The short version: A mylar bag that never sucks down can be sealed correctly. An oxygen absorber removes only the oxygen fraction of air, about 21%, so roughly four-fifths of the air stays in the bag by design [1][2]. Most of what gets repeated about mylar and absorbers is contradicted somewhere in the manufacturers' own documentation. Below are six beliefs with the correction quoted from the people who make the product.
Every claim here is quoted or paraphrased from manufacturer technical pages, a freeze dryer OEM, or a university extension service. Where two manufacturers disagree, both positions are shown rather than resolved. Sublime Pantry has not bench-tested any of this.
The six at a glance
| The belief | What the manufacturer says | Source |
|---|
| Shrinkage proves a good seal | "You may not always see that vacuum-packed look even though the oxygen has indeed been removed." | PackFreshUSA [1] |
|---|
| Vacuum sealing removes the oxygen | "Air still contains oxygen — even after vacuuming." | PackFreshUSA [6] |
|---|
| Thicker mylar is a better oxygen barrier | "It is only the aluminum on the surface that keeps the oxygen out." | PackFreshUSA [9] |
|---|
| A FoodSaver cannot seal mylar | "If you have a FoodSaver, that is a very easy way to seal the smaller gallon or quart bags." | PackFreshUSA [1] |
|---|
| Mylar keeps out rodents and insects | "Mylar is not strongly resistant to insect penetration and not resistant at all to rodents." | USA Emergency Supply [4] |
|---|
| A sealed, oxygen-free bag makes food safe | "This low-oxygen environment is just right for the development of several pathogens." | NC State Extension [13] |
|---|
Myth 1: If the bag doesn't suck down, the seal failed
The belief. A finished bag should pull tight around its contents. A bag that still looks puffy means the seal leaked or the absorber was dead.
The correction. PackFreshUSA states it directly: "You may not always see that vacuum-packed look even though the oxygen has indeed been removed" [1]. The arithmetic is in the same document; the absorber takes the oxygen only, so "1/5 of the air volume will be gone but 4/5 of the air will always remain" [1]. Wallaby Goods names the belief as a misconception in its FAQ: "It's a misconception that a 'vacuum-packed' look confirms the oxygen has been removed" [2]. Discount Mylar Bags adds the food-type variable, noting that powdery foods like flour will not shrink and some freeze-dried items may not either, depending on air content [3].
Worth knowing that Wallaby's own blog says the opposite of its FAQ, describing a properly sealed bag as one that "will often feel slightly 'sucked in'" [5]. That contradiction sits inside one manufacturer's documentation and neither page reconciles it.
What to do instead. Test the seal mechanically. USA Emergency Supply publishes the pull test: "Try to pull the mouth of the bag open. If it doesn't come open… then your seal is good" [4]. Combine that with a check on the absorber itself before it goes in. PackFreshUSA's tactile test is that a good absorber "feels soft and powdery," and where an indicator pill is included, it should read pink or red rather than dark blue [1]. Shrinkage, if it happens, is a side effect. It is not a test.
Myth 2: Vacuum sealing removes the oxygen, so absorbers are optional
The belief. Pulling a vacuum evacuates the bag, so the food is stored without oxygen and an absorber is belt-and-suspenders.
The correction. Vacuum reduces the volume of air, not its composition. PackFreshUSA sums it up in four words: "Vacuum != oxygen-free," alongside the fuller statement that "Air still contains oxygen — even after vacuuming" [6]. Wallaby reframes vacuuming as preparation rather than protection: "This is not vacuum sealing; you're simply removing large pockets so the absorber will have less work to do" [7]. The gap between the two approaches is large. IMPAK Corporation publishes that with an absorber in place "the oxygen level in the surrounding atmosphere is effectively reduced to approximately 0.01%" [8], a figure Wallaby also cites for its own absorbers [2]. No manufacturer in this set publishes the residual oxygen figure for vacuum alone.
