Jun 21 2008
After food has been freeze dried what else can be done to preserve it?
The freeze drying process removes 98% of the water from food, stopping bacterial growth as well as killing insects and their eggs.
Beyond freeze drying further to preserve food and increase shelf life, oxygen is the main enemy. If the food is stored in a way that it is not exposed to oxygen, the shelf life can reach 25 to 30 years. Shelf life here refers to the food maintaining it’s properties of nutritional value, taste, and appearance. It may still be safe to eat beyond this time but the aforementioned properties are degraded.
Some freeze dried food producers use oxygen absorbers to extend shelf life.
Oxygen absorbers are materials that chemically react with oxygen in the environment they are in, combining with the oxygen and thus removing it from that environment. The most commonly used material for absorbers is iron in the forms of iron powder or iron carbonate. Both combine with oxygen very effectively.
Once oxygen absorbers are exposed to oxygen they will continue to react with it until the material is fully “oxidized” meaning it can not absorb any more oxygen. For this reason they need to be very carefully sealed and stored so that they are not consumed before their intended use.
The application in which oxygen absorbers are used for freeze dried food storage is to place them in the can of food before it is vacuum sealed.
The idea is that any oxygen that leaks into the sealed container over years of storage will be absorbed by it, rather than the oxygen reacting with the freeze dried food and degrading it.
There are two types of oxygen absorbers commonly used. One type, Multisorb Technologies’ FreshPax Type-B requires some moisture from the environment it is in to be present to work and is used for moist foods like bread and processed meats. Type-D absorbers contain there own moisture source and are thus suited to dry foods like freeze dried food.
You may remember the old adage Aristotle proclaimed in 350 BC, “nature abhors a vacuum”. So any vacuum packed container will over time be invaded, if ever so slightly, by the surrounding air and with it the 21% of air that is oxygen.
So while the oxygen absorber will extend the shelf life by absorbing the oxygen in the air that is present initially during packing as well as the air that leaks in over time, eventually the absorber will be “maxed out”, that is it will be fully oxidized and can not absorb any more oxygen.
I have seen the guarantees for shelf life for this type packed freeze dried food at 10-15 years. This period may be a reflection of the limit of the process and process controls that the producer of the food uses, as well as that of the oxygen absorber.
Nitrogen packing or “nitro-pak” on the other hand takes a different approach to dealing with oxygen “enemy”.
Rather than relying on the properties of the container to fight the invading air trying to get in, the container is flushed with nitrogen or packed in a nitrogen environment. As a result the sealed container has the same or slightly higher pressure but with nitrogen and not air. This means that air is not fighting to get in. There is no abhorrence so to speak.
Thus the period that the food remains unexposed to significant concentrations of oxygen is much longer and thus the possible shelf life is longer.
What is the longest shelf life for nitrogen packed freeze dried food?
Mountain House, the commercial brand of Oregon Freeze Dry which has been around for over 40 years, states on their web site regarding their #10 cans of freeze dried food;
“Our foods will have the longest shelf life available…up to 30 years!”
It may be the result of superior process controls, not only the nitrogen packing process, that makes them feel comfortable making this statement.
There is information online on how you can nitro-pak foods yourself with some equipment but I would be leery of assuming your process control would be on the same par and have the same shelf life.
In any case “nitro-pak” freeze dried food has the longest shelf life for any type of commercially available stored food I have seen.
References on oxygen absorbers, nitrogen packing, and freeze dried food;
Wikipedia – Oxygen Absorber
Walton Feed – A Short Lesson on Oxygen Absorbers
Nitro-Pak – Freeze Dried Food Reserves