Why are yolks yellow? The end of the Endian War.
The Animositys between these two Parties run so high, that they will neither eat nor drink, nor talk with each other. It began upon the following Occasion. It is allowed on all Hands, that the primitive way of breaking Eggs, before we eat them, was upon the larger End: But his present Majesty's Grand-father, while he was a Boy, going to eat an Egg, and breaking it according to the ancient Practice, happened to cut one of his Fingers. Whereupon the Emperor his Father published an Edict, commanding all his Subjects, upon great Penaltys, to break the smaller End of their Eggs. The People so highly resented this Law, that our Histories tell us there have been six Rebellions raised on that account; wherein one Emperor lost his Life, and another his Crown. Many hundred large Volumes have been published upon this Controversy: But the books of the Big-Endians have been long forbidden. (Swift)
It may surprise many a city-dweller that the gold color of yolks in our eggs is largely artificial: the industry adds prodigious amounts of xanthophyll carotenoids, such as lutein, to chicken feed. This is not done for the sake of yolk alone: it has been discovered that consumers prefer chickens with yellowish skin to their paleskin brothers and sisters. People have this fixed idea that yolks must be gold-yellow. This used to be a regional preference which became global quite recently.
...The carotenoids in the hen’s feed make the yolks yellow. Not all carotenoids find their way into the yolk. Beta-carotene, for example, is converted to vitamin A and metabolized; it has no effect on yolk color. Canthaxanthin is different: birds only convert about 30% of it into vitamin A. The rest is stored in the egg yolk. They prevent the oxidation and destruction of fragile, vital substances such as vitamins in the egg. Our preference for golden yellow egg yolks is rooted in history. Pale yolks were a sign of sick hens, worm infestation, or poor feed. Only healthy, well-nourished hens store carotenoids in their yolks. Bright golden-yellow yolks show that the hens are well supplied with lutein or canthaxanthin. Where the color of egg yolks is concerned, Europeans are not unanimous. 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. http://www.yellow-egg.com/wEnglish/das_gelbe_im_ei/Der_Eidotter.shtml?navid=18
This divide no longer exists. Contrary to the folk wisdom, there is no established connection between the health of the embryo and the amount of carotenoids present in the yolk. Moreover, there is no connection between the health of the hen and the concentration of nonessential carotenoids in their eggs:
...Carotenoids can be important antioxidants and immunostimulants for developing embryos and adult birds, and they are often incorporated into colorful sexual displays as signals of individual quality. The allocation of carotenoids to egg yolks and feathers can be affected by a female's physical condition, which can in turn affect offspring growth and survival. We examined relationships between yolk and feather concentrations of carotenoids and various indices of female quality (i.e., body condition, heterophil-to-lymphocyte ratio, hematocrit, date of nest initiation, egg mass, reproductive success). Carotenoid concentrations and female condition variables were not significantly related to the reproductive traits measured.
