A story about a 14-year-old McDonald's hamburger that hasn't rotted is going viral.

David Whipple, the man who saved the burger, says that he shows the artifact to his grandchildren as an example of how fast food is packed with preservatives and chemicals.

But McDonald's Canada offered an alternative explanation on its question and answer page after a customer wrote in to ask why the company's burgers didn't rot.

"The reality is that McDonald’s hamburgers, french fries and chicken are like all foods, and do rot if kept under certain conditions," Dr. Keith Warriner, the program director at the University of Guelph's Department of Food Science and Quality Assurance, told McDonald's.

Here's his explanation:

“There have been a lot of online videos and photos touting the fact that when left out for an extended period of time, a McDonald’s hamburger does not rot and that this is because they are laden with chemicals. The reality is that McDonald’s hamburgers, french fries and chicken are like all foods, and do rot if kept under certain conditions.

Essentially, the microbes that cause rotting are a lot like ourselves, in that they need water, nutrients, warmth and time to grow. If we take one or more of these elements away, then microbes cannot grow or spoil food.

In the example of a McDonald’s hamburger, the patty loses water in the form of steam during the cooking process. The bun, of course, is made out of bread. Toasting it reduces the amount of moisture. This means that after preparation, the hamburger is fairly dry. When left out open in the room, there is further water loss as the humidity within most buildings is around 40%. So in the absence of moisture or high humidity, the hamburger simply dries out, rather than rot.

With moisture loss, we take away an element required by microbes to grow and cause spoilage. So to spoil a McDonald’s hamburger, we simply need to prevent the moisture loss. This can be done through wrapping it in cling film to prevent moisture from escaping, or storing it within a high humidity environment, such as a bathroom (notice black mould on your bathroom windows but not in your bedroom). If you try doing the same experiment with a homemade burger with similar moisture content as a McDonald’s hamburger and under similar conditions, you’ll probably get the same results.”

Food blog A Hamburger Today also did some rigorous experiments which confirm that the phenomenon of undecayed burgers isn't unique to McDonald's. From AHT:

"Turns out that not only did the regular McDonald's burgers not rot, but the home-ground burgers did not rot either. Samples one through five had shrunk a bit (especially the beef patties), but they showed no signs of decomposition."

Kim Bhasin contributed to this story.