Here's a good reader question that we've often pondered:
Is it safe to leave butter out of the fridge in a butter dish for a few days? In the fridge it's just too hard to spread but I don't want to eat it after it's gone bad. How long is too long?
This question is subject to many different opinions. Many cooks say that they wouldn't touch butter if it's been out of the fridge more than an hour or two; others leave it out for up to a week. We found that official USDA guidelines assumed butter should be refrigerated and only softened "ten to fifteen minutes" before use. Which is all well and good for those who like their butter cold and hard, but we like ours very soft and spreadable! Is it all right to leave it out on the counter to soften?
Butter is made from pasteurized milk so the chances of bacteria growth are relatively low in fresh butter. Also, there is a low water content in butter, and often there is added salt as well, which also inhibits bacteria.
The more immediate issue is rancidity, where the butter develops an off taste and smell. The rate of butter going rancid can be much reduced by always covering butter left out to soften. You want as little air in contact with the butter as possible.
One of the classic ways of preserving butter at room temperature is the French butter keeper. It keeps the butter in a small pot immersed in cold water. You can safely leave it out on the counter for cool yet completely spreadable butter.
• Butter Bell Crock, $19.95 at Butter Bell
Ultimately, we can't tell you exactly how long you can leave butter out. We have no problem with leaving butter out to soften as long as it is well-covered. It usually doesn't take us more than a couple days to go through a stick anyway! We've never had rancid butter in that time. But if you're concerned about freshness and bacteria, we'd suggest using a French butter keeper for soft yet still cool butter.
Here's one more answer on the question from the MU extension:
(Top image: Faith Durand)