A number of casual, relatively unscientific experiments with McDonald’s food have turned up a disturbing discovery: The food seemingly never decays. A Happy Meal still looks kinda appetizing after 143 days on a shelf; a 12-year-old hamburger looks relatively similar to a new one; one burger traveled around the country in a woman’s purse for more than four years. But it turns out it’s not—as widely theorized—chemicals and preservatives that are to blame for McDonald’s immortal food.
An expert tells Salon that fat is the real culprit: Both McD’s fries and burgers are very high in fat (50% of the calories in the fries are fat, and as much as 54% for the burgers), which means they’re low in moisture, which means there’s less room for mold to grow. (All that salt—nature's great preservative—on the fries doesn’t hurt either.) The buns contain preservatives, but no more than what you’d find in supermarket white bread. So if you’re looking to avoid McDonald’s, do it to avoid the fat and the salt—not the chemicals.