When you package ground beef with nitric oxide gas, it looks kind of dull at first, but over five days in the store, it turns a bright, fresh-looking red and stays that way—just like beef packed with carbon monoxide—because the gas slowly turns the meat’s natural pigment into a stable red color.

When beef is packed with a gas called nitric oxide, it stays pink inside even after being cooked all the way through—unlike beef packed with other gases—because the nitric oxide binds to the meat’s natural pigment and locks in the pink color.

Putting a tiny bit of carbon monoxide in the plastic wrap around ground beef keeps it looking bright red for longer—up to five days—better than other gas mixes, because it locks the meat’s natural red color in place.

When you package ground beef with nitric oxide gas, it leaves behind more nitrite and nitrate chemicals than other gases like carbon monoxide or plain oxygen—and when you cook the meat, even more of these chemicals show up, maybe because the gas breaks apart inside the meat.