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.
The most obvious sign an oxygen packet is 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% of 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.
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.
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)
Some conditions are better than others for the speed at which an oxygen absorber works. For example, in a very dry climate, it is a bit slower whereas, in a humid climate, it works faster. Also, there are different types, the standard is 48 hours but our BZ Type is 10 hours and our S Type is 10 Days
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.
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).
The easiest way I’ve found to store oxygen absorbers is to use a small mason jar with a gasket 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. The second best to use 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.’