Carbon dioxide (CO2) and nitrogen (N2) are commonly used in packaging both fresh and shelf-stable foods, in order to extend their usable shelf lives. Fresh foods are outside the scope of this work so attention shall be focused only on those foods suitable for use in storage programs.
The most common use of these gasses is for excluding oxygen (O2) from the atmosphere contained inside of a storage container (called head gas). When oxygen levels can be dropped below 2% the amount of deleterious oxidation reactions in stored foods can be greatly decreased resulting in longer palatability and nutritional shelf lives. Actually achieving this low oxygen content is not a simple matter when limited to the equipment and facilities typically available in the home. With careful technique and proper packaging materials it is possible to achieve useful results though.
In order for either gas to be used most effectively to gain the longest possible shelf life it is recommended that it be contained inside of packaging with high barrier properties to prevent it from diffusing out over time or allowing oxygen to infuse in. Examples of this kind of packaging are Mylar and other high barrier property plastics, metal cans and glass jars. Buckets made of HDPE plastic are relatively poor gas barriers and will, over time, allow oxygen to infuse into the container. In order for foods to be kept for their maximum shelf lives the containers would need to be re-purged every three to four years. Foods that are particularly oxygen sensitive, such as dry milk powders, should not be stored in HDPE without a secondary gas barrier. It is possible to use HDPE buckets alone when gas purging if a shorter rotation period is used. An example would be using wheat in four to five years instead of the eight to ten that would be achievable if a high barrier container were used.
Purging efficiency can be greatly improved when used with a vacuum device. By first drawing down the head gas of the container and then flooding with the purging gas much more oxygen can be removed from the container. Repeating the process once more will improve removal efficiency even more. If a true vacuum pump is not available, the suction end of a home vacuum-cleaner can be made to serve and still achieve useful results. With careful technique, oxygen levels can be dropped to be 0.5-2%. Finely textured materials such as grain flours and meals, dry milk powders and similar textured foods will purge poorly and are better packaged with oxygen absorbers. Instructions for vacuum usage are given in A.5.1 Using Mylar Bags. Instructions for gas purging are given below in B.1 Dry Ice and B.2 Compressed Nitrogen.
A less common, but important use for carbon dioxide is fumigation. This is the killing or retarding of insect life contained in a product. Many chemical fumigants are available to do this but are not thought desirable by many who have foodstuffs they want to put into storage. CO2 is not as certain as the more toxic fumigants, but it can be made to work and will not leave potentially harmful residues behind. It is possible for nitrogen to work in a similar manner, but it must be in a head gas concentration of 99%+ whereas carbon dioxide can be effective over time at levels as low as 3%. The precise amount of time necessary for the gas to do its work will vary according to the specific species and growth stage of the insect along with the temperature and humidity level of the product being fumigated. In general, the more active the growth stage and the warmer the temperature the more effective CO2 is in killing weevil infestations. The gas also exhibits bacterial and fungal inhibiting properties, but for our purposes this will be of little moment since all foods should be too dry to support such growth in the first place.
The procedure for fumigating foodstuffs with carbon dioxide is precisely the same as the one used in purging oxygen from storage containers mentioned below. The only change is that for the fastest effectiveness the sealed container should be left in a warm place for a week or so before moving it into its final storage location. The gas is still effective at cooler temperatures, but because insect life is slowed by lower temperatures the carbon dioxide takes longer to complete its mission.
NOTE: Both Mitsubishi Gas-Chemical, maker of the Ageless line of oxygen absorbers, and Multisorb, manufacturer of the FreshPax D 750 absorbers, state the their products should not be used in a high carbon dioxide head gas environment. There are absorbers that will work well in high carbon dioxide atmospheres but they require an external moisture source which would make them difficult to use for our purposes.
Misc.Survivalism FAQs maintained by Alan T. Hagan, firstname.lastname@example.org
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