Is the color of an egg yolk a reliable indicator of nutruient content, or more related to the another factor (ie type of chicken or feed)?
The farm raised eggs I get locally have almost orange yolks, whereas most store bought egg yolks are more of a pale yellow (very noticeable when cracked next to each other). However, some brands seem to have much darker/orangey yolks, and I'm wondering if all things being equal, my policy of opting for the darker yolks is valid. They seem to taste a little better, but that may just be my own confirmation bias. Thoughts?
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The yellow color comes from carotenoids. Carotenoids are antioxidants and are used up during periods of immune stress, so any sick/distressed animal will have significantly less carotenoids in its body. In the case of hens, less to store in their eggs (and skin). However, commercial operations know we are getting wise to the yolk color correlation and feeding caretonoid additives to boost the colors. So I suppose color isn't that reliable when dealing with commercially farmed eggs. But if comparing farm fresh eggs, I think it would be valid.
I had free-range laying hens last year and fed an omega-3 supplement to our hens. The egg yolks came out reddish-orange, but the taste was the same. I quit after the bag was done, the color was too weird for me. I switched to a locally produced feed and the color returned to a more natural, golden yellow color. The taste is definitely superior to store bought eggs - when a raccoon killed our hens our whole household had a hard time adjusting to store eggs again, even the organic, free range brands.
It is possible to feed chickens things like marigolds and other foods specifically high in beta carotene in their feed simply to "fake" the desirable orange egg yolks rather than the orange color coming from the chickens foraging a wide range of pigmented foods, increasing the nutrient content of the eggs. It's a good idea to look at several factors. As B86 mentions, a nice, thick shell is often indicative of a more healthy chicken and thus a more nutritious egg.
Also, the thickness of the egg white (judged by how fast/far it spreads in the pan and how "tall" the cooked egg ends up) and the ease to break the yolk could also be used as an indication of the freshness of the egg, another desirable quality that could effect nutrition. Lastly, There is a greater richness of flavor in the pastured eggs in comparison to a conventional egg. So, rather than simply using the yolk color to judge, look to see how many of these factors line up. The more that do, the more that you know that you are getting a very fresh, high quality and nutritious egg.
I find that the pastured eggs I buy generally are more orange but I've also bought omega 3 enriched eggs and their color was certainly more orange, so maybe it's the dha? I think you can tell a healthy egg by the strength of the egg white not so much the color of the yolk. The store bought eggs I buy occasionally, have egg whites like water and the pastured ones have an egg white that stays together more like jelly.
I am not 100% sure but I would assume the darker the colour the better; however, what I have noticed is that a good indicator is the strength of the shell. The harder the shell is to crack the better the egg. I eat eggs straight from the farm and they're good solid eggs as my mum would say. On the occasion I buy own-brand supermarket ones I notice they are much more fragile. I would presume indicative of non-organic/free range/battery hen eggs. Get eggs from the butchers or local markets if you can not just for health reasons but taste!! Also, duck egss are awesome and Goose eggs...drool