Butter is a dairy product, meaning it is made from the milk of mammals — usually cows.
It is made by churning milk or cream until it separates into buttermilk, which is mostly liquid, and butterfat, which is mostly solid.
Butter is unique among dairy products because of its very high fat content. While whole milk contains just over 3% fat and heavy cream contains nearly 40% fat, butter contains more than 80% fat. The remaining 20% is mostly water.
Unlike other dairy products, it doesn’t contain many carbs or much protein.
This high fat content is what makes butter so thick and spreadable. However, when it is kept in the fridge, it becomes hard and difficult to spread.
This leads some people to store butter at room temperature, which keeps it at the ideal consistency for cooking and spreading.
Because butter has a high fat content and relatively low water content, it is less likely to support bacterial growth than other types of dairy products.
This is especially true if the butter is salted, which lowers the water content further and makes the environment inhospitable to bacteria.
According to the United States Food and Drug Administration (FDA), while most types of bacteria would be able to survive on unsalted butter, there is only one type of bacteria that can survive the conditions of salted butter.
In one study to determine the shelf life of butter, scientists added several types of bacteria to the butter to see how well they would grow.
After three weeks, the bacterial content was significantly lower than the amount added, demonstrating that butter doesn’t support most bacterial growth.
Therefore, regular, salted butter has a low risk of bacterial contamination, even when kept at room temperature.
In fact, butter is actually produced with the expectation that consumers will not keep it in the fridge.
However, unsalted and whipped kinds are a different story.
Although butter has a low risk of bacterial growth, its high fat content means it is vulnerable to going rancid. When a fat spoils, you can tell it should no longer be eaten because it will smell and may be discolored.
Fats go rancid, or spoil, through a process called oxidation, which alters their molecular structure and produces potentially harmful compounds. It also results in off flavors in any foods made with the rancid fats.
Heat, light and exposure to oxygen can all speed up this process.
Yet it has been demonstrated that it may take anywhere between several weeks to over a year for oxidation to negatively affect butter, depending on how it is produced and stored.
Unsalted, whipped or raw, unpasteurized butter is best kept in the fridge to minimize the chances of bacterial growth.
Salted butter does not need to be stored in the fridge since the risk of bacterial growth is so low.
Studies have shown that butter has a shelf life of many months, even when stored at room temperature.
However, it will stay fresh longer if it is kept in the refrigerator. Refrigeration slows down the process of oxidation, which will eventually cause butter to go rancid.
For this reason, it is generally recommended not to leave butter out for more than a couple of days or weeks in order to keep it at its freshest.
Additionally, if the temperature of your house is warmer than 70–77°F (21–25°C), it is a good idea to keep it in the refrigerator.
If you prefer to keep your butter on the counter, but don’t expect to use the whole package soon, keep a small amount on the counter and the rest in the fridge.
You can store larger amounts of butter in your freezer, which will keep it fresh for up to one year.
While certain types of butter should be kept in the fridge, it is fine to keep regular, salted butter on the counter.
Here are a few tips you can follow to make sure your butter stays fresh when stored at room temperature:
There are plenty of butter dishes specifically designed to meet most of these needs, but an opaque plastic storage container also works well.