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Food Storage FAQ

Comprehensive Food Storage FAQ v. 0.2.0, 08/30/2018

What is an Oxygen Absorber?
An oxygen absorber is a small packet of material used to prolong the shelf life of food. They are used in food packaging to prevent food color change, to stop oils in foods from becoming rancid, and also retard the growth of oxygen-using aerobic microorganisms such as fungi.

The active ingredient is an iron oxide powder, which when it chemically reacts (IE. rusts) removes oxygen from the atmosphere.

How do I know when an oxygen absorber is working?
The most obvious sign an oxygen packetis working is that it gets warm. When continuously exposed to oxygen, some can get so hot as to be uncomfortable to touch, and will often form condensation on the inside of the outer package.  On some occasions, typically when it's very dry, an absorber might not get warm.  However, most oxygen scavengers have a ridiculously low failure rate.  If you are calling to tell us 'my absorbers didn't work', in 99.9% cases that's wrong.  An anecdote to go with that claim:

We have had several cases where customers went so far as to simply tape up (or staple, or wrap in paper towels) their oxygen absorber bag and send it back with a nastygram attached.  In a couple of those cases, even after several days in the mail, we were able to put several absorbers in some random food storage project we were working on, and they worked perfectly.

In the vast majority of cases, if your 'oxygen absorber didn't work' it's because the seal on your Mylar bag is compromised.  We recommend for the average 1 gallon bag a 2" seal.  All those little seams you see when you iron a Mylar bag are potential 'straws' that will let air in, so you need a good large seal to make sure those straws don't make it from the top of the seal to the bottom.

How do I know when an oxygen absorber is used up or no good?
This is one of the most frequently asked questions we get. The easiest way to tell if an oxygen absorber is good is to pinch the packet. If it feels ‘soft’ or powdery, the iron oxide powder is still in its original state and it is good. If it feels ‘hard’ or like a solid wafer in the packet, it is completely spent and should be replaced.

(An update to this question 8_30_18):  Please note that the manufacturing process of oxygen absorbers has changed and improved over the years since we started selling them.  While this may still hold true for many or even most oxygen absorbers, for many others it isn't.  They will simply stay as a powder for their entire life, while good and used up.

At the end of the day, they only way to know whether an absorber is good is if it gets warm, and to buy them from someone you absolutely trust to discard old absorbers when they aren't working any more.  (That's us; I regularly throw away oxygen absorbers, desiccant and Mylar bags that aren't 100%...better that than risking someone's food storage)

How long should it take for my absorber to remove all the air from my bag?
Some conditions are better than others for the speed at which an oxygen absorber works. For example, in a very dry climate, it might take up to a week for an absorber to fully activate. In a warm, humid climate it might take only 48 hours.

How many oxygen absorbers should I use? (Updated)
For 1-gallon bags, you should use 1 500cc or 2 300cc oxygen absorbers. For 5-gallon bags you should use 1 2000cc and 1 500cc oxygen absorber. You should adjust this number up a little bit if you are storing less dense foods, such as pasta or some lentils, because the bags will contain more air even when full in comparison to very dense foods such as rice or wheat.

Please note I’ve changed this answer somewhat. Over the last 8 years since I did the original FAQ, I’ve worked with 100’s of businesses and thousands of customers who are storing everything from dried goods, freeze dried food to hops and cannabis to pharmaceuticals. While a 300cc oxygen absorber is enough in most cases, there are more fail (not many, just more) cases using just 300cc. Things like freeze dried food, which naturally have a lot of internal airspace, should definitely have 2. However, that is also slightly overkill, so the easiest ‘new’ answer is 1 500cc per 1 gallon bag. This amount will cover far more cases with far fewer failures.

Do I need to use oxygen absorbers with everything I store?
Most foods will benefit in longevity when using oxygen absorbers. However, they are unnecessary when storing sugar or salt. In some cases, using an absorber with these foods will cause significant clumping, although it won’t harm them otherwise. Also note that some foods may not store well for long periods of time no matter the method used (for example flour, yeast and some spices).

