[thedoc (OP)]

Alan asked the Naked Scientists:
   
Why is butter kept in the fridge but peanut butter does not need to be?

I can't make all your radio answers so I would like a reply via my email please.

Many thanx
Alan

What do you think?

[Bored chemist]

I think a big part of the answer is that peanut butter has a very low water content and that stops most bacteria.

[alancalverd]

Proper (not "lite") butter has a fairly indefinite life at room temperature but melts in summer. Not a problem if you use it primarily for cooking (especially ghee, which has a very low water content) but unpalatable as a spread on bread. Even soldiers complain about mass-catering sandwiches made with brushed-on melted butter: it soaks through the bread and makes the sarnie sticky on the outside.