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I was separating egg whites today and one of the yolks broke and contaminated my bowl of whites. I started from scratch, but I'm wondering if I really needed to; would a tiny bit of yolk (say, 1/8 tspn in 4 egg whites) mixed in with the whites make a big difference?

To provide some context, I was about to whip the whites with castor sugar to make almond macaroons.

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Yes, it matters a lot. When you are separating egg whites, it is for whipping them into a foam. This foam is a protein-based foam, relying on protein ends hooking into each other. Even small traces of fat will prevent the foam from forming. Egg yolks contain high amounts of fat. Once an egg yolk breaks in your whites, you have to start the separation anew, because it can prevent your foam from forming. Also, don't use plastic bowls for whipping egg whites (their surface retains some fat molecules even after washing, giving you a less stable foam) and only whip with a cleanly washed whisk or mixer attachment (not one you have just used for something else, not even if you wiped it clean).

To prevent big mishaps, just separate each new egg in a teacup and only add the new white to the old whites after it has separated cleanly. Else you are in big trouble if you are separating a lot and the last egg contaminates the whole whites with yolk. And a single contaminated egg is easily reused for a quick egg-and-feta sandwich or something else.

Contamination the other way round isn't so worrisome. You still want to work as precisely as possible, as yolk-only recipes will often have somewhat worse texture if eggwhite is included, but small contaminating amounts are usually not noticeable in the finished product, even in foams (zabaglione, mayonnaise). This is because yolk foams are fat-based, and small amounts of protein don't prevent a fat foam from forming.

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Excellent answer. – Chris Cudmore May 12 '12 at 13:04
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I would add that it is especially important for something like macaroons, meringhe, etc. where a stiff foam is very important. You may get away with it if just lighly beating egg whites for a cake, though. – nico May 12 '12 at 13:58
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Nice answer. Pastry chefs use copper bowls for making meringues, as copper ions transfer to the egg whites and help stabilise them, preventing overbeating. – ElendilTheTall May 12 '12 at 18:28
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@ElendilTheTall you don't need copper. Acid does the same job as copper ions. So most recipes call for a pinch of cream of tartar (very weak acid, no taste). And it doesn't color the foam the way copper does. – rumtscho♦ May 12 '12 at 19:04
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@Cerberus I suspect that it depends on the type of plastic too. My measuring cups seem to be made of PLA or some similar hard plastic, while the plastic which always feels "greasy" is usually nylon-based. But I never tried it myself, only read about the problem in sources I consider reputable (I think McGee mentions it too), and of course for most applications, you don't want the stiffest meringue possible, just soft peaks. – rumtscho♦ May 12 '12 at 19:06
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