WHAT IS AN OXYGEN ABSORBER?
Oxygen absorbers are a relatively recent food storage tool whose arrival has been a real boon to the person wanting to put up oxygen sensitive dry foods at home. The packets absorb free oxygen from the air around them and chemically bind it by oxidizing finely divided iron into iron oxide. This removes oxygen from being available for other purposes such as oxidative rancidity and respiration by insects, fungi or aerobic bacteria. The practical upshot of all this is that by removing the free oxygen from your storage containers, you can extend the storage life of the foods inside. Not all foods are particularly oxygen sensitive but for those that are the absorbers truly simplify getting the job done.
The absorbers themselves have only a relatively short life span, roughly about six months from the time they were manufactured for the types that do not need an external moisture source. They don’t suddenly become ineffective all at once, it’s just at that point you will begin to notice (if you can measure it) that the absorbers no longer soak up as much as they would when they were new. Better to use them while they’re fresh.
HOW ARE OXYGEN ABSORBERS USED?
In order to make the best use of your absorbers you need to know three things:
#1 – Is the food I want to put by particularly oxygen sensitive for the time I want to keep it in storage? Whole grains that have not been polished or hulled such as wheat, corn, and rye are not especially oxygen sensitive. If you intend to use them up in five years or so, there’s no great advantage to using oxygen absorbers, unless used to deter weevil infestations. The same for most beans and peas. Processed or high fat grains and legumes such as oats, barley, brown rice, soybeans, peanuts and split peas would benefit from their use if they are to be kept for more than a year. Whole grain products such as whole wheat flour and rolled oats would as well. Refined grain products such as white rice, white flour, degerminated cornmeal will keep fine for a year or so, possibly longer, without oxygen absorbers if kept dry and protected from weevils. Dry milk, dry eggs, dry meats, and many kinds of dehydrated foods and any kind of freeze dried foods would benefit from oxygen absorbers. Foods with an easily transferable fat content should not be used with oxygen absorbers, nor should they be used with foods that are high in moisture or with free liquids in the storage container. These should be preserved using pressure or boiling water bath canning as appropriate.
#2 – Will the packaging I want to use seal air-tight and is the packaging material itself a good gas barrier? Obviously if the container won’t seal air tight you’re wasting your time trying to use oxygen absorbers but the barrier properties of a container stump many folks. Canning jars with good lids, properly sealed #10 (or other size) cans, properly sealed Mylar bags, PETE plastics with appropriate lids or caps, military surplus ammo cans with good gaskets, and many other types of packaging will seal air-tight and provide good barrier properties against oxygen infusing through the packaging material. Non-laminated flexible plastic packaging (bags, sheets, etc.), HDPE plastic buckets and any kind of non-laminated paper or cardboard container have poor gas barrier properties. “Poor” is a relative term, though, and if you’re going to use the food up in two or three years, even oxygen sensitive foods can be kept in unlined HDPE buckets if you use an appropriately sized absorber and make sure the bucket is well sealed. You’ll be using the food before sufficient oxygen has been able to infuse through the walls of the container to make a significant impact.
#3 – What is the volume of the container and how much air volume remains after I’ve filled it with food? This is important to know if you want to make the most efficient use of your absorbers and be certain your food is adequately protected. Taking the question in two parts, here is how to determine the answer:
A. Absorber capacity is rated by the amount of oxygen in milliliters that each will absorb so you’ll need to know what the volume of your container is in milliliters. The table below gives conversions between common U.S. container sizes and their milliliter equivalents.
|Pint jar (16 fl oz)||475 milliliters|
|Quart jar (32 fl oz)||950 milliliters|
|Half-gallon jar (64 fl oz)||1,900 milliliters|
|#10 can (112 fl oz)||3,300 milliliters|
|One gallon jar (128 fl oz)||3,800 milliliters|
|Five gallon pail (640 fl oz)||19,000 milliliters|
|Six gallon pail (768 fl oz)||22,800 milliliters|
|Fifty-five gallon drum (7,040 fl oz)||208,175 milliliters|
|Fluid ounces x 29.57 = milliliters = cubic centimeters|
Now multiply the volume of your container times the 21% (0.21) of the atmosphere that oxygen constitutes and you’ll come up with the volume of oxygen, in milliliters, that your container holds when it’s empty.
