Different kinds of sake require different storage conditions. Unpasteurized sake, known as nama sake, should be refrigerated even before it’s been opened, as should the more delicate ginjo and daiginjo sakes. Most other sake types are perfectly fine to be stored at room temperature so long as they aren’t in direct sunlight. Once a bottle has been opened, though, all sake needs to be refrigerated regardless of what type it is. Refrigeration slows down the oxidation process, which keeps the sake fresh longer.
If you’re not sure if your sake has gone bad, there are some signs that you can look for. Color is a big one. Filtered sake should be clear. When it’s gone bad, it will take on a slight yellow tinge. It won’t look like orange juice, but the coloration should be noticeable enough. Another way to tell if the sake is past its prime is by smell. Bad sake will smell a little like sour vinegar instead of the fruity, fresh aromas it had when it was first opened. You can use sake that’s gone bad for cooking if you like, but it may be better to just toss it at this point.