of egg yolk seems high, at 6.5 mg/100 g, the yolk of an egg is about 15 g. Calculated, the iron intake from eating an egg yolk is about 1 mg, which is only 1/3 of the iron content of lean pork, so the contribution to iron supplementation is not significant. In addition, the iron in eggs is affected by the high phospholipid protein in the egg yolk, and the absorption rate is only about 3%. [4]

Although you cannot count on egg yolks to supplement iron, egg yolks are rich in high-quality protein, lecithin, riboflavin, carotene, vitamins and other nutrients, and are still a recommended food for baby food.

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Can’t eat egg yolk if it turns green?

This statement is wrong. The egg yolk turns green due to prolonged heating. It is not toxic and can be eaten safely.

If you boil eggs for too long, the yolk will turn green. Especially for the tea eggs sold at breakfast shops, the yolk will turn green, and you can see a thick layer of green outside the yolk when you bite it open.

Some health programs mentioned that if you boil eggs for more than 10 minutes, the yolk will turn blue, which will affect iron absorption. Some even say that such egg yolks are poisonous.

In fact, it is normal for the surface of the egg yolk to turn dark green after boiling eggs for a long time. This is because eggs contain cysteine ​​and iron. When heated for a long time, cysteine ​​will decompose into hydrogen sulfide, which will react with the iron in the egg yolk to produce dark green iron sulfide, so the surface of the egg yolk turns dark green.

As for the claim that it affects iron absorption, there is actually no need to worry. After all, the iron content of an egg is very low, and you can't expect to supplement iron by eating eggs. If you want to supplement iron, it is more reliable to eat red meat, pork liver, duck blood and clams. In addition, such eggs are not poisonous.

If you want the egg yolk to prevent it from turning dark green, put the egg in cold water, start timing for 3 minutes when the water boils, then turn off the heat and simmer for 5 minutes without opening the lid.

Are free-range eggs more nutritious?

The nutritional value of local eggs and foreign eggs is comparable.

For young people, it doesn’t matter what kind of eggs they eat, whether they are local or foreign. But for the older generation, most of them prefer local eggs, that is, free-range eggs, believing that free-range chickens can roam around and eat everything, and local eggs are more nutritious.

In fact, there is not much nutritional difference between local eggs and foreign eggs.

Some researchers compared the nutritional differences between eggs from caged chickens and eggs from free-range chickens in rural areas. The results showed that the trace elements of zinc, iron, and calcium in free-range eggs were higher than those in caged eggs, but the magnesium, copper, manganese, and selenium content was lower than that in caged eggs. The crude protein and crude fat content in free-range eggs were higher than those in caged eggs, but the difference was not large. [5]

Overall, the nutritional value of the two is comparable, so there is no need to worry about choosing one. Just eat whichever you like.

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Are brown-shell eggs more nutritious?

This statement is not scientific. The nutritional value of brown-shelled eggs is not much different from that of white-shelled eggs.

Some people prefer to choose red-shell eggs when buying eggs, believing that their nutritional value is higher than that of white eggs. In fact, the nutritional difference between the two is not big. The color of the eggshell is mainly determined by genetic factors, so there is no need to worry about it when buying.

Dark yolk = good nutrition?

This statement is prejudice. The color of egg yolk is affected by factors such as egg-laying period, food, and hen breed, and has nothing to do with nutrition.

Many people like eggs with darker yolks, believing that eggs with darker yolks are more nutritious.

In fact, the depth of egg yolk color is affected by many factors.

▶ Egg-laying period: When a hen first starts laying eggs, the color of the egg yolk is generally darker. As the hen lays more eggs, the color of the egg yolk gradually becomes lighter. This is because as the hen ages, the intestinal absorption capacity of carotenoids gradually decreases, and the color becomes lighter. [6]

▶ Food: The color of the egg yolk of free-range chickens is related to the food the hens eat when they roam, such as corn, insects, and vegetable leaves. Free-range chickens that often eat corn will have darker egg yolks. This is because the lutein in corn plays an important role in the deposition of egg yolk color. On the other hand, the color of the egg yolk of caged chickens is related to the feed they eat. If the carotenoid content in the feed is high, the egg yolk color will be darker. [6]

In addition, different hen breeds, human lighting intervention, environmental factors, etc. will also affect the color of the egg yolk.

Are double yolk eggs produced with hormones?

This is not the case. Double yolk eggs are caused by abnormal hormone secretion in the hen itself. The cost of injecting hormones is high and it is not conducive to egg production.

Different people react differently to eating a double yolk egg.

Some people would dance with joy: "Wow! I actually ate a double yolk egg. I must be really lucky today!"

Some people said: "Oh my god, double yolk eggs, can't they be caused by hormones?"

In fact, double yolk eggs are not caused by hormones given to chickens, but are a kind of deformed eggs. In the same poultry breed, double yolk eggs are generally about 50% larger than single yolk eggs, and can usually be distinguished from the size of the eggs.

The reason for the double yolk egg may be due to the abnormal hormone secretion of the hen itself, which leads to an abnormal number of yolks discharged by the hen. A normal hen discharges one yolk at a time and can lay a normal egg, but if two yolks are discharged in succession, a double yolk egg will be laid.

As for the claim of injecting hormones, it is not very reliable. It may cause the hen's health to deteriorate, which is not conducive to egg production. Moreover, the cost of injecting hormones is not low.

If you are worried, don’t go out of your way to buy double yolk eggs. Just buy eggs normally. If you come across a double yolk egg, it’s your “lucky day”!

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in conclusion

Raw eggs are not more nutritious. Ordinary raw eggs may even carry bacteria and increase the risk of food poisoning. Eating one egg yolk only provides 1 mg of iron, which is not the best choice for iron supplementation. The green yolk is caused by long-term heating and is not toxic. The nutritional value of free-range eggs is comparable to that of ordinary eggs. Red-skinned eggs are affected by genetic factors and have similar nutritional value to ordinary eggs. The dark color of the yolk is affected by factors such as the laying period, food, and environment, and has nothing to do with nutrition. Double yolk eggs may be caused by abnormal hormone secretion in the hen itself, not because of hormone injections.

References:

[1]https://www.cfs.gov.hk/sc_chi/programme/programme_rafs/programme_rafs_fm_01_22.html

[2] Wang Xueshuo, Cui Shenghui, Xing Shuxia, Ding Hong. Hazard analysis, contamination investigation and prevention of Salmonella in catering food[J]. Chinese Pharmaceutical Affairs, 2013, 27(09): 974-979. DOI: 10.16153/j.1002-7777.2013.09.006.

[3] Chinese Nutrition Society. Dietary Guidelines for Chinese Residents[M]. People's Medical Publishing House. 2022

[4]. Yang Yuexin, Ge Keyou. Chinese Nutrition Science Encyclopedia 2nd Edition (Volume 1) [M]. People's Medical Publishing House, 2019: 482

[5] Bai Jian, Li Qiang. Comparative study on the nutritional content of eggs raised in cages and eggs raised in cages[J]. Poultry Science, 2012(01):9-11.

[6] Shen Manman, Wang Ying. Analysis of factors affecting egg yolk color deposition[J]. Guangdong Feed, 2014, 23(06): 43-45.

Author: Xue Qingxin, registered dietitian

Reviewer: Zhang Na, Associate Researcher, Doctoral Supervisor, Peking University School of Public Health

Planning丨Zhong Yanping

Editor｜Li Mengxin