Guide
Mylar Bags and Oxygen Absorbers for Freeze-Dried Food
October 21, 2026
Freeze-dried food leaves the machine with almost no water in it, and it will start pulling moisture back out of the air the moment you open the door. Oxygen is the other enemy: it turns fats rancid and fades color and flavor. Good packaging deals with both. For long storage that means a thick mylar bag, the right oxygen absorber, and a proper heat seal, done the same day the batch finishes.
The short version
| Bag size | Dense food (rice, powders, crumbles) | Loose pieces (most fruit, veg, meat) | Very airy (puffed candy, whole berries) |
|---|---|---|---|
| 1 quart | 100 cc | 300 cc (or two 100s) | 300 cc |
| ½ gallon | 300 cc | 300 cc | 500 cc |
| 1 gallon | 300 cc | 500 cc | 750 cc |
| 5 gallon (bucket liner) | 2,000 cc | 2,500 cc | 2,500 + 750 cc |
These line up with the common “300 cc per gallon for dense food, 500 cc for loose food” rule that bag sellers use, and they are what our calculator gives with no extra margin. For other containers, or to add a safety margin, the oxygen absorber calculator works it out from the air left in the bag.
Why the absorber size depends on the food
An oxygen absorber is a small packet of iron powder. It reacts with oxygen and locks it up as iron oxide. A “300 cc” absorber can take up about 300 cubic centimeters of oxygen. Air is about 21% oxygen, so one 300 cc packet clears the oxygen from roughly 1,400 cc of air.
What matters is how much air is in the sealed bag, not how much food. A gallon of rice leaves maybe 35 to 40% air between the grains. A gallon of freeze-dried strawberry slices is mostly air, often 60% or more, because the pieces keep their shape but weigh almost nothing. Puffed candy can be 80% air. So the same gallon bag needs a bigger absorber for strawberries than for rice.
Overshooting is harmless. An absorber that runs out of oxygen to grab just stops. Undershooting means oxygen is left in the bag, which shortens shelf life for anything with fat in it. When in doubt, go one size up.
Bags: thickness and windows
Utah State University Extension recommends 5 to 7 mil mylar bags without clear windows, paired with oxygen absorbers, for the best long-term protection. Windowed bags let more air and moisture through, and light degrades food over time. A few practical notes:
- Thicker bags puncture less. Freeze-dried food has sharp edges (think corn, crisp apple, chicken pieces). 7 mil bags survive handling better, especially for meat.
- Size the bag to the food, not the other way around. A bag that is a quarter full has more headspace air and needs a larger absorber. Smaller bags also mean you open less at a time.
- Vacuum sealer bags are fine for weeks or months in the pantry but are not an oxygen barrier for years. Plastic slowly lets oxygen through.
- Glass jars with absorbers work well for short and medium storage, but keep them in the dark.
Sealing
Mylar does not seal with a vacuum sealer’s thin strip as reliably as with a wider heat bar. Owners use an impulse sealer or a hair-straightening iron. Either way:
- Fill the bag, leaving about 2 inches clear at the top.
- Wipe the inside of the seal area. Powder or crumbs in the seal are the most common cause of slow leaks.
- Drop the absorber in last, right before sealing.
- Press out as much air as you can by hand, then seal across the full width.
- Check the seal by trying to pull it apart gently. It should not separate.
- Label: food, date, and whether it needs cooking after rehydrating (raw-cooked meat, for example).
Within a day or two, a properly sealed bag with an absorber usually pulls in a little and feels firmer. That’s the oxygen being used up. A bag that stays puffy for a week may have a leak.
Handling absorbers without wasting them
Absorbers start working as soon as they touch air. A pack of 100 left open on the counter can be spent in a few hours.
- Open the pack only when your bags are filled and lined up.
- Take out what you need for the next 15 to 20 minutes and reseal the rest immediately, ideally in a small mason jar or a mylar bag with its own seal.
- The pink indicator pill in many packs turns purple or blue once oxygen gets in. If it has changed before you start, the remaining absorbers may be partly spent.
Mistakes that show up months later
- Bagging food that was not fully dry. The bag goes soft, the food goes chewy, and in the worst case mold grows. Check dryness before bagging, especially thick pieces in the center of the tray. The dryness check uses tray weights to catch this.
- Using an absorber with a desiccant. A desiccant (silica gel) absorbs moisture, not oxygen. You don’t need one if the food is properly dry, and some absorbers work less well in very dry conditions anyway.
- Bagging the next morning. Food left out overnight takes on moisture from the room. If you can’t bag at once, leave the trays in the closed machine; on a Harvest Right, the final dry can keep running until you’re ready.
- Storing bags somewhere warm. Heat speeds up quality loss. A cool, dark closet beats a hot garage every time.
For the cost side, the cost per batch calculator adds bags and absorbers to the electricity, so you can see what packaging adds per pound of food.
Sources: Utah State University Extension, “Can I Freeze-Dry That?” (2026); Penn State Extension, “Let’s Preserve: Freeze-Drying”. The absorber chart is a rule of thumb, not a lab standard.