http://www.bioone.org/doi/abs/10.1525/auk.2011.10216
...Metabolism of carotenoids in animals, including birds, is poorly understood. We know that, generally, after ingestion carotenoids are absorbed in the small intestine together with other fat-soluble nutrients and delivered with portomicrons to the liver. We also know that the liver incorporates carotenoids into very low density lipoproteins, where they are delivered to the egg yolk. It is not known exactly to what extent carotenoids are metabolized in the liver. While the efficiency of absorption and assimilation of carotenoids are known to be affected by components in the diet and general health of the hen, the relationships are complex. For example, factors related to diet other than carotenoid content, genetics, hormonal status, parasites, sex, age, and season, to name a few, may act independently or interact with each other. While such relationships have been documented, it is not known to what degree they are causal in influencing carotenoid absorption or assimilation. Similarly, little is known as to what degree variation in carotenoids could be used to indicate current or past physiological states. Most avian research on carotenoidshas focused on applied issues in the rearing of captive or domesticated species: zookeepers must maintain the bright colors of display animals, and the poultry industry must satisfy consumer demand for pigmented meat and egg yolks. http://www.ebd.csic.es/jnegro/articulos%20negro/Carotenoids%20in%20eggs%20and%20plasma%20of%20red-legged%20partridges%20Effects%20of%20diet%20and%20reproductive%20output.pdf
Carotenoids are deposited in eggs of various animals: fish do it, amphibians and reptiles do it, even invertebrates, including bees (but not educated fleas) do it. The function of these carotenoids remains a mystery. Because carotenoids are antioxidants (retardants of lipid peoxidation, inter alia) it is generally assumed that their role is to protect some important target from oxidation. A quick look at a chicken egg sheds doubt in this rationale: there are many barriers to oxygen diffusion. Furthermore, there is little evidence that such function exists in fish, so another explanation was suggested for the fish: that the carotenoids serve as the reserve for vitamin A. Since birds cannot convert the carotenoids that they deposit in yolk to vitamin A, this explanation does not apply to them. The latest idea is that the benefit is conferred to hatchlings rather than the embryo:
...Carotenoids in the diet of the laying hen are incorporated into the egg yolk and subsequently into the liver and other tissues of the chicken embryo. The aim of this study is to compare the effectiveness of pre-hatch (from the hen's diet) with that of post-hatch (from the progeny's diet) supplementation with carotenoids on the carotenoid status of the chick during the first 4 weeks of post-hatch life. Hepatic carotenoid concentrations in chicks from enriched eggs maintained post-hatch on the control diet were sustained at higher values compared with chicks from control eggs that were fed post-hatch on the carotenoid-supplemented diet, for at least the first 7 days. However, by 14 days, the latter group had overtaken the former in terms of liver carotenoid levels. Since the antioxidant and immunostimulatory roles of carotenoids are likely to be especially important during the immediate post-hatch period, maternal dietary intake of carotenoids may have important ramifications for the viability of the offspring. http://www.ncbi.nlm.nih.gov/pubmed/15886038
So we are back to the mysterious benefit. I'm almost sure that this benefit is imaginary. Long time ago, the first birds (like their reptilian ancestors) had the ability to freely convert carotenoids to vitamin A; perhaps there was scarcity of beta-carotenes in their diets. With the advent of flowering plants, this scarcity disappeared, and they largely lost the ability to convert carotenoids (mammals do not have it at all, for the same reason). Transferring carotenoids to yolks is a vestigiality, conferring no real survival benefit today. It may have value for some birds (the carotenoids color feathers), as their hatchlings can be born properly colored (though this advantage disappears within days, as there are carotenoids in the diet). Chicken hatchlings are yellow, after all. Perhaps such camo had its uses in the habitat of the ancestors of our domesticated hens. But that was some time ago. Presently, the storage of carotenoids in eggs has no biological function. But that's only a prologue to the story of the golden yolk:
Long, long time ago, the Mediterranean was the center of learning, where great discoveries about Nature had been made. At some point an unknown sage generated the idea that yellow yolks are healthy and pale yolks are bad for you. This was not unreasonable: undernourished hens cannot extract and deposit enough carotenoids. The result of this connection was selection for hens depositing more carotenoids, and so southern Europeans developed preference for bright-yellow yolks. Meanwhile, the Barbarians up North had never made this connection and happily consumed pale yolks, unaware of the collosal benefits of the golden ones.
Both of these clans immigated to the US, where the two ancient traditions collided head-on. The outcome of this war would be uncertain had the farmers stuck to the traditional chicken feed, but the New World diet was heavy on carotene-rich corn, and that fated the yolks to become gold-yellow, to the delight of southern Europeans. The age-tested wisdom about the superior quality of golden yolk eggs spread across the New World and became our national standard. From America it spread via England to the rest of Europe and now there is nay a civilized country that does not spend billions of dollars on carotenoid supplies to chicken feed so we can have the healthiest, yellowest eggs on our plates!
Finally, the sunny side is up - forever.
