Fat: The Foam Foiler

There is a popular myth which holds that even a speck of egg yolk, butter or oil can make an egg white foam all but impossible to form. This is, um…a myth. Sure, you ought to go out of your way to prevent fat or other oily and/or soapy substances from coming in contact with your egg whites, but a dot of errant yolk certainly won’t hurt anything. But that raises the question: exactly how does fat undermine an egg white foam?

Well, you remember how I talked about how proteins form a reinforcing mesh around air bubbles. Part of the reason protein molecules collect so readily on the surfaces of bubbles is because they have a love-hate relationships with water. Some regions along their length love it (are hydrophilic, in other words) and others hate it (hydrophobic). The surface of a bubble is therefore the perfect spot for them: they can stick their hydrophilic bits in the watery egg white, dangle their hydrophobic parts in the air, and bond to one another side-to-side in the bargain. Ahh…

The thing about fat molecules is, they have both hydrophobic and hydrophilic parts too, and so are likewise attracted to bubble surfaces — where they compete with proteins for a spot. The difference is that unlike proteins, fats don’t bond to each other side-to-side to form reinforcing networks. So they just get in the way, keeping protein molecules from finding one another. Until those proteins get a chance to do their bonding, fats should be denied an invitation to the bubble party.

3 thoughts on “Fat: The Foam Foiler”

  1. Thanks for clearing up something I have always wondered about — whether, as most cook book writers claim, even the tiniest molecule of fat will ruin a meringue. Here is another “reality or myth” question for you: When you make pate a choux, after you have heated the dough, do you really have to add the eggs one at a time, incorporating each before adding the next? I read someplace on the Internets that you didn’t, and so the last time I made a Paris Brest, I dumped in all the eggs at once. It took a little more mixing to get the dough to the right consistency, but it seems to have come out fine. It rose like a champ in the oven, and when finished, tasted the same as the other time I made it, when I followed the Sacred Traditions. Now, I don’t know what to believe: My cookbooks, or my own lying eyes.

    1. I think the main reason cookbook writers suggest adding the eggs one at a time is that it makes the mixing easier. As you know from making this type of dough/batter, it plays a game of slip-and-slide around the inside of the pan even when there’s only one egg involved. Add all the eggs at once and you’ll eventually get them all worked in, but as you pointed out, it’s more work. The extra work won’t hurt the batter at all — it needs the developed gluten and indeed might benefit from the little bit of extra gluten you might have activated. Good question as usual, Reader Lee!

  2. So cool that you mentioned this…I’ve always thought that if I got just a bit of yolk in my whites, they were history. This weekend, when prepping some whites macarons, (which aren’t the most forgiving to begin with) I accidentally broke a bit of yolk into the bowl with them Since I was low on eggs, I decided to “chance” it anyway and they whipped up just fine!

    Another cooking myth dispelled. 🙂

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