Talk to a backyard chicken farmer long enough, and you'll eventually hear about the color of their egg yolks, and how those from the grocery literally pale in comparison. You'd think hipster hen keepers invented brightly colored yolks, but consumers have shown yolk color preferences for at least a century. And egg producers have been catering to this trend for about as long.
"The consumer demands highly colored yolks in 'fancy' eggs throughout the year, and the eggs with pale colored yolks, so frequently found on the market during the winter months, are the object of much complaint, particularly in cities," wrote Dr. Leroy Palmer in a 1915 paper in the Journal of Biological Chemistry.
Palmer sought to identify the chemical culprits behind yolk color, and found a class of carotenoids called xanthophylls. Nearly 100 years later, Palmer's conclusion is still widely accepted.
Carotenoids are pigment molecules produced mainly by plants, and are only available to animals via diet. They include the building blocks of vitamin A, and many have been shown to have antioxidant capabilities.
Historically, pale egg yolks were often taken as a sign of malnourished hens, because most sources of chicken food contains carotenoids. But this is not always the case. In many African countries, white corn is typically a part of chicken feed. White corn (or maize) is low in carotenoids and hens that eat it can produce eggs with yolks so pale they're almost off-white, despite the chickens being well-nourished and healthy.
By the same token, richly colored yolks are not universally preferred. Consumer demand for various yolk colors varies by region. Germans prefer darker orange yolks, while egg eaters in the UK prefer something a bit paler. In Italy, eggs are named according to the color of their yolks, with yellows being referred to as giallo dell'uovo, and the orange yolked eggs called rosso d'uovo.
The DSM Nutritional Products yolk color fan resembles the paint color fans from the hardware store. It contains 15 shades along the yellow-to-orange spectrum, and is the industry standard for assessing and comparing yolk color. Egg producers use the yolk color fan to zero in on and maintain the desired yolk color. Many xanthophylls are routinely used to micromanage yolk color, including marigold leaves, yellow maize, alfalfa, orange peels, algae, carrots and annatto seeds. Synthetic pigments can be more powerful than naturally derived pigments, but consumer preferences for natural ingredients in feeds have kept the industry focused on natural sources, or liquid extracts thereof.
The manipulation of yolk color isn't just something practiced by large-scale producers. Once, at a farmers market in Santa Fe, I watched farmer Matt Romero crack a pair of eggs into a demonstration pan. The yolks were a spectacular shade of sunset-red.
"I save all the old red chile that doesn't pass our cleaning stage," Romero explained. "Any chile in the field that's damaged, we feed it to our chickens. They eat the seeds first, then little by little they get to the flesh. The active dye [a xanthophyll found in chile] is called zeaxanthin. We call them red chile eggs."
I went straight home, set aside a few eggs for later comparison, and began feeding my hens a paste of chile powder and fish sauce. It worked.
Considering how easy it is to manipulate egg yolk color, a recent NPR story concluded that judging eggs by their yolks' color "won't tell you anything" about their nutrient content. But it's not quite so simple. Many of the xanthophylls added to enhance yolk color are actually good for you, such as ß-cryptoxanthin, which is both a precursor to vitamin A and an antioxidant, and plays an important role in bone health, among other benefits. Lutein, another yolk-brightening xanthophyll, has antioxidant qualities as well, and it accumulates in the retina, where it is thought to protect against age-related sight degeneration.
But before you get too excited about the nutritional possibilities of richly-colored yolks, consider that two servings of butternut squash per month will give you more than your monthly dose of ß-cryptoxanthin. By weight, pumpkins have about 717 times more ß-cryptoxanthin than the average-yolked egg. One cup of kale, meanwhile, has 79 times the lutein of two eggs.
Although richly pigmented yolks are not guaranteed to be significantly healthier, if you know anything at all about where the eggs in question came from, the color of the yolk can in fact be a clue into its nutrient content. Hens that forage on rich pasture would presumably eat a lot of bugs and green plants. This diet would be full of carotenoids, including xanthophylls, and bugs and plants make for a healthy diet. Hens that are healthy and have had their fill of vitamins will be in a position to stash the leftover nutrients in their eggs, while hens that are nutrient-deprived will not. So, bright yolks really could be a sign of healthier eggs, especially if the farmer isn't intentionally pigmenting them.
Meanwhile, if a carton of eggs billed as "free-range" contains pale yolks, that could mean that the eggs were produced in a way that barely satisfies the minimum requirement to merit the free-range label. That is, the chickens were given access to a postage stamp-sized patch of dirt next to their massive barn. In this case, yolk color can be used to help determine if a brand of free-range eggs is worth the extra money.
But if you bring home cheap eggs from the supermarket, only to discover yolks blaring like the sun from your pan, you'll know those yolks are the equivalent of a sprayed-on tan.