PS: BTW, yellow & yolk were the same thing in Old English:
yolk :: O.E. geolca, geoloca "yolk," lit. "the yellow part," from geolu "yellow" (see yellow).
yellow :: O.E. geolu, geolwe, from P.Gmc. *gelwaz (cf. O.S., O.H.G. gelo, M.Du. ghele, Du. geel, M.H.G. gel, Ger. gelb, O.N. gulr, Swed. gul "yellow"), from PIE *ghel- "yellow, green".
It may surprise many a city-dweller that the gold color of yolks in our eggs is largely artificial: the industry adds prodigious amounts of xanthophyll carotenoids, such as lutein, to chicken feed. This is not done for the sake of yolk alone: it has been discovered that consumers prefer chickens with yellowish skin to their paleskin brothers and sisters. People have this fixed idea that yolks must be gold-yellow. This used to be a regional preference which became global quite recently.
...The carotenoids in the hen’s feed make the yolks yellow. Not all carotenoids find their way into the yolk. Beta-carotene, for example, is converted to vitamin A and metabolized; it has no effect on yolk color. Canthaxanthin is different: birds only convert about 30% of it into vitamin A. The rest is stored in the egg yolk. They prevent the oxidation and destruction of fragile, vital substances such as vitamins in the egg. Our preference for golden yellow egg yolks is rooted in history. Pale yolks were a sign of sick hens, worm infestation, or poor feed. Only healthy, well-nourished hens store carotenoids in their yolks. Bright golden-yellow yolks show that the hens are well supplied with lutein or canthaxanthin. Where the color of egg yolks is concerned, Europeans are not unanimous. 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. http://www.yellow-egg.com/wEnglish/das_gelbe_im_ei/Der_Eidotter.shtml?navid=18
This divide no longer exists. Contrary to the folk wisdom, there is no established connection between the health of the embryo and the amount of carotenoids present in the yolk. Moreover, there is no connection between the health of the hen and the concentration of nonessential carotenoids in their eggs:
...Carotenoids can be important antioxidants and immunostimulants for developing embryos and adult birds, and they are often incorporated into colorful sexual displays as signals of individual quality. The allocation of carotenoids to egg yolks and feathers can be affected by a female's physical condition, which can in turn affect offspring growth and survival. We examined relationships between yolk and feather concentrations of carotenoids and various indices of female quality (i.e., body condition, heterophil-to-lymphocyte ratio, hematocrit, date of nest initiation, egg mass, reproductive success). Carotenoid concentrations and female condition variables were not significantly related to the reproductive traits measured.
http://www.bioone.org/doi/abs/10.1525/auk.2011.10216
...Metabolism of carotenoids in animals, including birds, is poorly understood. We know that, generally, after ingestion carotenoids are absorbed in the small intestine together with other fat-soluble nutrients and delivered with portomicrons to the liver. We also know that the liver incorporates carotenoids into very low density lipoproteins, where they are delivered to the egg yolk. It is not known exactly to what extent carotenoids are metabolized in the liver. While the efficiency of absorption and assimilation of carotenoids are known to be affected by components in the diet and general health of the hen, the relationships are complex. For example, factors related to diet other than carotenoid content, genetics, hormonal status, parasites, sex, age, and season, to name a few, may act independently or interact with each other. While such relationships have been documented, it is not known to what degree they are causal in influencing carotenoid absorption or assimilation. Similarly, little is known as to what degree variation in carotenoids could be used to indicate current or past physiological states. Most avian research on carotenoidshas focused on applied issues in the rearing of captive or domesticated species: zookeepers must maintain the bright colors of display animals, and the poultry industry must satisfy consumer demand for pigmented meat and egg yolks. http://www.ebd.csic.es/jnegro/articulos%20negro/Carotenoids%20in%20eggs%20and%20plasma%20of%20red-legged%20partridges%20Effects%20of%20diet%20and%20reproductive%20output.pdf
