ISO: Mac and Cheese

My mother made my father Mac and Cheese from scratch, she used sharp chedder and baked it in the oven. One day she curdled the dish...not sure if it was the cheese or the milk or both. My father LOVED it so this is how she continued to make it for him. I know that the dish when finished was more white and chunky then the orange color of the sharp cheese. She passed away this summer and my father asked me if she had ever shown me how to fix the mac and cheese "his" way.....she hadn't. His birthday is coming up the beginning of Feb. and I'd like to make this for him....but I don't know how she curdled it...Would anyone know how I can curdle mac and cheese made from scratch????