How can I stop my cheese seizing? 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.
This is a combination of the type of cheese and too much heat. Some cheeses melt more readily (mozzarella for example), but all of them will seize up if they are heated too much too fast - the proteins 'curl up' and separate from the fat and water in the cheese.

To combat this, you should chop or grate the cheese up to speed up melting, and lower the heat a little before adding it. You should also avoid using cheese straight from the fridge.