Quite a few recipes require tossing cheese with some hot ingredient until it melts (like pasta), or melting cheese into something (sauce, soup, etc.). Usually this works fine, but sometimes the cheese "seizes", where it balls up into hard, rubbery curds, and won't melt or break down for anything. Further cooking only makes the cheese curds firmer.
I've had this happen both with commercial cheeses and with my own homemade cheeses.
Does anyone know what causes this? Ingredient or process in making the cheese? Type or degree of heat? Something else? I've already ruled out other ingredients in the dish, since I've had this happen with identical recipes, except the brand of cheese was different.