[cypher]

I think mites in substate are pretty rare, I have heard of people who suspected mites came from substrate but it's not something I've ever had a problem with, I've never frozen substrate & it's not something I worry about.

I do freeze all my hamster food mix, any dried plant material like leaves or flowers, cork & wooden items that go in the cage. Although you may not see any mites, weevils, moths etc there is the possibility of eggs being hidden & freezing should prevent any problems with that.
I freeze mine for at least a week, I usually buy more than one bag of food & the ones I'm not using just stay in the freezer until needed.
You do need to put it into a plastic box of some sort & most importantly keep it in the box until completely defrosted so no moisture gets into the food as it's defrosting.

Messing with their food hoard is stressful for them, you really shouldn't remove it unless it has got wet food like veg in it or has been peed on. Bugs are unlikely to be attracted into the cage by the stash as it's usually pretty well hidden away.