Grandmas know best, and like most, mine was an incredible woman - she worked in a munitions factory during WW2, owned a successful bakery, and helped me grow into the handsome and modest man I am today.
She also kept her butter in the fridge, which is good enough for me.
However, it’s not good enough for a few people around here, as other equally trustworthy old dears leave theirs on the worktop in a little pot.
Apparently “it's never killed anyone”, so I guess we’ll have to look into it further…
Proper old fashioned butter is made almost entirely from dairy, and like milk, it deteriorates pretty quickly if left at room temperature.
It’ll be alright in an airtight butter dish for a week or so, but after that, it’s going to start to taste a bit sour. Store the same dish in a fridge and it’ll last for up to six months without going bad.
“…there is a very much greater deterioration when butter is stored at 20° and even 10° than when it is kept at O°”. Pacific Rural Press, Volume 85, March 1913
So, for longevity and taste’s sake, store it in the fridge. Old farmers and my Gran say so.
Salted butter lasts a bit longer out of the fridge than the normal variety, but it still goes off much quicker than if it was properly cooled.
As with non-salted, keep it in the fridge until you need to use it.
Margarine has a load of oil and other stuff in it that butter doesn’t, so lasts a lot longer out of the fridge without going bad.
However, there’s still some dairy in it, so it will eventually go mouldy if left out for too long.
If you go through a pack every week or so, you’re fine leaving it out. For occasional users, stick it in the fridge where it’ll last for months.
Despite advocating correctly cooled condiments, I’ll concede that trying to spread hard butter straight from the fridge is a nightmare.
The easiest way around this is to take a few days’ worth of butter out of the fridge and keep it in a butter tub somewhere cool in your kitchen.
You’ll get soft butter for easy spreading without ruining the rest of it.