Same with potato chips and a lot of other foods that have some "air" in the sealed bag.
Earthbound Organics invented the process, which allows greens to store and ship for about 13 days, in contrast to non-nitrogen packed greens, which wilt and brown within 48 hours of picking.
Earthbound was also able to stay ahead of the competition by coming up with new ways to pack produce, removing oxygen and adding nitrogen to the bag so greens stay fresh longer and developing harvesting equipment specifically for gathering baby lettuces.
http://www.mercurynews.com/mld/mercurynews/news/breaking_news/13759311.htm (AP business news wire, 31 Jan 2006)
Nitrogen makes up nearly 80% of the atmosphere, and it is the oxygen that makes vegetables brown though oxidation of an enzyme called polyphenol oxidase. Upon oxidation, it produces quinones that are yellow in color, leading to browning. (It's not just food that needs to be protected from oxidation; I learned about this when I was a grad student in history and we were trying to preserve papers and books that had a high lignin content. 19th century paper pretty much falls apart after about 90 years, thanks to the polyphenol oxidase in the paper. Rare, old books and papers are kept in a nitrogen vault much of the time.)
An article on lettuce and browning from a big lettuce producing city's paper is here:
http://sun.yumasun.com/artman/publish/articles/story_22601.phpNitrogen is relatively safe. Carbon monoxide, not so much. As far as meat goes... I'd rather they cryovac'ed it, but the problem with that is the color is off, and people think meat should be bright red. When something is cryovac'ed, there's no oxygen for the hemoglobin to react with, and thus the meat stays the purply-brown of veinous blood instead of arterial blood red. Cryovacing removes all gases from around the meat (think about those big briskets and pork loins) instead of replacing it with some other gas. No styrofoam needed, either.