Q: What's the deal with salted and unsalted butter?
Butter comes two ways: salted and unsalted. Salt is added to butter for flavor and as a preservative so it will have a longer shelf life. Salt, however, can sometimes overpower the sweet flavor of the butter and can also mask odors. Additionally, the amount of salt added to salted butter varies from manufacturer to manufacturer, so it's hard to know how much extra salt you're adding to a recipe. Using unsalted butter allows the chef to control the amount of salt in a recipe.
If you have no choice but to use salted butter in a recipe, the rule of thumb is to omit about 1/4 teaspoon salt per 1/2 cup (1 stick) of butter.
Unsalted butter has a short shelf life because it contains no preservatives. If you buy unsalted butter and do not use it right away, it is best to freeze it. If properly wrapped so it won't pick up any odors, butter can be frozen for around six months. Just remember to defrost the butter overnight in the refrigerator before using it.
-Food Network Kitchens