Can sugar really “cook” egg yolks?

Reader Lily writes:

Joe, the other day I left some egg yolks that I was about to whip into pastry cream sitting in a bowl with some sugar for too long. The yolk sides that were touching the sugar turned pale and hard. My instructor said that it was caused by sugar cooking the eggs because sugar and yolks together create heat. I’m skeptical, but what do you think?

Ooh! I love this question! Lily, it’s common kitchen lore that egg yolks and sugar react to create heat. You’re right to be skeptical of that claim, though it certainly can appear that a yolk that’s in contact with sugar has been cooked (especially if the yolks are a nice, deep yellow). In the picture above I left an egg sitting on some sugar for about twenty minutes. You can see that there’s a ring of lighter colored yolk along the bottom where the two are touching. What’s causing that?

Sugar, as I’ve discussed on many occasions before, is a hygroscopic substance. Which is to say, it absorbs water. It absorbs it from the air, but it’ll also absorb it from an egg yolk if the two are in contact, right through the yolk’s membrane (that there’s called osmosis in science-speak). An egg yolk contains a mixture of water, fat and protein with a few sugars and other miscellaneous nutrients mixed in. Take the water away and the long stringy protein molecules get closer to one another, eventually to the point that they coagulate into clumps. So that’s what you’re seeing there: concentrated egg protein.

I’ll add that once egg proteins clump up like this there there’s no reversing the process. My best advice is to keep your eggs and sugar separated until you’re ready to whip!

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16 Responses to Can sugar really “cook” egg yolks?

  1. Linda says:

    I still remember that warning from my old FoodTV obsession days when the shows actually taught you something instead of trying to sell you cookware. I never put the eggs and sugar together until I’m ready for that part. I knew not to do it…now I know WHY. Another lesson learned here! Thanks, Joe!

    • rainey says:

      I sooooo miss those days!

      Sara Moulton Forever!!!

      • Linda says:

        I agree, Rainey. A sad lost to those of us who appreciated a good show about cooking or baking instead of wanting hype and marketing instead.

        Another warning about egg yolks–don’t try to freeze them raw. They will become chewy and rubbery. If you want to freeze them you will have to 1) float them in water (so I’ve heard) or 2) make them into something like curd or German Chocolate Frosting that can freeze.

        • candide001 says:

          when i freeze raw yolks i simply beat them with a fork with either a bit of sugar or salt. i’ve never had problems using them in baking once thawed. same thing for whole eggs. simply beat with sugar or salt. i do this all the time when eggs are on sale.

  2. OB says:

    I once found by accident that if you leave yolks out in the open for a short period of time (without sugar), they get dry and hard (anyhow). So sugar or no sugar, that yellow stuff gives you quite a bit to worry about. Or how about the time when I first attempted Robert’s Absolute Best Brownies from David Lebovitz’s “Ready for Dessert” and added yolks straight out of the fridge into my warm chocolate-butter mixture only to have the batter weep (I was weeping also) – I can be so stubborn (sometimes) that I went ahead and baked what can only be described as the world’s largest fried chocolate cookie!!! Which was kind of a waste seeing as I was using some fancy schmancy Swiss chocolate made from fine Bolivian beans…

    • joepastry says:

      Yes, those membranes dry out fast, no question. You can’t leave them sitting around, and if you store them they need to be covered tight!

      - Joe

  3. Jen says:

    Hi Joe, this is in effect just dehydration, no? I.e. the same thing would happen if the yolks were sitting on salt or flour?

    • joepastry says:

      Hey Jen!

      The protein clumping is certainly a result of moisture loss, a loss that’s greatly accelerated by the sugar, so it’s dehydration on steroids. I believe something similar would happen with salt, but not with flour which doesn’t have the same ability to draw out moisture through the yolk’s membrane. But I should test one with some salt to be sure!

      - Joe

      • Dani says:

        I once watched and old lady in China somewhere make dried egg yolks by placing them on top of a thick layer of salt. The process with salt happens even faster. If I remember correctly, in about 30 minutes her yolks were completely dry (and they were from duck eggs so they are larger by volume). Apparently that is a popular practice for preserving eggs in the province where she lived, though I have no clue what they do with the whites.

        • joepastry says:

          Why not? Though I doubt I’d snack on the while watching football games. I’d be up for trying one though!

          - Joe

  4. Annie says:

    My boss came back from a trip to Spain with a small flat box filled with sugary yellow discs that looked quite similar to Sunkist Fruit Gems (the round lemon ones). He said they were candies given to him by one of the locals, that they were a regional specialty. We eyed them suspiciously, since something about their texture seemed quite different from normal candies…Then hunted for a dictionary to translate “Yemas de Santa Teresa.” The ingredients? Yemas y Azucar. Egg yolks and sugar. Raw egg yolks nestled in granulated sugar until they were mostly dehydrated and “cooked.” He wouldn’t try one, so we didn’t either!

    • joepastry says:

      Hm…yeah, I admit I’d be hesitant as well. Who knew these things were delicacies?

      - Joe

  5. Dani says:

    I need to make a correction: I did a little research and the process apparently takes about 48 hours for the yolks to dry completely through the core. I assume the lady was transfering them elsewhere to finish drying after they formed that dry skin on the outside. I also learned that a mixture of salt and sugar is the preferred way of doing it because combined they have a much higher hygroscopicity than either one alone.

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