Should Your Mylar Bags Contract?

It’s been a while since I did a too-long post, so this one’s been cooking for a while.  Being more active in Facebook groups has made me start to re-examine every bit of advice I’ve both given and received as ‘conventional wisdom’ over the years.  I’ve put up over a hundred bags of food recently, and I’ve gone back over my thousand+ other bags and it’s made me change my thoughts on one bit of advice I’ve been reading recently. 

The question I’m posing today is an easy one: should your Mylar bags contract when using an oxygen absorber?   The reason I felt the need to write this is to clarify some information I see people giving out on this subject.  Additionally, there was a gentleman in one of my earlier threads that said essentially none of his bags contracted and he thought it was OK because of what people were saying, and that started to worry me.  We (as a community) may have let the contraction issue slide too easily, and here’s why.  One of the most common answers to the question is a variation of : ‘they don’t have to, as air is only 20% oxygen, and that’s all that’d being absorbed.’  While true, that’s not nearly a complete answer.

TL; DR:  My new concise answer to this question is now:  Barring powders, yes, your Mylar bags should contract if using an oxygen absorber.  If they don’t (or haven’t), I suggest you start checking your food.  Of the dozens of food types, both dry goods and freeze dried foods I have stored (a partial listing at the bottom), the only ones where you can’t see some contraction are powders:  mainly flour and eggs.  There are a couple of other food types that show less contraction, such as bread. Food in 7 Mil bags show less contraction than in 5 Mil bags, due to higher tensile strength and less ‘printing’ (This is where you can run your nail down a Mylar bag and see a line).

My concern is that some of the food storage supply companies may be deflecting from the fact that their absorbers are under-weighted (see experiment in a few paragraphs) by saying that its OK for your food to not contract.  And some of the freeze dry and prepper user base has picked up on that and there’s very little concern over bags not contracting.

So I did an experiment with samples I recently purchased from one of the most popular food storage companies.  I’m not going to name names, because I’d rather have folks reproduce this on their own than take my word for it if they feel my methodology has some holes.

Experiment Part A:

1:  2 brands of Oxygen Absorber, Brand A and Brand B; the Brand A oxygen absorbers are rated 300cc, and the Brand B oxygen absorbers are rated 400cc.  Generally speaking, 300cc is considered the right amount to clear a 1 gallon bag filled with food. The 300cc Brand A absorbers weigh approximately .303 ounce per packet; the 400cc Brand B absorbers weigh approximately .094 ounce per unit.

Oxygen Absorbers Oxyfree on Left, Wallaby On right
Oxygen Absorbers – Brand A on Left, Brand B on Right

2:  20 8”x12”x4” 9 Cup 5 Mil (2.25 quart) Mylar Bags; both absorbers should have 0 issues with clearing this size bag, and this experiment should have ended here.

3:  Half the Bags are filled with Macaroni (approx. 2.75 lbs), and half with Quick Oats (approx. 2 lbs, they are way less dense).  Each Brand of Oxygen Absorber was used in 5 bags of Macaroni, and 5 Bags of Quick Oats.

I thought the initial part of the experiment would be a 1 and done.  Either all of the bags would contract or all of the pasta bags would contract (but not the oats, because I’ve heard for 15 years they often won’t, and it’s not something I’ve added to my storage until just recently) and that would be the end of the experiment.  However, when I came in the next day to check, something I didn’t expect had occurred.  All 10 bags of the Brand A oxygen absorbers (the ones I generally use) had fully contracted, all 5 of the brand B pasta bags had contracted, but all 5 brand B oats bags had either not contracted or ran out of capacity before they fully contracted.  A few of the brand B oat bags had the beginning stages of contraction, but not completely.

Contracted Mylar Bags for Long Term Food Storage
Brand A Contracted Bags

What does this tell us?  My thought is that the Brand B absorbers, which you can tell from the pictures are very small compared to the ones I normally use, had enough capacity to clear the Pasta, but not enough capacity to clear the Oats.  So what does THAT tell us?  A 300cc rated absorber, at exactly its rating, should clear 1.5 liters of air of oxygen.  A 400cc rated absorber, at exactly its rating, should clear 2 liters of air of oxygen.  The 2.25 quart bags I was using = 2.13 liters of air.  Meaning, both absorbers should be able to almost clear the bags with NOTHING IN THEM.  The fact that the 400cc absorber struggled to clear full bags of oats is a strong indication that its given capacity rating is suspect. (and potentially dangerous if people are relying on them in  2 quart+ bags) 

Wallaby Absorbers Uncontracted bags
Brand B, Half the Bags Contracted, Half Didn’t (or didn’t complete)

