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!