You boil low-fat milk products
The Result: The milk curdles or "breaks," yielding grainy mac and cheese, ice cream, or pudding.
The Fix: If you're new to lighter cooking, you may not know that even though you can boil cream just fine, the same is not true for other milk products, which will curdle. The solution is to cook lower-fat dairy products to a temperature of 180° or less.
Use a clip-on thermometer, hover over the pan, and heat over medium-low or low heat to prevent curdling. And if it curdles, toss and start again. One alternative: Stabilize milk with starch, like cornstarch or flour, if you want to bring it to a boil; the starch will prevent curdling (and it'll thicken the milk, too).