My recent investigation into the endless cycle of queso enjoyment promoted a question in the comments. After I described a bubbling plate of molten cheese primi timpano asked:
A little surprised at the bubbling cheese as I thought cheese heated to these temperatures would result in broken proteins. Does Velveeta have ingredients/properties that prevent this? The Home Sick Texan has a good article and recipe on queso in which she recommends using a bechamel sauce with the cheese, which allows her to use traditional cheeses.
Ah, the broken proteins. Velveeta had a field day poking fun at poor cheddar heated well past its melting point and broken down into curdled milk solids suspended in a sea of orange oil. Velveeta, as featured in the commercials, melts into a sumptuous velvety substance that pours in soft folds, all after a simple pass through the microwave.
The key to cheese melting is moisture. Cheeses with high moisture contents (brie, tallegio, fontina) are softer and melt at lower temperatures -- perfect for oozing grilled cheese sandwiches. Cheeses that have aged a while and have lower moisture content (cheddar, Swiss) melt at higher temperatures and break easily. These require a little help if you want to melt them and not end up with a clumpy, oily mess.
Starchy ingredients, like the flour used to set the bechamel referenced in the question, coat the milk proteins and act as a stabilizer -- cornstarch and arrowroot work too. By incorporating cheese into a starchy sauce you can make a "queso" out of nearly any cheese you like.
As for Velveeta, it's not cheese. Even the USDA, which lets processors label pink slime as beef, required Kraft to find another label. (They went with "cheese food.") Velveeta is the ultimate high-moisture melter because whey, the liquid that separates from the curd in the cheese-making process, is actually reincorporated back into the cheese -- it's hyper-hydrated -- and emulsifiers, which act similarly to the flour in a bechamel, are already included. And that that's why you can boil Velveeta away for days.