I was making a small batch of chocolate cake and decided to flavor it with peppermint extract (as a substitute for vanilla extract). Foolish me added the extract directly to the eggs (not beaten). A small portion of the white became opaque, as if it had been cooked. What happened here?
Your egg whites were cooked by the alcohol in the extract. Cooked, in this case, means denaturing, which means unfolding the protein molecules. There are many ways to denature proteins. Acid, such as vinegar, will denature egg proteins, which is why some people suggest adding vinegar to the water to poach an egg (a practice I disagree with, but that's another subject).
Ceviche, as another example, is fish which is cooked (denatured) in acid (such as lime juice). Although heat is not used, the fish is still cooked. (Note: the acid will kill some bacteria but is not as effective as cooking with heat, so be sure to use fresh fish from a source you trust.)
This is similar to how burning means combining with oxygen, so you can burn metal (rust) without heat (most easily by adding water).
Would another extract (such as vanilla) have the same effect? Mar 27, 2020 at 4:19
6It's the alcohol in the extract that is denaturing the albumin, so any extract that is alcohol-based would do the same.– myklbyklMar 27, 2020 at 4:23
3While effects of denaturing with heat and with other chemical elements (acids and alchol) are the same, applying heat makes it easier to remove bacteria and other harmful components, while the same effect with alchol or similar would be impratical, if not impossible.– bracco23Mar 27, 2020 at 13:53
Oh! I guess, that denaturing egg proteins by vinegar is taking place in making mayonnaise too! Mar 27, 2020 at 14:02
7“Although heat is not used, the fish [in ceviche] is still cooked and safe to eat.” — In virtually all ceviche I’ve ever eaten the fish is still essentially raw. Only a very thin outer layer is cooked in any material way. Of course if the fish was fresh it’s still safe to eat, same as sashimi or seared tuna. The food safety is also completely unrelated to whether the fish itself is denatured. Mar 27, 2020 at 16:22
The egg whites contain proteins that depend on water to stay soluble. The peppermint extract is no doubt alcohol based. Thus the alcohol denatured the proteins. However there isn't enough alcohol to denature all the egg white, it will just denature the egg white that it first comes into contact with. After that the water in the egg whites dilutes the alcohol.