why does the sauce separate in scalloped potatoes?

You most likely used too much butter. Too much will cause it to become "unstable" and break apart. There shouldn't be anything off with the taste...just mix it up before you serve it. Hope this helps!

You should always make sure that your rue is hot before you add any cold liquid to it. I use white rice flour in my rue instead of wheat flour. I find it binds better and doesn't separate. Here is a recipe for a basic white sauce, it is the method that I use for scalloped potatoes, mac and cheese, white wine sauce, and so on. You can adjust your ingredients as needed.

Melt in a medium saucepan over medium heat 3 heaping TBSP of butter or margarine.

Whisk in until well blended and crumbly 2 to 3 TBSP of all purpose flour or white rice flour.

Make sure this part is hot but not burning.

Slowly whisk in 1 cup of milk or cream and then one cup of chicken broth.

Add whatever seasonings and cheese that you want to the pot. Keep whisking as you do this.

Thin the sauce with more milk or broth as needed. It is easier to thin a sauce than it is to thicken it.

Remove from heat. Pour over your potatoes when you are ready.

I adjust my seasonings to whatever dish I happen to be making. A tiny pinch of nutmeg and some Italian herbs go really nicely in scalloped potatoes. I par cook my potato slices before I add them to the dish. Make sure you strain them well if you do this though. I layer the potato slices with the sauce as I would a lasagna.

I really hope that helps you out :)

I've seen this and I think it's the potatoes having a lot of water. Also, keeping them covered too long can create too much steam and make the sauce separate.

If the cheese is really greasy, this can make it worse.

the water separates from the milk you are not dong anything wrong .