My whole life, I’ve been putting mustard in the fridge, but recently I thought about it and I have no idea why. It’s not kept refrigerated in supermarkets or restaurants and when I got older and wiser (i.e. married) I found out that I was an idiot to put peanut butter in the fridge like my parents had taught me to. So does mustard need to be refrigerated? Can it go off if not?
I think if you check the bottle, it says, “Refrigerate after opening.” (I don’t have a bottle handy to check.) Mustard, like some other foods, is vacuum sealed, so stores can keep in on the shelf unrefrigerated. Once the seal is broken, though, refrigeration keeps it fresh longer. Mayo is unrefrigerated in the store, but I wouldn’t try keeping it out once it’s open. I don’t know whether mustard actually goes bad very quickly, but it can dry out.
As for peanut butter: the reason to refrigerate peanut butter is to keep it from separating as much. In “real” peanut butter, the oil will separate out, and you have to stir it before using it. When we had homemade peanut butter around, my mother even stored it upside down in the refrigerator to make it easier to use. Processed peanut butter (Peter Pan, Jiff, Skippy, etc.) have ingredients to keep them from separating, so you can store them in the cupboard.
It seems to me that peanut butter separated MORE in the fridge and when it did separate, the oil gelatinized in the cold environment… I always had to dig under the oil to get to the pretty chunky peanut butter and I always wondered how my friends had such creamy peanut butter… then I found out that like an idiot, I actually thought my parents knew what they were talking about. 
I’ve bought lots of non-vaccuum sealed mustard, so I don’t think that’s the answer. Is there anything in mustard to go bad? It’s pretty much mustard powder and vinegar, isn’t it?
I’m pretty sure it’s the other way around, but now I have to buy some good peanut butter and run a test.
That’s exactly why my mother kept it upside down!
I’ve never seen non-vacuum sealed mustard, whether in a jar with a screw-on lid or a plastic squeeze bottle. I don’t know if mustard can go bad – I’ve had jars of mustard in my refrigerator for a long time. (It separates as well.) It does dry out, though, especially if the bottle/jar is not air-tight.
Mustard says “refrigerate after opening” because it gets runnier at room temperature, like ketchup, and won’t go exactly where you want to squirt it on your hot dog. Consumers call up French’s and complain, “Hey, I squirted your product on my hot dog and it wouldn’t stay in those nice squiggly lines”, so French’s tells you to refrigerate it after opening.
I have been kicking around this planet for 40-something years, and I have never encountered a jar or squirt bottle of spoiled, green-stuff-on-top mustard. As a matter of fact, I spent many years working as a waitress and fast-food clerk, and we were always told that the ketchup had to be refrigerated overnight but the mustard didn’t, and that this was because ketchup will spoil eventually if you always leave it out at room temperature but mustard, never.
[hijack]
The Straight Dope on peanut butter: I have also never put a jar of peanut butter in the fridge. If you do that, there’s no way you can spread it on bread. It’s also more difficult to eat it with a spoon if it’s really hard. You don’t get that desirable “mouth-glued-shut” effect.
Peanut butter is practically 100% oil and oil doesn’t really spoil, in the sense of getting mold on it. I’ve never seen spoiled peanut butter. That’s not to say that if you left a jar of it up at the cabin for 3 years, it wouldn’t eventually spoil.
I have here with me now three jars of peanut butter.
Jif Extra Crunchy says “No Refrigeration Required.”
Peter Pan Reduced Fat Creamy has no indication either way of refrigerate/do not refrigerate.
Skippy Creamy also does not indicate whether or not to put it in the fridge.
(And yes, thank you, it IS a challenge doing the grocery shopping for these people…)
Well, I think we’ve resolved the mustard issue, so now on with the peanut butter hijack!
But if you’re dealing with homemade or “natural” (see below) peanut butter, you have to mix the oil back in anyway. There’s now way I’d want to just pull out a spoonful of separated peanut butter.
I think you’re right that oil doesn’t spoil in that sense, but it can get rancid. I think that may be from non-oil bits spoiling, but I’m not sure. Anyway, this is from the Adult Peanut Butter Lovers’ Fan Club site:
So, there is no reason to refrigerate peanut butter from a freshness stand point. I still think refrigeration will slow the separation process, but I need to test it sometime. Refrigeration does make the peanut butter much harder, so that’s definitely a factor.
Mustard won’t get spoiled at room temperature. Like almost anything else, it tastes better at room temperatute. It is cheap enough to be replaced if something happens to it. What’s the problem?
From Rich Hall’s Sniglets:
Ah, yes, a similar question to the chocolate syrup thread I started a while back.
Strangely, it IS indeed safe to keep mayonnaise in the cupboard; according to an article I read, the acidity of the vinegar kills anything truly nasty. I would never, ever keep mayo anywhere other than the fridge, though.
Incidentally, I keep both my mustard and my ketchup (but never my peanut butter or chocolate syrup) in the fridge.
although typically, mustard is made from vinegar (or some other acidic thing) and mustard (seed/powder), both of which are not particularly good hosts for germs, there can be other ingrediants in there that would need refrigeration (eggs for one - in one recipe I have for mustard, there’s cooked eggs in it), so I’d read first.
I like to make my own mustard; it’s pretty simple and tastes better than the store bought in my opinion. It is basically ground mustard seed and some type of liquid, like vinegar or wine, and maybe some other flavorings if you want to get fancy. Freshly prepared mustard is very sharp and somewhat hot but mellows over time, at room temperature. To stop the process when your mustard reaches the desired level of piquancy, you refrigerate it. Otherwise it will get bland. I believe the process involves some sort of enzymatic action started by mixing the liquid in, but I’m working from shaky memory.