What to do instead. Treat the two steps as doing different jobs. Discount Mylar Bags' instruction for long-term storage is the short form: use oxygen absorbers plus mylar [3]. If you vacuum, do it to collapse headspace before sealing, and size the absorber for the bag as if you had not.
Myth 3: Thicker mylar is a better oxygen barrier
The belief. A 7 mil bag holds oxygen out better than a 5 mil bag, so gauge is the spec to shop on.
The correction. PackFreshUSA attributes the barrier to one layer only: "It is only the aluminum on the surface that keeps the oxygen out" [9]. Its numeric version is blunter, noting that 7 mil mylar provides no more oxygen protection than 2 mil mylar [9]. What thickness buys is puncture and abrasion resistance. On the same page, PackFreshUSA also addresses two related worries: creases at the folds are "tiny breaks in the foil layer… not holes," and "their barrier properties are not significantly affected" [9]; and a bag that passes light in the flashlight test is not thereby defective, because "inside a metalized bag in a cool, dark storage place, there is virtually no effect from light" [9].
This one is contested. Blue Alpine Freeze Dryers specifies heavy gauge for freeze-dried loads: "thicker bags (at least 7 mils, 14 mils total)" with "a thicker layer of aluminum (at least 12 microns)" [10]. Note that the second half of that spec is about the aluminum, which is the layer PackFreshUSA credits. The publishers do not address each other.
What to do instead. Buy gauge to handle sharp-edged contents, bags that get moved, and anything stacked. Do not pay for gauge expecting a longer shelf life from it. If a vendor publishes an aluminum layer thickness, that is the number closer to the barrier claim.
Myth 4: You need an impulse sealer; a FoodSaver won't work on mylar
The belief. Mylar demands a dedicated impulse sealer, and a household vacuum sealer cannot make a real seal on it.
The correction. The largest packaging vendor in this set says otherwise. PackFreshUSA: "If you have a FoodSaver, that is a very easy way to seal the smaller gallon or quart bags" [1]. Its instruction is to "turn the pump off and just use the sealing feature," because "oxygen absorbers do a much better job of eliminating the oxygen" [1]. The same page treats household tools as ordinary practice: "A hair-straightening iron or household iron are favorite ways to seal the larger bags" [1]. Wallaby ranks the impulse sealer as recommended rather than required, stating: "While an impulse sealer is recommended to maximize seal strength, a clothing iron on the wool setting, or hair straightener can also be used" [2]. Harvest Right, the freeze dryer OEM, agrees: "Then, simply seal the bag with an impulse sealer or even just a hot iron or flat iron" [11].
The real constraint is thickness, not machine type. Dura-Pack notes that "Mylar bags are thicker than standard vacuum bags" and advises you "check if your vacuum sealer can handle them" [12], without publishing a mil limit.
What to do instead. Try the sealer you own on an empty bag first, with the pump off, and pull-test the result [4]. Buy an impulse sealer for seal consistency and for wide bags, not because the alternatives are said not to work.
Myth 5: A mylar bag protects food from rodents and insects
The belief. Sealed foil-laminate bags are a pest-proof package on their own, so they can sit on an open shelf or a garage floor.
The correction. This is stated in the strongest terms of anything in the set. USA Emergency Supply: "Mylar is not strongly resistant to insect penetration and not resistant at all to rodents" [4]. The same source gives the structural conclusion: mylar has "poor puncture resistance so it must be used as an interior liner for more puncture resistant containers" [4].
What to do instead. Treat the bag as a liner and give it a rigid outer container, such as a food-grade bucket, a lidded tote, or a sealed bin. Store it off the floor. Neither Harvest Right nor Blue Alpine publishes pest guidance in its storage documentation [11][10], so the extension and vendor position above is what there is to work from.
Myth 6: Sealed and oxygen-free means the food is safe
The belief. Removing oxygen removes the spoilage risk, so anything sealed this way keeps indefinitely and you will see or smell it if something goes wrong.
