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What does splashing in a shot of white vinegar to the simmering water do when poaching an egg?

Is it for taste or is it supposed to react in some way with the albumen?

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I just learned a great trick that I suspect works even better than vinegar for helping set the eggs nicely... Julia Child recommends boiling the egg (in shell) for 10 seconds before cracking it into your poaching liquid. It's just enough to help the outer layer start to set. –  Allison Mar 20 '11 at 8:53
    
We do them in oil lined cling film, add a bag clip to seal the egg inside, and bob's your uncle! Means you can do 10 at a time if you want and they all come out perfect too!! –  ferdiesfoodlab Mar 11 '13 at 23:27

3 Answers 3

up vote 13 down vote accepted

Egg whites need to be heated up to a certain temperature in order to coagulate ("set").

Lowering the pH (increasing the acidity) of the cooking liquid is one way to lower the temperature required for coagulation of the egg whites. So, in a way, this does prevent "feathering" of the eggs, but not because of any direct reaction; rather, the reason the eggs feather less is because they have less time to feather, because they don't need to get as hot.

Any acidic liquid will have a similar effect. White vinegar is probably the most effective, but you could also add lemon juice or wine to the poaching liquid. In fact, eggs poached in red wine sauce ("Oeufs en Meurette") is quite a popular preparation method.

For reference, the coagulation temperature is also proportional to the salinity (add salt to lower the coagulation temperature, add sugar to raise it), and inversely proportional to the number of eggs used (more eggs = lower coagulation temperature).

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All of that makes sense .. except for the number of eggs. I just don't get that one. The only thing I can think is that it's only an indirect relationship -- more eggs lower the cooking temp, and lower temp cooking results in lower coagulation temperature (which I know is true for custards) –  Joe Jul 22 '10 at 1:55
    
@Joe: It's not cooking temperature, it's actually the concentration of proteins (and thus, the number of eggs in the same amount of liquid). Have a look here - see the section on Concentration near the end. Something to keep in mind next time you're poaching - do as many at a time as you can! –  Aaronut Jul 22 '10 at 2:24
    
Actually, I'm starting to question myself here... I know for a fact that this is true with any egg solution (diluting the egg raises coagulation temperature). But I am pretty sure that it also applies to poaching, because some of the egg does disperse into the water and cause a "concentration" effect. I might be wrong - if somebody can convince me, I'll delete that part of the answer as not relevant to poaching. –  Aaronut Jul 22 '10 at 2:35
    
Any wisdom as to how to get the vinegar to not smell? –  Marcin Oct 17 '10 at 18:08
    
@Marcin: I've never had poached eggs smell vinegary using the relatively small amount of vinegar. But if you're worried, try one of the other acids, like lemon juice. –  Aaronut Oct 17 '10 at 19:20

It's supposed to help with cooking the albumen in such a way that it doesn't get all feathery at the edges.

I have no idea what the actual chemical reaction is, though.

I've also heard of people adding a pickle to the poaching liquid instead of vinegar directly.

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Vinegar and salt both help the proteins (albumin) to denature (unwind) more quickly and link up to form a network of proteins, thus setting the egg. The quicker the proteins denature the less feathering there will be around the edges and the nicer looking the egg.

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