eggspoaching
When I poach eggs, an off-white foam/scum forms on the surface of the water and sometimes attaches itself to the egg. Does anybody know what causes it, and how I can avoid it?
EDIT: I use malt vinegar rather than white vinegar, which I think discolours the foam/scum and makes it look more unpleasant, so I think I'll change to white vinegar.
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).
In Poland some soups (notably żurek or a sorrel soup) are served with a hard boiled (separately) egg added.
Best Answer
The scum is similar to that which forms on stock; it is made of proteins which have detached from the larger protein matrix in the egg, forming a secondary protein matrix on the surface of the water. I know of no way to avoid it, but there is absolutely nothing wrong with it. If your concern is cosmetic, I wouldn't worry; it largely disappears as the bubbles pop. If you really want to get rid of it, give your poached eggs a quick rinse under warm water before drying and plating.