I ordered 20/50/100 Oxygen Absorbers, I’m definitely not going to use that many…what should I do with the rest?

The easiest way I’ve found to store oxygen absorbers is to use a small mason jar with a gasketed lid.  You’ll know you have a good seal because the absorbers will pull the pop-top down.  Try to use the smallest jar possible to minimize the work the absorbers you are storing need to do to clear the jar.  You can also re-vacuum seal the absorbers in their original or another oxygen barrier bag. Second best to using a mason jar is to use a Mylar bag. Please note, oxygen absorbers will NEVER cause a bag to contract, no matter how many are in a bag. See my answer below about ‘why didn’t my bags get hard.’

What is Mylar?
BoPET (Biaxially-oriented polyethylene terephthalate) is a polyester film made from stretched polyethylene terephthalate (PET) and is used for its high tensile strength, chemical and dimensional stability, transparency, reflectivity, gas and aroma barrier properties and electrical insulation.

A variety of companies manufacture boPET and other polyester films under different brand names. In the US and Britain, the most well-known trade names are Mylar, Melinex and Hostaphan.

To be honest, Mylar isn’t any better or worse than many other sealant layers, such as branded films of LLDPE. It is just the most widely known.

Wait, your Mylar bags aren’t clear; what’s up?
The polyester film is combined with an aluminum foil layer and in some cases another sealant layer of LLDPE (Linear Low-Density Polyethylene); this allows for good heat-sealing, UV protection, incredibly low odor transmission, and high puncture resistance. Aluminum foil is the best barrier layer available in food storage packaging today.

How do I seal Mylar bags?
Please see this post and video about sealing Mylar bags. Mylar can be sealed with a variety of devices, including a hotjaw sealer, clothes iron, or hair straightener.

OMG, I can see pinholes of light coming through the Mylar bag, why is that?
All foil structure Mylar bags will have small pinholes in the foil layer. There is a measurement, ‘Pinholes per meter squared’, that is part of the specification of foil bags. Pinholes affect all foil structures, from a thin 2.5mil bag to the thickest 7-8 mil bags. The other transparent layers of the bag keep the integrity of these bags, and it is only very rarely (less than 1/100th of 1%) an actual ‘pinhole puncture’. Plus, some pinholes are so small they are invisible to the naked eye, but they are still there.

Why don’t my bags get hard when I use an oxygen absorber?
This is our third most common question. While there will often be compression of the Mylar bag after sealing due to the oxygen absorber, an absorber is only removing the 20% of the atmosphere in the bag that is oxygen, leaving the 80% that is nitrogen intact.

There is also a specific density of food to headspace (air) that is also required. As I mentioned above, you could put 100 oxygen absorbers into a small Mylar bag and it won’t contract. Obviously there is plenty of absorption capacity, but the packets themselves don’t have the required density/shape to allow contraction. In terms of headspace, as little as .1 liters of air can cause extreme contraction in one bag and mild contraction in another.

Please note also that 3.5 mil bags tend to show their compression a lot more, down to the shape of a kernel of wheat. 5 mil bags may ‘feel’ tight but show compression much less. A 7 or 7.5 mil bag may simply ‘feel’ compressed without much visible sign.

A tip: when sealing alufoil bags make sure you remove as much of the ‘headspace’ as you can; this is the area at the top of the bag you seal. Even a little headspace can use up much of the power of the oxygen absorber.

You Folded the Bags, eek!  Does folding hurt Mylar Bags?

The short answer is no, folding does not hurt Mylar bags.  The longer answer can be found here explaining why.

Can I store Hops in Mylar Bags?

Yes, much of the hops industry has moved to Mylar Bags for transport, storage and sale of their product. The barrier properties of a Mylar bag simply can’t be beat for the cost.

Can I store cannabis in Mylar Bags?

