Curdling cream

Cream may curdle when added to hot foods like soups, stews or casseroles. This can also happen when you add cream to foods with a high acid or salt content. This exposure actually denatures (changes the structure of) the milk protein and changes the cream from a smooth consistency to a lumpy, unappealing globs of fat.

How To Prevent Cream From Curdling
To help prevent this make sure to use "heavy whipping cream" with a fat content of 35% or more. Alternately you can stabilize lighter cream by mixing it with a small amount of corn starch before adding it to the rest of the ingredients.

Culinary term, substitutes do not apply