There’s no single answer for this. The simplest thing to do is to follow the wise counsel of the label on the peanut butter jar. The manufacturer has tested the peanut butter thoroughly, and if they recommend you store it in the fridge, there’s a reason for that. But there are also plenty of peanut butters that are perfectly fine if kept at room temperature and say nothing on the jar about storage after opening. Hence the confusion.
The two main benefits of storing PB in the fridge are straightforward enough. First and foremost, refrigerating some peanut butters keeps the oil from quickly separating out. Also, peanut butter can go rancid relatively quickly if stored in a warm place, so stashing it in the fridge extends its freshness.
In our experience, it’s typically the “natural” and/or organic peanut butters that list a refrigeration requirement on the label. Many of these are made of peanuts and nothing else. That may sound great, but the lack of other ingredients means the oil is more likely to seep out and head for the surface. Refrigerating the peanut butter inhibits the oil from separating out, which is a nice bonus – unless you like stirring up the jar every time you use it.
The downside of refrigerating peanut butter is obvious. It’s not easy to spread in its cold state, which is a pain when making peanut butter sandwiches on soft, fresh bread.
Our home economists have proposed a compromise solution. Store your jar of peanut butter in the fridge if the label says to do so, but keep a small airtight container in a cool pantry area filled with enough peanut butter to last you a few days.