By day 137, the food looked pretty much the same, KTXL-TV, Sacramento, reported.
Some were quick to assume McDonald's is using preservatives to create the appearance of "fresh" food.
But scientists say preservatives aren't the main reason why the Golden Arches' food won't break down -- it's fat.
Fries and burgers sold by McDonald's are high in fat -- more than 50 percent of the calories found in McDonald's french fries come from fat, the same amount as in the restaurant chain's burgers.
"Anything that is high in fat will be low in moisture," Washington State University Professor Barry Swanson said.
Low moisture on a surface means mold can't grow, which means the food can't decay.
And don't forget the salt.
A McDonald's double cheeseburger contains 1,150 milligrams of sodium, while a large order of fries contains 350 milligrams.
Sodium acts as a preservative, contributing to a low decay rate.