What happened to my sauce??
Several times I've made a basic marinara sauce and shortly before it's done I've added a little whole cream without a problem. Last I night I made the same thing but added half and half since I had some on hand. As soon as I began to stir it in, it started to seperate and instead of a nice creamy sauce, it had little grainy clumps in it. It was awful! What happened? Was it too hot? Or can't I use half and half in a sauce like that?
My guess is the whole cream has enough fat to blend smoothly while the milk content of the half & half was high enough for it to curdle on contact with the acid in the sauce.
Too big of a temperature difference can cause it to separate or seize up. Next time, add a little bit of your sauce into the half and half, and then add that into the pot of sauce.
Heavy cream has a higher fat content (at least 36% by law, less for "ultra-pasteurized" and whipping cream) than H & H (10-18%) and doesn't separate when heated, even in an acid. See the link below for an explanation of emulsification, casein proteins, etc.
The lower the butterfat content, the more likely the product is to curdle, which is what happened to your sauce. Heat also affects H & H and it has to warmed before being added to a hot liquid. When it "breaks" in coffee, it's due to the acidity and also indicates that the H & H is not very fresh.
Stick with heavy cream and you'll be fine.
Here's another thread on this subject: