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I was recently packing my own dehydrated meals for hiking using mylar bags and oxygen absorper packs. The oxygen absorbers come in packs of 10. Generally the process is to prep all your bags and move quickly once the absorbers are opened, but I want details on "how quick".
How long do you have between exposing the oxygen absorper packs to air and sealing them in the mylar?
Depends somewhat on what type of absorber you're using, but they're most commonly made from iron powder, which are good for hours between first opening and sealing in your bags, but that time can be extended to days, weeks or longer if you keep the unused ones in an airtight container like a jar, or some other container with a rubber seal.
Oxygen absorbers remove oxygen from their surrounding atmosphere by chemical reaction. Simply explained, most are filled with an iron and salt compound that rusts when it's exposed to oxygen and humidity, and the oxidization eats up all the O2.
They are pretty much miniature hand warmer heat packs, and will actually become warm to the touch when they are working. This isn't a fast process, it takes about 4 hours for the packets to reach their maximum absorption rate, and they will absorb oxygen until all the iron in them has oxidized. Once all the oxygen is gone, the reaction stops, but it can reactivate once introduced into the air again.
An absorber is considered spoiled or "loaded" when all the iron in them has oxidized. Complete oxidation of 1 g of iron can remove 300cm3 of oxygen in standard conditions, which amounts to a volume of air of almost 1,500cm3 or 1.5L (~50oz) since oxygen only makes up about 20% of the composition of air. If you have a larger container you want to absorb all of the oxygen out of, you could save some money by using a hand warmer in place of several smaller packets in a larger bucket, but the hand warmers aren't exactly food grade, even though I'm sure they won't hurt anything.
New oxygen absorbers are good and usable for about a year as long as they are sealed, so it's recommended to only stock up on what you plan on using in a year.
This may not be an exact answer but I figured I would share it just in case you hadn't seen it.
They say 10 to 15 minutes is the window you have to get them in the Mylar and seal it.
From Backdoorsurvival.com :
The most important precaution is to limit the exposure of unused packets to air. Take out only what you are going to use in the next 15 minutes or so and seal the rest up in a jar with a screw top lid. Don’t put them in a zip lock bag because they will immediately suck up the residual oxygen and become useless.
A good rule of thumb is to use one 300cc oxygen absorber for each gallon of product. For larger containers, you can purchase larger, 2000cc oxygen absorbers which are ideal for 5 or 6 gallon buckets.
Be mindful of the little pink pill. Most reputable vendors will include a little pink pill with their package of absorbers. If the pill is blue, the absorbers are toast so don’t use them. However, if they are just starting to turn – not quite pink and not quite blue – they are probably okay since the change of color can happen in as little as 10 or 15 minutes.
Another good test of their viability is to pick one up an hold it. It may feel warm. It will also feel soft and powdery, like a little pillow. If it gets real hot and uncomfortable, it is in full out working mode and has probably been exposed to the air for too long to be usable. In this case it may also start to feel hard and brick like. Toss it.
Oxygen absorbers themselves have a limited shelf life, even when sealed. Only purchase an amount that you will use within a year."
Full article can be read here
Every other thing I've seen, read, or been informed of says 1-4 hours but I don't believe that, I wouldn't want to use one that had been out for over an hour.
When acid washing a steel gas tank, the tank becomes completely bare metal. It will look great! Nice and clean metal surface. If you do not neutralize the sulfuric acid left on the metal, it will "flash" rust and within a few hours the surfaces of the gas tank will become completely and thoroughly rusted again.
The salt in an oxygen absorber is a catalyst that acts the same way that the un-neutralized acid would when cleaning a gas tank. In an oxygen absorber the iron is probably cheap iron shavings and they are small making them much easier to rust. I would think that 1 to 4 hours is probably correct because of the salt and ambient moisture in the air.
Moisture is also required for the oxygen absorber to work. The solution to this problem is to put your oxygen absorbers in a vacuum bag and only open it to get the number of packets you need out of the bag. Then immediately vacuum seal the remaining absorbers. This should pull a vacuum on the bag removing most of the oxygen. Then get your food storage fixed up and sealed as soon as possible. Also some moisture is required for the reaction to occur. So put an uncharged moisture absorber in the vacuum bag with your unused oxygen absorbers. Periodically, as you are using them, take an extra one out and dissect it to determine the current condition of your remaining unused absorbers.
Because moisture is required for the reaction of the absorber to remove oxygen it is common practice to put the moisture absorber in the bottom of your storage container with food and put the oxygen absorber at the top of the container, on top of the food. This will allow the oxygen absorber to react with the moisture, oxygen, and salt longer. If you put them together the moisture absorber can inhibit the effectiveness of the oxygen absorber.
