When I was still learning about making cheese sauce at home, I figured it wouldn’t matter when I added the grated cheese to the hot milk – either before or after I’d thickened it with flour. I even thought it might be better to add the cheese before thickening. After all, as it melted it would add liquid to the sauce, and that might mean it would need more flour than I’d estimated for the milk alone. So the cheese went in early, and as the mixture heated up I saw how wrong I’d been. The cheese turned weirdly stringy and wouldn’t blend smoothly with the other ingredients. Though the sauce tasted okay, the texture was frankly off-putting.
The cheese had curdled. In other words, it had begun to break down into lumps of its various materials, and it was not going to get back together again. Dairy products do this, and so do eggs. The less fat, the more risk there is of curdling. Sometimes it’s done intentionally, as in turning milk into cheese, using enzymes to start the process. But way too often, it happens accidentally to home cooks simply because their food gets too hot.
So, what to do about curdling? Your options are limited. The best thing to do is prevent it in the first place by heating eggs, milk, and other dairy over low heat, keeping an eye on them, and stirring or whisking often. This sounds like a pain but really only takes a few minutes, and it’s worth it.
Sour cream curdles especially easily, and when this happens you lose the thickness that makes it so good for sauces. According to About.com, you can “discourage” this by adding flour, one tablespoon to each half cup. (I’d guess this will also help thicken it if needed.) My old standby, blending flour, is probably best for this.
Wikipedia says strained yogurt – the “Greek” yogurt you see in stores, or even yogurt you’ve drained overnight through a coffee filter – is much less apt to curdle. This may make plain strained yogurt a better bet than sour cream for sauces.
Confusingly, though fat in a dairy product cuts the risk of curdling, too much fat in an egg-based sauce can actually cause the eggs to curdle (again according to Wikipedia). So be sure to follow the recipe even if you think it could use more butter or oil.
If your cooking is already curdling, act quickly to get it away from the heat. You may not be able to reverse the curdling, but you can minimize it. A few seconds in the food processor may help the texture, even if it’ll never be quite perfect again. If it’s any consolation, it probably still tastes fine.