Yes, as above, the new best practice for storage of cannabis is either in large or small Mylar bags. Large 5 gallon Mylar bags (18”x28” to 20”x30”) are often used for flowers and storage. Small bags, sometimes clear front or and sometimes solid silver or colored are used in almost every dispensary and by every cannabis distributor.

I see another seller saying imported Mylar bags are no good and the American ones are better, is that true?

The short answer is no. The long answer is very long, but you can read it below or skip to the end!

The sellers saying this are selling almost exclusively on Amazon. In the early 20-teens, a new kind of infomercial came out, mostly online, that told people how to market and sell stuff on Amazon. Part of those courses teach sellers to ‘private label’ products that others are selling, and simply make the listings better, prettier, with nicer pictures and good bullet points on why you should buy their products over the originals. I’ll admit many of these sellers make prettier listings than I do!

I’ll also be very clear…there isn’t a whit of difference between a US made and an imported bag. They are made from the exact same film stock, the same layers of Mylar, polyethylene, and aluminum foil, in the same thicknesses. The main culprit on Amazon simply had to find a way to justify that the cost of his bags is 20-30% higher, and the thickness is up to 20% lower than ours and is waving the ‘made in the USA’ banner to do that.

Now don’t get me wrong, I love Made in the USA products, so let me clarify. We sold only USA bags for many years…but honestly, the bags made by American manufacturers were simply not as good as what we sell now. They had a higher error rate on both orders and product. They had lower mil rates and inferior specs. If you want to support American companies, awesome, I do too! If you want the best product for your food storage? Don’t assume an American company makes that, because more often than not, they don’t. The error rate on Asian shipments is infinitesimal compared to American manufacturers. We manufactured 3 full CONTAINERS of product in Asia before we had a single problem in a shipment. In the early days when we were ordering single pallets of products from American manufacturers, we had problems on every order. Every single one. In two cases, they produced the wrong product and shipped it.

Another point on this question is that any seller who claims this just doesn’t know much about packaging. Mylar is ONLY made by a consortium that includes a foreign company (Teijin and Dupont). So any company that says they are selling ‘USA made’ Mylar bags may be speaking about the actual physical assembly of the bag, but very rarely are the films in Mylar bag manufacturing made in America (I know of only one plant in the US that makes it, and they don’t sell to this small Amazon seller).

Finally, one of the sellers who is saying these things on Amazon is purchasing from an American company that imports millions of Mylar bags from Asia annually. We’re talking many containers a month. There is no way the seller has chain of custody documents that can guarantee his particular Mylar bags are made in America.

Why did I spend so much time and space on the above answer?

Integrity. One thing many of my customers have noticed over the years is that I answer questions, emails and phone calls to the best of my ability (some months are not as good as others). And my answers are NEVER predicated on you buying anything from me. Many times, I actually steer people away from buying something because what I sell won’t help them achieve their goal.

I want people to store food. Whether its in boxes, glass jars, beach buckets, coffee cups or anything else, I just want people to be prepared. My answers to questions are based on this fact. I’ve referred folks to other businesses when mine wasn’t best suited to serving them. The LDS storehouse in Hendersonville used to refer customers to us because they knew we cared about not just the product, but the process of storing food.

I spent so much time on the above question because the seller mentioned is questioning my integrity. I wouldn’t sell you bags if I couldn’t sell you awesome bags. I wouldn’t sell you oxygen absorbers if I didn’t carry the most reliable brand of oxygen absorber. And when I do make a mistake, I usually follow it up with something like ‘oh crap, I made a mistake, here’s what I’m going to do to try to make it right.’

How do I contact ShieldPro or Discount Mylar Bags?

Email is always easiest for us. I still work in the warehouse every day and so its easiest for me to get to your question or issue if you email me. Our email is

However, if you do need to call us, that’s cool too! 615-945-0762

If you have a question not answered by this FAQ, let me know and I’ll update and republish it!

Thank you so much for being my customers all these years. I’ve been amazingly blessed to be in this business and able to work with many of the most industrious and imaginative folks you can imagine.