When I'm separating egg whites, how much does it matter if a little yolk gets in?

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.

I just stumbled upon this to see if I ruined my angel food cake when some egg yolk leaked into my whites. I spooned out as much as I could but there was still a little in the whites but I didn't have enough eggs to start over. Gave it a go, and I was able to get stiff peaks. Took a tad longer than normal but I got stiff peaks nonetheless.

I know that this question was from over a year ago, but id just like to point out that i make meringues at least once a month and often a little bit of yolk gets in.

This is never a big issue!!I just whisk the egg whites like normal and almost always it is fine.

Good Luck with future meringues!!

I had the same problem this morning as I was trying to make waffles, but I still got stiff peaks. I had a significant amount of yolk in my whites and what I did was I tried to scoop as much yolk as I could out of the whites with a spoon and, even though there were still some wisps of yolk leftover, the whites still became stiff peaks quite quickly (of course I was using the second highest setting on my electric mixer). Waffles were delicious!

It depends on the amount of yolk. 1/8 tsp of yolk to 4 whites may be a bit on the high side, but using a spoon and a damp paper towel to remove as much yolk as possible generally reduces the amount of yolk down to an amount that works fine. (You can sometimes lift yolk off the top with a damp paper towel, otherwise you can drag it up the side of the bowl with a spoon and then wipe it off the side with the paper towel.)

SeriousEats did a test where they compared whites with no yolk, whites with a trace amount of yolk (1 drop per 100 grams), whites with a larger amount of yolk (3 drops per 100 grams), and whites whipped in a bowl that had been wiped down with a thin sheen of vegetable oil. Only the whites with a larger amount of yolk failed to form stiff peaks. The other three tests formed virtually identical stiff peaks, although the whites with a trace of yolk took longer to form peaks.

Before discarding, you can give this a try. Worked for me today (I was making tiramisu).

1. Lift as much of yellow specks from the egg whites in your bowl as you can and then start beating it, adding little sugar at a time.
2. Even after beating long enough if you do not see the anti-gravity feature of the meringue, just let your bowl sit undisturbed for some time (maybe half an hour).
3. Once you are back, you should see a foam-like surface on the bowl.
4. Scoop the foam up carefully with a spoon (don't dig too deep, else the liquid might come in too) and now hold the spoon upside down.
5. If the foam doesn't fall off, yippee, there you have your meringue!!
6. Collect as much of this foam as you can(remember to check for the anti-gravity feature) and add it to your cream mixture (in case of tiramisu), very gently mixing it in. You will find that the cream slowly thickens.

Ofcourse, this method will not produce as much meringue as expected out of x number of eggs, but, it does come in handy.

Well I have had a little contamination from a broken yolk on two occasions but both times pressed on and had no problem getting the mix stiff enough in a normal timescale and the end product was perfect. I was making pavlova. So in my mind I have disproved the old wives tale that even minute amounts of yolk contamination make a foam impossible to sustain. I think the sugar beaten in stabilizes it strongly.

A speck of yolk shouldn't matter but more than that you should start over because you'll be disappointed in the final result I speak from decades of baking

It does matter!! I had this problem and I whipped and whipped and would not form anything. I didn’t have more that 3 drops of yolk!!

Honestly it doesn't really matter. If you want to separate the yolk, then you should break the egg and then slowly tip the yolk between the two shell halves. The other way is a bit more messy; first rule wash your hands. Then break the egg. Tip it into your hand and then roll it between your hands the white will filter out between your fingers. I have found that the best thing is to have 2 bowls, one for whites and one for yolks. This way if things get messed up then you won't ruin a whole meal.

If you are going to beat the egg whites to peaks, a teeny tiny bit of yolk wont spoil the effort IF you add 1/4 tsp of cream of tartar. This is my experience.