Last night I tried to make Saveur's Finocchio al Forno (Fennel baked in cream), but it being the summer (and thus me being a little more calorie conscious with a beach trip coming up) I thought to replace the heavy cream with some light cream... what could possibly go wrong? Well, you see above a nice luscious and creamy fennel ready to go into the oven for an hour (after it gets dotted with butter)... and what you see below:
... is clearly not rich and creamy. That, my friends, is curdled cream... though not bad milk in your fridge curdled (i.e. not smelly or rotten)... simply the curds and the whey separating (as in cheese making). While very much a novice cook, I am certainly experienced enough to know this would happen (and as I was pouring in the light cream I started to worry)... there are threads in every cooking forum and entire recipes and articles devoted to keeping mac and cheese or potatoes au gratin from curdling.
Milk fat is a powerful stabilizer, and it's very dicey to cut the milk fat of any dairy (including cheese) dish that will be exposed to high heat. What I should have done is either use 1) evaporated milk which, while lower in fat than cream, has already been cooked and is incredibly stable, or 2) use guar or xanthan gum as a chemical stabilizer.
Of course, I didn't do that, but maybe you will next time you're trying to lighten up a creamy dish. I actually do plan on trying the evaporated milk method in a mac and cheese in the near future, so I'll report back with the results.