Answer: Carbon Monoxide

Red meat browns with age as the blood in the muscle tissue oxidizes. Despite the fact that it’s perfectly safe to cook and eat after it no longer looks bright red and fresh from the slaughterhouse (assuming it has been properly stored and handled), the browning process is off-putting to picky consumers.

To avoid American consumers leaving perfectly good meat on the shelf, the meatpacking industry uses a clever trick: they treat and package red meat with a mixture of carbon monoxide, carbon dioxide, and nitrogen. The carbon monoxide is specifically included in the mixture to prevent the browning that is so off-putting to consumers (without the carbon monoxide in the mixture, the meat would take on a gray/purple cast thanks to the other compounds in the mixture).

Although the process is benign (breathing in carbon monoxide is very dangerous for humans, but eating treated meat has no effect), not everyone finds the process acceptable. The process is banned in many countries and the Health Science Institute has called for a cessation of the practice in the United States on the ground that the treatment doesn’t actually inhibit microbial growth, but it does give older meat the appearance of freshness which can lead to consumers eating meat that looks fresh from the butcher block but is unsafe to eat.