The correction. Low oxygen is a selective environment, not a sterile one. NC State Extension: "This low-oxygen environment is just right for the development of several pathogens, but one to be specifically concerned about is botulism" [13]. You cannot inspect your way past it; "these disease-causing bacteria may not change the color or look of the food" [13]. Sealing also does not reclassify a food: "Perishable foods that need to be refrigerated or frozen without vacuum packaging still need to be refrigerated or frozen once sealed" [13], and "vacuum packaging is not a substitution for the heat processing of home canned foods" [13]. Discount Mylar Bags states the manufacturer version. Oxygen absorbers should never be used with moist foods, because of botulism risk [3].
The long shelf-life numbers carry a condition attached. Wallaby's framing: "Foods with less than 10% moisture content such as cereals and grains can last up to 30 years… when stored correctly" [2]. That figure is about dry goods, stored dry.
What to do instead. Keep this method to properly dried and freeze-dried foods and to shelf-stable dry goods. Anything that needed refrigeration before sealing still needs it after [13]. Follow your food-preservation authority's guidance on what is safe to store dry; we describe what these sources say and do not give safety advice.
What we could not source
- Residual oxygen after vacuum sealing alone. Every manufacturer asserts that vacuum is not oxygen-free [6]; none publishes the number. NC State does not specify what percentage remains [13]. Only the with-absorber figure of about 0.01% is published [8][2].
- A mil-thickness limit for household vacuum sealers. Dura-Pack raises the thickness constraint but publishes no specific limit for any sealer type [12], which is the practical question behind Myth 4.
- Harvest Right on suck-down and pests. The OEM's storage guide does not address whether a vacuum appearance indicates a proper seal, and gives no rodent or pest guidance [11].
- Blue Alpine on bag reuse and pests. Its storage guide covers gauge and sealing; it does not address reuse or pests [10].
- Whether absorbers have a shelf life. Manufacturers split rather than converge. PackFreshUSA holds that absorbers do not age and only fill up on exposure [1]; Discount Mylar Bags publishes an unopened shelf life of six months to two years depending on storage conditions [3]. We record both and reconcile neither.
Next
Sold by Sublime Pantry: 100cc Oxygen Absorber Refill — absorbers on their own, in sealed ten-packs; that size pairs with a pint bag and smaller.
Sold by Sublime Pantry: 100cc Oxygen Absorber Refill — 100 Count. Sublime Pantry has no affiliate relationships, sponsorships, or paid placements as of publication. We sell packaging; where a product we sell appears, it is labeled.
Common questions
My mylar bag never shrank. Did the seal fail?
Not necessarily. An oxygen absorber takes only the oxygen fraction of air, so about four-fifths of the air remains and PackFreshUSA notes you may not see the vacuum-packed look even when the oxygen is gone [1]. Powders like flour and some freeze-dried items will not contract at all [3]. Pull-test the seal instead: if the mouth of the bag will not come open, the seal is good [4].
If I vacuum seal, do I still need an oxygen absorber?
Manufacturers say the two do different jobs. Vacuum removes air volume, not oxygen: “Air still contains oxygen — even after vacuuming“ [6]. Wallaby describes vacuuming as removing large pockets so the absorber has less work to do [7]. With an absorber, IMPAK publishes a residual oxygen level of about 0.01% [8].
Is a thicker mylar bag a better oxygen barrier?
PackFreshUSA says the barrier is the aluminum layer, not the gauge, and that 7 mil mylar provides no more oxygen protection than 2 mil [9]. Thickness buys puncture resistance. Blue Alpine nonetheless specifies at least 7 mils with at least 12 microns of aluminum for freeze-dried food [10]; the two publishers do not reconcile.
Can I seal mylar with a FoodSaver or a hair straightener?
PackFreshUSA calls a FoodSaver ”a very easy way to seal the smaller gallon or quart bags“ with the pump turned off, and lists a hair straightener or household iron for larger bags [1]. Wallaby treats an impulse sealer as recommended rather than required [2] and Harvest Right names a hot iron or flat iron [11]. Dura-Pack's caveat is thickness: check whether your sealer can handle it [12].
