The Food Lab: Here's Why McDonald's Burgers Don't Rot

Long story short: McDonald's burgers don't rot because they dry out, that's it. If you think a McDonald's burger that doesn't rot is unnatural and gross, you should also count saltine crackers, beef jerky, hardtack, croutons, dried beans, or pretty much any nutrient-rich, shelf-stable food in your pantry as unnatural and gross. The reason a McDonald's burger doesn't rot has nothing to do with chemicals, lack of nutrition, or anything else you should be scared of. It all comes down to water activity.

See, a McDonald's hamburger is small and thin, giving it a very high ratio of surface area to volume. It is cooked well-done on a very hot griddle. These factors contribute to rapid moisture loss, resulting in a burger that dries out long before it can start to rot. Moreover, the burgers are cooked in a food-safe environment to a very high temperature that kills any bacteria, and are thus relatively free of any agents of decay to begin with.

http://www.seriouseats.com/2016/02/the-food-lab-why-mcdonalds-burgers-never-rot.html