Why does peanut butter not have to be refrigerated and other foods like jelly have to be?
The peanut butter I buy says to refrigerate after opening…
Bacteria grow where there are water and food. Bacteria like to eat sugars.
Peanut butter is essentially ground peanuts, salt, and maybe some flavouring or oil. There’s negligible water, and not enough sugar. The bactera can’t grow.
Cite: umm… well, I work in the place where they make Reese Peanut Butter Cups… does that count?
It’s got a pretty high oil content, which might have something to do with it. Then again, perhaps the bacteria share my belief that the stuff is just too revolting to touch. 
Also, the jars are barriers against bacteria entering, except those times when the jar is open.
Peanut butter can be stored without refrigeration because it contains less than 1% moisture and therefore cannot support the growth of bacteria or molds. PDF cite
I guess I should specify… we were designing a transfer system for peanut butter a couple of weeks ago, and one of the thigns you always discuss in designing a transfer system for food is whether there’s a risk of microbiological contamination (“Micro Issues”). In this case, the senior engineers notified me that the risk of this was very low, because, well, for the reasons I gave above. Bacteria very rarely grow in PB, because it’s just ground peanuts and salt, with maybe a couple of extra things.
Now, some commercially available peanut butters have a lot more in them- they may be sweetened, or have chemical flavourings or whatever added in. (IMO, those are to Peanut Butter what ‘Orange Drink’ is to Orange Juice) For these, there may be more moisture, and more things that the bacteria can metabolize. Maybe that’s why some recommend refrigeration. Or maybe the manufacturer is afraid that you’ll get the stuff wet.
So, that’s the story. In general, PB can’t support bacteria becasue it has very little moisture, and some lack of the right nutrients for the bacteria. But mostly just no moisture.
Doesn’t it also contain a preservative - or several? It doesn’t spoil because of bacteria but it can sure get rancid as I learned to my dismay a year or so ago.
Peanut butte? What’s that, a mountain of peanut?
Yes – related to the hill of beans which played such a vital role in world politics and Hollywood films.
My PB needs refridgeration so it won’t seperate. It has no additives. No sugar.
Oh, well, there’s that.
But separation is no big deal. You can just mix it back together.
Remixing is a big pain in the …uh butte. Until a year or so ago, I bought it from a U-grind-it place (which has since closed, sadly) and I refrigerated it immediately and it never separated. Needless to say it had no sugar, no salt, no preservative.
And yet you eat Vegemite? And don’t pretend that you don’t, I can sense you salivating at the mere mention of the word “vegemite” all the way on the other side of the planet. “Vegemite on toast. That’d be just the thing right now!” you’re saying to yourself.
Regular grocery store peanut butter needs no refrigeration (like Peter Pan, Skippy, etc). The “natural” peanut butter you find in health food stores (no added sugar or anything) says “for best quality refrigerate to avoid separation”. But I don’t think it’s a health thing.
Most commercial peanut butters are NOT just peanut butter and salt. I am looking at two peanut butter jars, with the following ingredients:
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peanuts, dextrose, partially hydrogenated vegetable oils, salt;
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peanuts, sugar, dextrose, hydrogenated vegetable oil, salt, molasses, mono- and diglycerides.
Neither of these peanut butters contains antimicrobial preservatives; as others have explained, peanut butter does not have enough moisture to support bacterial growth (these do have enough sugar, I dare say). The presence of hydrogenated vegetable oils (fat, basically) and mono-and diglycerides (fat-related solids), however, explains why these do not need to be refrigerated, and “natural” peanut butter does: separation and rancidity.
Separation happens when the peanut solids “fall” through the liquid peanut oil. The above ingredients mix thoroughly with the oil and thicken it, preventing separation.
Rancidity happens when oil molecules oxidize; this happens far more easily to unsaturated oils (like peanut oil) than to saturated fats (like hydrogenated vegetable oil). Most peanut butter therefore takes more time to go rancid than “natural” peanut butter. Even the most “unnatural” peanut butter, however, will keep longer in the fridge.
As a matter of fact, at least some manufacturers remove the peanut oil and replace it with other, more stable, oils.
Courtesy of “The Food Network”, IIRC.
Some people think it does - discussed here (amongst other things), tangentially here, by Jif here and by Skippy here.
Try telling that to a Canadian!
I’ve found that if I buy Smucker’s Natural peanut butter (just ground peanuts) remixing is a pain in the neck. But–the stuff I get out of the grinder from the health food store never separates enough to need much more than a few swishes of the knife, and I never keep it in the refrigerator. I think the grocery store stuff sits on the shelf longer and gets solid or something.
I have a theory that if I ever managed to mix the grocery store stuff sufficiently, it would act like the health food store stuff, but since I really hate dealing with that solid lump of peanut under two inches of oil, I’m not going to try it until I can’t get fresh ground anymore.