(I owe Kenny Harris big for this one.)
I love marmalade, but I love it in phases: I’ll go for months without being the right mood for it, and then BAM I’ll have a late-evening craving for a slice of toast, dripping in butter and smothered in Chivers Olde English. Until last year, I’d go to the cupboard, grab the aged jar from behind the little-used condiments, only to have my desires thwarted by layers of semi-sentient fungus.
Fortunately, there is a very simple way of making your marmalade last longer: don’t let it come into contact with any oily substance, i.e. butter (or margarine). I have no idea why this works; I’m just delighted it does. Use a separate knife or spoon to extract your marmalade instead of the one you buttered your toast with, and your marmalade will stay mould-free for months.
I don’t know if this works the same for jam. Jam doesn’t last long enough in our household to provide an adequate experimental baseline.