Even though I always have a stick of butter sitting in my fridge, I could never really tell you when I bought it or how old it is, even as I slice off a hunk of that butter for my toast. The butter in my fridge could be months old, for all I know, which begs the question: Is butter still safe to eat after all that time? What is the shelf-life of butter? And can butter go bad?
The good news is that butter is a fairly resilient food even though its made of milk, which isn't exactly known for its long shelf-life. The low water content of butter means bacteria, which are what cause food to spoil, have a hard time growing on it. In fact, butter can safely sit out at room temperature for up to two weeks without going bad.
And if you keep that butter in the fridge, you'll be able to extend its lifespan for a long time. According to the US Department of Agriculture's Food Safety and Inspection Service, butter can be safely stored in the fridge for up to three months. You can also store butter in the freezer, for later use, for between six to nine months. A brochure from the USDA called "How to Buy Butter" from 1995 recommends that you, "Freeze butter not intended for use within 2 or 3 days."
But a three-month-old block of fresh butter that's been sitting in your fridge won't taste the same as butter you bought the day before, especially if you don't store butter properly. That's because butter is finicky, and its taste can change depending on where it's kept. As Harold McGee writes in On Food and Cooking: The Science and Lore of the Kitchen, "its delicate flavor is easily coarsened by simple exposure to the air and to bright light, which break fat molecules into smaller fragments that smell stale and rancid." He continues, "Butter also readily absorbs strong odors from its surroundings," like, for example, that leftover takeout food sitting in your fridge.
This is why, if you are storing butter in your fridge, you want to keep it in an airtight container, "preferably with the original foiled paper and not with aluminum foil," notes McGee, explaining, "direct contact with metal can hasten fat oxidation, particularly in salted butter." That butter storage technique is seconded by the Dairy Farmers of Canada, who explain on their website, "The foil laminated paper helps prevent spoilage from exposure to light and air, and also protects butter from picking up the flavor of other foods."
That's why you don't want to store butter in the vegetable crisper or on top of meat; it'll pick up any strong odors. The best place to store butter in your fridge, if you want it to last for as long as possible, is in the coldest part, according to the butter manufacturers Challenge Dairy—which generally means the back, in either its original or a covered container.
You'll know if your butter has spoiled because it'll smell rancid. You might also see some discoloration and changes in texture. Mold is also another really good sign that your food has turned. But really, if you've been keeping that stick of butter in your fridge for a month or two—especially if its unused—chances are good that you'll be OK using it. Or, you know, you could always just churn your own butter when you need it and stop worrying about expiration dates all together! (That's a joke. You don't have to do that.)