I did some digging and tapped an expert to find out. Apparently, there are a lot of people out there who have ancient butter in their fridge, including this person who put four year old butter on her baked potato. Fortunately, scientists have actually studied this. They meticulously monitored butter kept in the fridge and found that it can last about six months with no gross bacteria growing on it and no changes in texture or flavor. In another study, scientists found that after 10 months, there actually still wasn't enough harmful bacteria on refrigerated butter to lose any sleep over it. So it turns out when kept in the cold, butter can last for a long damn time.
- Benjamin Chapman, PhD, Benjamin Chapman, PhD is a professor and food safety extension specialist at North Carolina State University and the host of the podcast Food Safety Talk.
But what happens when you're like the lady who ate four-year-old butter? Will it leave your stomach, er, churning? (Sorry! Sorry!) The first thing to know: Don't panic—old butter can't kill you. "Butter is a special type of food," says Benjamin Chapman, PhD, a professor and food safety extension specialist at North Carolina State University and the host of the podcast Food Safety Talk. Dr. Chapman explains that since butter is structured with the fat on the outside and the bulk of water on the inside, the fat serves as a protective layer of sorts, preventing bacteria to form. "The microbes that we're concerned about when it comes to food safety need water to grow," he says. "It won't grow on fat." If you have salted butter in your fridge, Dr. Chapman says you're even more in the clear. "Microbes don't like salt," he says, saying they're less likely to grow in foods with a high concentration of the mineral.
You may have heard of someone eating old butter and then getting a stomach ache or experiencing digestive distress. Dr. Chapman says this can happen, but not why you may think. "There's actually nothing in the butter that's causing this, but if you eat something that tastes sour or not like you were expecting it to taste, it might lead to a physical response such as gagging or wanting to throw up," he says. In other words, you can literally think yourself sick.
This brings up an important point: Even though eating old butter can't make you sick, Dr. Chapman says it can go rancid. He explains that while butter doesn't have an expiration date, it does have a "best sell date" and once that passes, the taste, color, and texture can change. (If your butter tastes sour, chances are it's definitely past its sell by date. "Rancidity has nothing to do with microbes or safety," Dr. Chapman says. "What's happening is oxygen is binding to the fat and changing the chemical components, which can lead to a terrible taste but it won't make you sick."
So there you have it. Eating old butter isn't going to cut your life short after all. It's just going to taste gross. Now you're moldy bread on the other hand...
Originally published September 28, 2018. Updated September 9, 2020.
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