Carotenoids are deposited in eggs of various animals: fish do it, amphibians and reptiles do it, even invertebrates, including bees (but not educated fleas) do it. The function of these carotenoids remains a mystery. Because carotenoids are antioxidants (retardants of lipid peoxidation, inter alia) it is generally assumed that their role is to protect some important target from oxidation. A quick look at a chicken egg sheds doubt in this rationale: there are many barriers to oxygen diffusion. Furthermore, there is little evidence that such function exists in fish, so another explanation was suggested for the fish: that the carotenoids serve as the reserve for vitamin A. Since birds cannot convert the carotenoids that they deposit in yolk to vitamin A, this explanation does not apply to them. The latest idea is that the benefit is conferred to hatchlings rather than the embryo:
...Carotenoids in the diet of the laying hen are incorporated into the egg yolk and subsequently into the liver and other tissues of the chicken embryo. The aim of this study is to compare the effectiveness of pre-hatch (from the hen's diet) with that of post-hatch (from the progeny's diet) supplementation with carotenoids on the carotenoid status of the chick during the first 4 weeks of post-hatch life. Hepatic carotenoid concentrations in chicks from enriched eggs maintained post-hatch on the control diet were sustained at higher values compared with chicks from control eggs that were fed post-hatch on the carotenoid-supplemented diet, for at least the first 7 days. However, by 14 days, the latter group had overtaken the former in terms of liver carotenoid levels. Since the antioxidant and immunostimulatory roles of carotenoids are likely to be especially important during the immediate post-hatch period, maternal dietary intake of carotenoids may have important ramifications for the viability of the offspring. http://www.ncbi.nlm.nih.gov/pubmed/15886038
So we are back to the mysterious benefit. I'm almost sure that this benefit is imaginary. Long time ago, the first birds (like their reptilian ancestors) had the ability to freely convert carotenoids to vitamin A; perhaps there was scarcity of beta-carotenes in their diets. With the advent of flowering plants, this scarcity disappeared, and they largely lost the ability to convert carotenoids (mammals do not have it at all, for the same reason). Transferring carotenoids to yolks is a vestigiality, conferring no real survival benefit today. It may have value for some birds (the carotenoids color feathers), as their hatchlings can be born properly colored (though this advantage disappears within days, as there are carotenoids in the diet). Chicken hatchlings are yellow, after all. Perhaps such camo had its uses in the habitat of the ancestors of our domesticated hens. But that was some time ago. Presently, the storage of carotenoids in eggs has no biological function. But that's only a prologue to the story of the golden yolk:
Long, long time ago, the Mediterranean was the center of learning, where great discoveries about Nature had been made. At some point an unknown sage generated the idea that yellow yolks are healthy and pale yolks are bad for you. This was not unreasonable: undernourished hens cannot extract and deposit enough carotenoids. The result of this connection was selection for hens depositing more carotenoids, and so southern Europeans developed preference for bright-yellow yolks. Meanwhile, the Barbarians up North had never made this connection and happily consumed pale yolks, unaware of the collosal benefits of the golden ones.
Both of these clans immigated to the US, where the two ancient traditions collided head-on. The outcome of this war would be uncertain had the farmers stuck to the traditional chicken feed, but the New World diet was heavy on carotene-rich corn, and that fated the yolks to become gold-yellow, to the delight of southern Europeans. The age-tested wisdom about the superior quality of golden yolk eggs spread across the New World and became our national standard. From America it spread via England to the rest of Europe and now there is nay a civilized country that does not spend billions of dollars on carotenoid supplies to chicken feed so we can have the healthiest, yellowest eggs on our plates!
Finally, the sunny side is up - forever.
PS: BTW, yellow & yolk were the same thing in Old English:
yolk :: O.E. geolca, geoloca "yolk," lit. "the yellow part," from geolu "yellow" (see yellow).
yellow :: O.E. geolu, geolwe, from P.Gmc. *gelwaz (cf. O.S., O.H.G. gelo, M.Du. ghele, Du. geel, M.H.G. gel, Ger. gelb, O.N. gulr, Swed. gul "yellow"), from PIE *ghel- "yellow, green".