I don’t know if any math geniuses out there want to help me with this, but even if the full bags of oats had 50% internal airspace, the absorbers would only have had to clear 1.07 liters of air, well below the 2.0 liters 400cc absorbers SHOULD clear.  The pasta bags specific density WAS high enough for them to clear, so we know the actual capacity of those 400cc absorbers is somewhere between the Food Density (my made up word to describe how much internal airspace a particular food has) of Oats x 2.13 liters and the Food Density of Pasta x 2.13 liters.  In any case, I think I can state with certainty that those 400cc absorbers don’t actually rate 400cc. Yes, that sentence is hard to read, but I think it says what I want it to.=)

BUT, this made me need to add an extra step to this process.  Because the Brand B absorbers did work on Pasta, I decided to re-test using 1 gallon bags, the size that both absorbers should be able to clear, assuming a Food Density of 60% or higher (meaning a 300cc is supposed to clear 1.5 liters in a 3.7 liter bag, or about 40%).

Part of me didn’t want to do this test, because I knew the likely result and I hate rebagging food.=)  But its ‘For Science!’  If the Brand B absorbers had trouble clearing a 2.25 quart bag, they weren’t going to clear a 4 quart bag.  And I know the Brand A 300cc absorbers have an actual rating north of 1000cc, or 4.0 liters, which is greater than the size of the bag empty, so I knew it wouldn’t be an issue clearing even a much bigger bag.

Experiment Part B:  I was running out of food to put up and the gallon bags are way bigger, so this is more ad hoc.

1:  Same absorbers as in Part A

2:  12 10”x14” 5 Mil 1 gallon (4 quart, 16 cups) Mylar bags.

3:  The bags are filled with either Flour (another food I’ve always heard won’t contract) or various kinds of pasta (Medium shells, Penne, Macaroni, all of which should have a bit of different Density).

Contracted Oxyfree Absorbers in Mylar Bags
Brand A 1 Gallon Test, all Bags Contracted

Results:  The results were exactly as expected.  All of the Brand A absorber bags were fully contracted, and none of the Brand B absorbers contracted at all.  Note about flour:  flour doesn’t have any ‘kernels’ or anything to contract around and show a nice pattern, but you call tell they contracted based on the little ‘bubble’ of air that sits at the top of the bag.  The Mylar on both sides of the bubble pulled together; its hard to see from the photo, but if you do it yourself you’ll see what I mean.  So yeah, the wisdom that says flour won’t contract seems accurate enough from a visual standpoint; you actually have to feel the bag to know it worked.

Wallaby 400cc not contracting 1 Gallon Mylar Bags
Brand B Absorbers did not contract any of the 1 gallon bags.

So what’s the verdict? 

  1. Almost every food I’ve ever done other than powders (flour and eggs) have visibly contracted.  Some were much milder than others (grilled cheese in small bags, for example)
  2. Even though the Pasta bags for the Brand B absorber contracted, I’m not confident they have enough capacity left to last 30 years to continue removing the very minor bit of oxygen passing through yearly.
  3. The 5 mil bags handled all the kinds of pasta just fine, even though I’d consider some of it ‘pokey’.  Another win for quality 5 mil bags.

I’m updating my documentation and FAQ’s to more clearly represent that in almost all cases, yes, your food should be contracting, with these addendums:

7 mil bags ‘print’ less than 5 mil bags, so your contraction won’t be as deep and noticeable as with 5 mil bags.

Flour will not look contracted when complete, you’ll need to examine the bags closely to know for sure, especially the ‘bubble’ at the top of the bag.

Oats have more internal airspace than macaroni.  This one surprised me considering macaroni is kind of hollow.=)

Smaller bags show contraction less, but it should still be noticeable. 

This is the list of foods I’ve done that have shown contraction, stored in 5x8x3 and larger bags:

Dry Goods: Rice, Beans, Oats, Pasta, Wheat

A small sampling of my Freeze dried foods:  Chili, chicken and rice, turkey pepperoni, shredded cheese, chicken nuggets, fish sticks, turkey kielbasa, turkey sausage, breakfast sausage, hash browns, tater rounds, sliced chicken, chunked chicken, apple fries, lasagna, strawberries, Skittles. And many more. My totes are all stacked and sealed, but looking into them I don’t see ANY that AREN’T contracted other than eggs.

Complete list of foods that don’t look contracted when properly sealed: Eggs (freeze dried), Flour (My assumption is other powders will show the same)

If you have food that’s not a powder you are confident is sealed correctly with a properly sized oxygen absorber and doesn’t show contraction, please let me know and I’ll do some testing.

Finally, and I stress this. These are my results. They have been consistent for these two brands of absorbers every time I’ve tested them (which is often), but that still doesn’t mean you shouldn’t do your own testing. If you would like to test the Brand A absorber, this link will let you purchase 1 pack (including shipping) for $6 (basically my cost and shipping, so I’m not making anything here) so you can test for yourself. And then please send me a note with your results!

Please let me know if you have any questions, comments, suggestions, or complaints! Thanks!

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06 2024

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