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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.

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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.

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A few tricks I've seen used in restaurants:

  • Add a bit of vinegar to the water (supposed to cut scum, keeps egg together better)
  • Increase the surface area of the pan (many restaurants use a large rectangular pan for poaching)
  • Use more water (reduce the ratio of protein bits to water)
  • Slide the eggs in more carefully and use fresher eggs (less protein separates)
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After much experimentation, I have found that the keys to avoiding the off-white foam/scum are:

  1. Use eggs that are as fresh as possible, as this greatly reduces the amount of scum.
  2. Use white vinegar instead of malt vinegar, as this avoids discolouring the scum, which makes it more visible.
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Instead of poaching, you could try steaming. It is less 'aggressive' but you should calculate about 20% more time for the same result.

Also, you could push a needle trough the shell at the flat of the egg. There is a small air chamber there and the pressure can escape without shedding egg-white. Be careful to not push the needle too far in, as you'll rupture the membrane...

With a fresh egg, the chamber is better defined, so you'll have less trouble there.

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