What's going on here?

11/24/09

What's going on here?

Filed under: Blog— by joe @ 03:02:56 pm Permalink

Yesterday's pictures of crème brûlée in progress elicited this response from reader (and apparently working cook) Dennis J.:

This is off-topic, but your picture of egg yolks and sugar got me thinking. Recently I left a few yolks sitting in a bowl with some sugar while I was doing other things in the kitchen. When I came back there were light yellow curd-like bits at the point where the yolks and the sugar were touching. I asked the chef what happened and he told me that raw sugar reacts with egg yolks to create heat, and the egg had "cooked". Is that true?

I've gotten this question a few times, but never answered it on the blog as far as I know. It's common kitchen lore that egg yolks and sugar react to create heat. The truth is that they don't, though it can appear that yolk in contact with sugar has been cooked, especially if the yolks are a 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. 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, to the point that they ultimately coagulate into clumps. Once they do that, there's no reversing the process. My best advice is to keep your eggs and sugar separated until you're ready to make your mix.

Thanks for the good question, Dennis!

UPDATE: Big Fat Dave points out that there's a term for the movement of water through a semi-permeable membrane: osmosis. You know, Dave, you're exactly right.

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