Once when I was making a pretty standard fondue (50% gruyere, 50% emmenthal, white wine) the cheese somehow "curdled" and became a mix of a slightly fondue-tasting liquid and a rubbery ball of cheese, completely useless for fondue purposes.
Why and how does this happen? Was it the quality of the cheese, did I overheat the wine before adding the cheese, or did I add too much/too little cheese at once?
Normally, you start adding the cheese when the wine is simmering and stir regularly as you add it. If necessary, you can add a tablespoon of corn starch dissolved in some white wine to homogenize the mixture.
Cheese sauces will curdle more easily if they are not acid enough.
I struggled with homemade mac-n-cheese until someone pointed this out to me. The wine that Zippy suggests is one solution to adding acid without undesirable taste.
White wine has a pH of 3 to 4 and is acidic enough to curdle milk and the milk proteins in cheese. The key to success is to choose a wine that is not too "dry", heat it first to drive off the volatile acids and then gradually add the grated cheeses while stirring constantly. If the cheese curdles you're done. I've never been able to reverse it. Start over.
As soon as the wine is simmering, you add a tablespoon of corn starch dissolved in some cold white wine. Stir in order to make a very light "sauce". Then you add the grated cheeses. So the best way to make cheese fondue is not to add a cornstarch solution afterwards as so many recipes tell you to do, but before you add the grated cheeses.
Add cheese more slowly. And more importantly: keep stirring. The Swiss give the fondue a little stir every time they dip the bread into the pot ... you get more cheese on your piece of bread, and help keep the cheese in the pot smooth.
