Long ago, I cut open a pack of dried yeast (one of the Gervins), used half of it, immediately sealed it again, and hoped it'd be OK next time I needed some. Nope. Mind you I did leave it quite a long time - at least 6 months.
So, even a brief exposure - say 5 minutes - to a non-protective atmosphere is enough to spoil dried yeast. Plus the time factor, obv.
Lucky I had plenty of other packs on hand really - doing the clear-the-freezer-coz-there's-stuff-ripening-that'll-need-to-go-in-it thing.
This brew is a mix of raspberry, blackberry, blackcurrant, redcurrant, and (red) gooseberry. Not necessarily in that order (by weight).
This weather is making our berries ripen early, but they're a bit small as they've not had the rain to plump them up. They seem to have the taste and sugar though.