Do Raisins become stale? What happens to them? How should I store them?
They absolutely do. They're "dried" but they're not really dry. They will continue to dry out until they become grainy and weird. Most dried fruit is this way, with the exception of freeze dried fruit, which has the opposite problem: once you open it, it absorbs moisture from the air, and gets gummy and gross.
We bulk purchase a variety of bulk dried fruits. Depending on their moisture level when you buy them they will keep for a long time (up to a year)
If they are moist and squishy (typical of apricots), they will need to be frozen to last past a few months
For reasons I don't know dried fruit does not improve with age. They don't taste 'more dried', they just lose flavour and texture. I have not had any go off, they are just not as nice
The exception being Arab figs, which seem to become more intense with storage?
Like most dry goods, store them in a cool dark place in an airtight container
I remember raisins being sold in packets with a little amount of (edible) petroleum jelly on them to keep them moist - 0.5% by weight, if I remember correctly.
My first degree was in Food Science (I graduated 33 years ago so I'm a bit out of date), and as part of my studies I worked for six months in what was called a 'Public Analysts' laboratory, in which we would check the chemical contents of food. Once I was asked how I would check the oil content of a packet of raisins - I would add the raisins to alcohol or ether then separate the solvent and evaporate it. What would be left would be the oil.