An example: A quart jar (32 ozs) is approximately 950 milliliters in volume. Multiply 950 x 0.21 (21%) and you get 199.5 milliliters of oxygen in an empty quart jar. This leads to the second half of the above question.
B. Determining remaining air volume in a container that has been filled can be difficult. Foods vary widely in their density and porosity from flour, which will pack tightly to elbow macaroni which is mostly air even if you pack it to just short of crushing. The following are three rough and ready rules that can be used and will work.
i> Foods that have a lot of open space between the food particles (called intersitial space) such as macaroni, pasta, instant dry milk, instant potato flakes, many coarsely chunky dehydrated foods, cold cereals, etc. should use one half the container volume as the remaining air space. Using the example above with the quart jar, there would be approximately 100 milliliters of oxygen remaining.
ii> Foods that pack more densely such as non-instant milk, dry eggs, flours and meals, grains with small kernels, dehydrated foods with fine particles and the like should use one-third the container volume as the remaining air space. Using the example above, there would be 66 milliliters of oxygen remaining.
iii> Alternatively, you could do what many of the commercial storage food packagers do and use the entire container volume. This is not as efficient as more closely determining remaining air volume but it does add certainty that your absorbers will soak up all available free oxygen and still leave some capacity to deal with any microscopic leaks or infusion through the packaging material.
NOTES: #1 — Both Multisorb and Mitsubishi corporations advise that their oxygen absorbers should not be used in a high carbon dioxide environment. This is apparently for reasons that the absorbers will also absorb carbon dioxide as well as oxygen and may run out of capacity before all of the oxygen in the container has been absorbed.
#2 — If you do choose to use oxygen absorbers in packing your food give some consideration to the sturdiness of your containers. In doing its job the absorber is going be removing the 21% of the atmosphere that oxygen constitutes. Since nothing is replacing the absorbed gas this will leave the storage container with a lower atmospheric pressure inside than outside. If the container is sufficiently sturdy this pressure differential will be of little consequence. For containers with thinner walls the pressure drop could cause them partially collapse or buckle, particularly if other containers are stacked upon them. Should this occur the entire stack could fall causing one or more to burst. Metal cans and glass jars should have no problems, but some plastic buckets made of HDPE have relatively thin walls which can buckle when the internal air pressure drops. To deter this, a liner bag of Mylar or other high gas barrier plastic should used. Heavier walled buckets won’t need a liner unless you’re trying to achieve the maximum possible shelf life. Seal the absorbers inside of the liner bag so that the pressure drop with not stress the walls of the container. Other containers should probably be tested or first flushed with an inert gas (N2) before the absorber is sealed in.
#3 — If the pack of absorbers you need to open contains more than you are going to use in fifteen minutes or so, you should minimize exposure of the remaining packets. This can be done by heat sealing the bag they came in with an iron after expelling as much air as possible or better yet by vacuum sealing the bag. You can also put the remaining absorbers in as small a jar or metal can as they will fit in and closing with an air tight lid.
#4 — The chemical reaction that absorbs the oxygen releases minor amounts of heat. This heat release is trivial in an individual packet but if they are piled one atop another as you’re using them they can warm each other and speed the absorptive reaction. This costs you capacity lost to open room air so it’s best to spread the packets in immediate use out on a tray so they lay atop each other.
#5 — If absorbers are sealed in a package with desiccants some thought should be given to how low the relative humidity will become. Silica gel will reduce humidity to approximately 40% which should not interfere with the absorbers oxidation reaction. Other desiccants, however, are capable of reducing relative humidity to very low levels. This might adversely affect your absorber’s ability to carry out its mission by removing moisture from the absorber package that is necessary to sustain the oxidation reaction. If you do use desiccants and oxygen absorbers in the same package, place the desiccant on the bottom, fill the package and then place the oxygen absorber on top of the food before sealing.