When I cooked salmon filets on the grill last night, after the filets had been on the grill for a while, I noticed that the salmon secreted through the surface of the filet a white liquidy substance that looked sort of like mayonaise or ranch dressing. What is it and why does it come out during cooking?
That white stuff is albumen, the same protein that makes the white of chicken eggs. The albumen is part of the salmon's blood, which means that your fillet was fresh. A trick for dealing with it was developed by Bruno Goussault while collaborating with chef Michel Richard from Citronelle in DC: soak the salmon pieces in an ice cold brine. The article in the New York Times that told the story recommends soaking the salmon fillets for ten minutes in a 10% brine (by weight). To make the brine, dissolve three tablespoons of Kosher salt for every cup of ice cold water and make as many cups as you need.
The 'white stuff' you see is fat being broken down during the cooking process.
one common reason why you see this is because you are over cooking the salmon.
If you're cooking your salmon to be well done, chances are you will see this. Gently wipe it off before serving for a better presentation. Although that might be tough with blackened salmon.