Mickey D’s is finally responding to the experiment! Here is what the link says:

Response to McDonald’s Happy Meal Food “Experiment”
MEDIA STATEMENT

“McDonald’s menu items are freshly prepared in our restaurants. It is not possible to provide a detailed explanation regarding these claims without knowing the conditions in which these food items were kept. That said, we strongly caution anyone from jumping to conclusions.
McDonald’s hamburger patties in the U.S. are made with 100% USDA-inspected beef. They are cooked and prepared with salt, pepper and nothing else – no preservatives – no fillers. Our hamburger buns are made from North American-grown wheat flour. Our world-famous French fries are made from potatoes and cooked in a canola-oil blend. These are the same foods that consumers buy every day in their local grocery stores – bread, meat and potatoes.

Bacteria and mold only grow under certain conditions. For example, without sufficient moisture – either in the food itself or the environment in which it is held – bacteria and mold and associated decomposition, is unlikely. If food is/or becomes dry enough, it won’t grow mold or bacteria. In fact, any food purchased from a restaurant or grocery store or prepared at home that lacks moisture would also dehydrate and see similar results if left in the same environment.
McDonald’s food safety and quality standards are among the highest in the industry. McDonald’s sources its ingredients from approved suppliers that adhere to strict standards for food safety and quality, including a controlled, well-maintained and clean environment throughout our entire supply chain. Together with stringent procedures for handling and storage, we work hard to minimize any exposure that would contribute to the presence of mold, bacteria or other microorganisms.”