I'd say that if you're only storing them for 2-5 years, and can live with lower germination rates after 3-4 years, almost anything works for short-term storage.
(I'm talking about normal seeds, not tropical seeds that need to stay moist at all times, and need to be planted ASAP after falling from the plant.)
(Maybe "sealed in plastic" protects them from
swings in humidity, for example rainy days and fog condensing when the fridge door is opened. But it also seals IN any humidity still in the seeds when you zip them shut, plus humidity released from the (living) seeds as they metabolize slowly. That's why I like some desiccant in the picture when I seal seeds in ZIP-Locs. But I've gone for years at a time without replenishing the desiccant, so I think it's optional except in humid areas.)
Just make sure the humidity always stays
below 50% RH (avoid mold) and hopefully usually below 30% RH (slow down the metabolism of the seeds so they don't consume all their stored energy just sitting on the shelf).
15% RH is the lower humidity limit. Normal seeds last the longest at that RH.
Kew Royal Botanic Gardens had the most convincing arguments about seed storage:
"Seed life span approximately doubles for every 10% reduction in seed eRH."
http://www.kew.org/sites/defau...
http://garden.org/ideas/view/R...
http://garden.org/ideas/view/R...
http://garden.org/ideas/view/R...
I'm always suspicious of humidity inside a fridge. Every time I open the door, I SEE some condensation, and that's only the droplets that I SEE. When I was storing seeds in the fridge, I had them sealed in Zip-Locs INSIDE a screw-cap jar, with desiccant in the jar. I never opened the lid until after all codnesation had dried from the outside of the jar, and the contents seemed up to room temperature.
BTW: the "crisper drawer" or "vegetable drawer" is the MOST humid part of the fridge. It's designed that way to keep lettuce crisp by preventing it from drying out.
The freezer, in modern self-defrosting fridges, actively dehydrates itself to reduce frost formation. "Freezer burn" is when food dries out and hardens in that aggressively dry environment. Yet water condenses on every package every time you open the freezer door. If you store seeds there, dry them WELL first, seal them VERY WELL to reduce long-term freezer burn, A.K.A. potent dehydration. I would double-bag them at the very least, or bag them inside a screw-cap-jar with a good soft gasket.
Then (if it were me) I would worry as much about condensation as about freezer burn.
Anyway, I don't store seeds in the freezer, so someone with more experience may be able to say: "freezer burn and condensation are all well and good, but as long as I kept them dry, my seeds lasted 2-4 years with no care at all, and more years than were easy to test in the fridge or freezer, so what's the problem?"
But, if you ever found insects or insect-holes in saved beans or other seeds, the freezer is good for killing insects and insect eggs. Dry the seeds well, very well, and double-bag them (at least - here is a great use for a desiccant!) After they are very dry and well-sealed, toss them in the freezer for at least three days. Kill 90% of any insects and eggs.
(The hard part is taking them out of the freezer without water condensing inside the bags. Hence desiccant in the inner bag.)
After you take them out, let them come up to room temp and all moisture to evaporate before you open the double-bags.
If it was worth killing bugs once, it's worthwhile to wait a week or two, then re-freeze them in the same double-bags, to kill most of the remaining 10% insects.
Seeds last longer in storage if they get several of these, the most important ones listed first:
1. Be DRY (15-30% eRH).
2. be uniformly dry, without humidity swings
3. be cool, without large temperature swings.
4. be dark
I think it was Joseph who pointed out the very first rule: keep hungry rodents away from stored seeds!
