Excuse me but isn't Alfredo a one and done sauce? Meaning one heat and eat. IMHPO it's a butter sauce that uses reduction, the cheese and a Liason to thicken it. I don't believe I've ever seen Alfredo Sauce re heat well. It's just not meant to be reheated. . Plus it's so simple to make why not just do fresh every time?
If it's that important to be able to reheat.... Don't use a microwave. Do it on the stove top and add the pasta and sauce to some already reducing cream or just ake a Bechemel but substitute some of the milk with cream and add some good quality Parm and thicken with a roux like the recipe (Bechemel) calls for it. I'd stay away from cornstach. No offense intended jigz. Just not a big fan of it's use. It has it's place in the kitchen just not in Alfredo sauce.....
If you're going to make Alfredo here's the recipe I was trained with;
Heavy Cream 8oz.vol
Butter whole unsalted. 2oz. wt.
Parmesano-Reggiano 8oz.vol.
grated fine
Nutmeg, fresh grated 1/8tsp
egg yolks 2ea
salt to taste
Fettuccini Noodles, Hot 12oz.wt.
In saute pan over high flame add cream and reduce by half. Reduce heat to med and add butter and swirl pan until incorporated. Add cheese and nutmeg and incorporate same way. Remove from heat and add egg yolks Incorporate to thicken off heat and add Fettuccini noodles. Toss well to coat noodles and adjust seasoning. Serve immediately.