Milk is an emulsion, just like mayonnaise, hollandaise sauce and vinaigrette dressings. Emulsions are inherently unstable and tend to break when exposed to high heat or when the ratio of fats to non-fats is disproportional. Curdled scalloped potatoes usually result from using less-than-fresh milk or milk with a relatively low fat content, such as skimmed or two-percent milk fat. You can prevent curdling in scalloped potatoes by using the freshest dairy, increasing the starch content or adding an emulsifier, such as lecithin.
Use russet potatoes in the recipe. Russet potatoes have a high-starch content that thickens the milk and creates an environment not conducive to separating. Russet varietals ideal for scalloped potatoes include Burbank, Ranger, Nooksack, Gem and Norkotah.
Substitute half and half or heavy cream for an equal quantity of milk. High-fat dairy resists curdling more capably than regular or low-fat dairy.
Mix 1 egg yolk with the milk. The lecithin in the yolk stabilizes emulsions.
Mix 1 tsp. of all-purpose flour per 1 cup of milk used. Similar to the starch in potatoes, the starch in flour binds the fats and non-fats together to create a stable emulsion.
Bake your scalloped potatoes at 325 degrees Fahrenheit. Moderately low baking temperatures prompt the potatoes to absorb the milk before it curdles.