That question from reader Amory. The answer is no — however that’s good initiative! Fermentation always provides more complex flavor, especially where dairy is concerned. The problem with using sour cream in a mascarpone recipe is that bacteria will already have begun producing lactic acid before you heat your cream. That means that proteins in the cream will have already begun to coagulate (clench up), and once that process has begun, there’s no reversing it. Heat won’t do anything to uncoil them, and in fact will only exacerbate the problem. Instead of coaxing balled up-proteins to open and form a net for fat (one that will later tighten with the application of acid), the heat will only make already-tight knots of protein even tighter, creating lots of itty bitty curds.
Ever wondered why you can’t add sour cream or yogurt to a hot sauce? It’s the same problem. The slightly-clenched proteins become completely clenched up, the matrix that makes them thick collapses, and they dissipate completely into liquid and tiny curds. Crème fraîche is a little more heat-resistant than the other fermented dairy products in that family due to all the fat it contains, however it too will curdle if the heat is maintained for very long.