Wiktionary
packaging gas
n. Any gas used to provide a modified atmosphere for a product.
Wikipedia
Packaging gas
A packaging gas is used to pack sensitive materials such as food into a modified atmosphere environment. The gas used is usually inert, or of a nature that protects the integrity of the packaged goods, inhibiting unwanted chemical reactions such as food spoilage or oxidation. Some may also serve as a propellant for aerosol sprays like cans of whipped cream. For packaging food, the use of various gases is approved by regulatory organisations.
Their E numbers are included in the following lists in parentheses.