“Why is there always yellow in [the] egg?”

egg yolkThe other day, one of the (4yr old) boys asked me Why is there always yellow in [the] egg?

“That’s a really good question,” was the best reply I could think of at the time (I was a little stunned by how excellent a question it really is). It was sparked by the cracking of eggs to make muffins for afternoon tea, so there were three eggs in the bowl at the time and I noticed we all stared at the eggs for a bit after he asked….

It was also Easter Tuesday, so one of the older, school-aged siblings happened to be with us (and doing the baking). I asked her what she thought. “It’s the chick,” she told him. “Oh,” he said… but I wasn’t personally satisfied with this question-answer moment (and I wonder, as well, if he was satisfied…). He does ask really good questions (last week: Why do baddies like money so much?) and I’d like to encourage him (I don’t think the answer is as important – being just information – as the process of wondering and theorising), but I need to think some more on this….

Meanwhile, back home on the ranch, as they say, I did a little research… Apparently (according to yellow-egg.com), and I hope they don’t mind me quoting the whole of their page:

picture“Why are egg yolks yellow?

This is one puzzle that is easily solved: the color of the yolk reveals what the hen has been eating. The carotenoids in the hen’s feed make the yolks yellow. They are found throughout the natural world, in fruit and vegetables for example, and are easily recognized by their yellow to orange-red color. The greater the quantity of these colorful substances in the hen’s diet, the stronger the shade of the yolk is. The hen ingests yellow pigments in corn or grass, for instance. A golden yolk is produced by red carotenoids from red peppers or by canthaxanthin, a substance found widely in nature.

The hen – a truly high-performance production unit

In the ovaries of one hen, several thousand egg cells wait to start out on the path to a finished egg. The yolk matures within seven to eleven days. After ovulation the yolk enters the oviduct, where it is enveloped in several layers of egg white. A thin shell membrane forms in the part of the oviduct known as the isthmus. Finally, the egg enters the shell gland, where the shell itself develops. Just before the egg leaves the hen’s body it is covered with a thin protective coating called “bloom”. Thus hygienically packaged, the little voyager sees the light of day. A hen egg takes about 24 hours to pass from a yolk to a finished egg. A hen lays about 280 to 300 eggs a year – a truly magnificent achievement.
» By the way: brown eggs take on their hue only in the final 5 hours of shell formation. «

 Why we love yellow egg yolks

pictureOur preference for golden yellow egg yolks is rooted in history. Pale yolks were always a sign of sick hens, worm infestation, or poor feed. Only healthy, well-nourished hens store carotenoids (preliminary forms of vitamin A) in their yolks. Bright golden-yellow yolks show that the hens are well supplied with essential carotenoids such as lutein or canthaxanthin. These protective substances are widely found in nature; they not only give the yolk its yellow color, but also prevent the oxidation and destruction of fragile, vital substances such as vitamins in the egg.

Europe is not unanimous

Where the color of egg yolks is concerned, Europeans are not unanimous. A real North-South divide can be observed. While the northerners prefer pale yellow yolks, the preference of consumers for golden-yellow yolks grows as we go further south. On the shores of the Mediterranean, only bright, orange-red yolks stand a chance of reaching the plate.

From chicken feed to yolk pigment

pictureNot all carotenoids find their way into the yolk. The well-known beta-carotene, for example, is completely converted to vitamin A and metabolized by the hen. Beta-carotene has no effect on yolk color. Canthaxanthin, another carotenoid, is different: Birds only convert about 30 per cent of it into vitamin A. The rest is stored in the egg yolk as a protective substance, causing the yolk to take on a golden-yellow hue.”


Actually, this is a really interesting website; they also explain:

“The construction of an egg

“A peep beneath the surface

pictureThe egg is one of Nature’s wonders. It really merits a peep beneath the hard surface that protects the precious energy cocktail within. The shell, which is strong because it is mostly made of calcium, is lined by two fine membranes. At the rounded end of the egg, the two membranes separate to form an air ‘cell’. The yolk resides in the center of the egg, surrounded by the egg white, or albumen. It is held in place by two little cords known as chalazas. These cords gradually become more elastic as the egg ages, and the yolk begins to move around. The egg white provides a protective envelope for the yolk and also has antibacterial properties.

Watch the movie!

» By the way: the calcium layer has around 10,000 pores, which allow the passage of air. Thus there is a constant exchange of air between the egg and its environment. Unpleasant smells can penetrate its sensitive interior, as well as putrefactive bacteria, mould spores, or salmonella.”

Get fit – with egg

pictureTop quality protein and a mixture of many vital substances mean that eggs are extremely nutritious. One large egg contains about nine grams of protein, eight grams of fat, valuable lecithin and all minerals and vitamins with the exception of vitamin C. Most of the vitamins are to be found in the yolk. The most important vitamin in hen’s eggs is vitamin A, together with its preliminary forms, the carotenoids.
pictureThis vitamin enhances our ability to see. It is needed in the retina for light/dark perception and also for distinguishing colors. Vitamin A also plays an important part in the immune system during growth and stabilizes hair, skin and teeth. In addition, a hen’s egg supplies all the vitamins of the B complex, which are responsible for the proper functioning of the metabolism, cell respiration and the generation of red blood corpuscles.
…One egg provides 26 per cent of our daily requirement of folic acid. This especially fragile B vitamin forms new cells and activates growth. Globally, folic acid deficiency is one of the most common vitamin deficiencies and often occurs with iron deficiency. Eggs are also packed full of minerals: calcium, magnesium, potassium, iron, zinc, iodine and fluorine make the egg one of the most nutritious foods available.”
Now I just have to work out the best way to help L with his question a little better…!

About backyardbooks

This blog is a kind of electronic storage locker for ideas and quotes that inform my research... literary research into fiction for young adults (with a special focus on New Zealand fiction). Kiwis are producing amazing literature for younger readers, but it isn't getting the academic appreciation it deserves. I hope readers of this blog can make use of the material I gather and share by way of promoting our fiction. Cheers!
This entry was posted in education around food and meals, Questions, Science education and tagged , , , , , , , . Bookmark the permalink.

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