The 'head gas content' is a listing of the gasses found inside a container. In the food storage industry this is important because oxygen oxidizes food over a period of time. Get rid of the oxygen and the food will store longer.
The food storage industry has never got together to set standards for packing food for long term storage. But rather, each company has made their own standards.
We have set very high standards for themselves, not only on the quality and condition of the products that go in the cans, but also in the removal of oxygen from the cans after they are sealed. After the oxygen has been removed you should store your food as cool as possible. To remove the oxygen, we use oxygen absorbers. We put 750cc oxygen absorbers in our #10 cans and 300cc absorbers in our #2.5 cans. This means that the 750cc oxy absorbers will absorb at least 750 cc of oxygen at sea level on a standard day and the 300cc absorbers will absorb 300cc. Actually, a safety factor is built into them, for they will absorb even more than this.
The latest test data dated the 9th of March, 1998, states, "This letter is to confirm that there was no detectable levels of oxygen in the samples of products you sent to us with the D-750 FreshPax oxygen absorbers in them. Our equipment measures down to 0.0009% of oxygen. Additionally, the oxygen absorbers had extra capacity still in them when the cans were opened.The D-750 is rather large for #10 cans. You would be better off with a D-300 in the large cans and a D-100 for the smaller cans..."
Even though the company who conducted the test suggested we reduce the size of the oxygen absorbers we use, we have decided it is better to overdo this kind of thing than under-do it. We have no plans in the future to use smaller oxygen absorbers.
Sometimes we get ask if it is better to purge the cans with nitrogen and then put in an oxygen absorber. Multisorb Technologies who make our oxygen absorbers stated, "Some companies do this as a cost saving measure so they can use smaller oxygen absorbers. Whatever way you do it, no oxygen means no oxygen and it doesn't matter how you do it."
The cans we use can easily withstand the small vacuum placed on them created by the removal of the oxygen. About 20% of the gas in the can is absorbed. Air naturally contains about 78% nitrogen which the absorber ignores. When the oxygen is removed in the can, the pressure drops about 20%, leaving only the nitrogen and traces of the other inert gasses that are naturally found in the air.