Correctly dried to under 3% residual moisture, sealed in 7 mil mylar with a correctly sized oxygen absorber and stored below 24°C in the dark, low-fat freeze-dried food will keep for 20 to 25 years. Get any one of those four wrong and you have a two-year food.
The four things that decide shelf life
- Residual moisture. Under 3%, ideally under 2%. Nothing downstream can rescue a batch that was not fully dry.
- Oxygen. Removed by an absorber sized to the bag, not by vacuum alone.
- Light. Mylar blocks it completely, which is most of why mylar rather than clear plastic.
- Temperature. Below 24°C. Every 10°C warmer roughly halves the life. A garage that hits 35°C in summer is not long-term storage.
Mylar thickness
| Thickness | Best for | Verdict |
|---|---|---|
| 3.5 mil | Single meals, weeks to months | Punctures too easily for stored food |
| 5 mil | Rotating pantry stock | Acceptable if handled gently |
| 7 mil | 20–25 year storage | The standard, and what we use |
| 7 mil in a bucket | Rodent-prone spaces | Best protection available |
Sizing oxygen absorbers
Absorbers are rated in cubic centimetres of oxygen they can scavenge. Air is about 20% oxygen, so a container of air needs an absorber rated to roughly a fifth of its volume — and in practice you round up generously, because an under-sized absorber is the same as no absorber.
| Bag or container | Absorber |
|---|---|
| 1 quart / 1 litre | 100cc |
| 1 gallon / 4 litres | 300cc |
| 2 gallon | 500–600cc |
| 5 gallon bucket | 1500–2000cc |
Absorbers begin working the moment they meet air. Once you open a strip, use them all within about an hour or reseal the remainder in a jar — a flat absorber looks identical to a fresh one, which is why buying individually packed absorbers is worth the small premium.
Sealing
You need a heat sealer with a jaw wide enough to cross the bag in one pass. A 12-inch impulse sealer handles gallon mylar in a single 1.5-second press; a narrow one needs two passes and the overlap is where seals fail. A hair straightener will seal 3.5 mil in a pinch and will not seal 7 mil reliably.
Leave a couple of inches of headroom above the food, drop the absorber in last, press the air out gently, and seal. Then check: run a finger along the seal, and squeeze the bag. A bag that hisses has not sealed.
Labelling
Contents, date sealed, and weight if you weighed it. Twenty-year storage means somebody who is not you may be opening it. Write on the bag before you fill it, not after.
Fat is the exception to all of this
None of the above buys you 25 years on cheese, nuts or fatty cooked meat. Fat oxidises rather than spoiling from moisture, and no absorber prevents that. Treat fatty food as a 2 to 5 year store and rotate it.