In the science of staleness (yes, there is such a science), you have “crust staling” and “crumb staling.” Crust staling is the process by which the outside of a loaf of bread goes from crisp to soft. Crumb staling is when the inside turns hard.
It’s easy to explain the first: Crust absorbs moisture from inside the loaf. Potato chips, which absorb moisture from the air, are all crust, so they completely soften.
Crumb staling is more complicated. Over the years, food scientists have published hundreds of papers on the subject. Many have come to believe the process has to do with starch within the gluten structure. “Starch granules exude amylose during baking,” says Bill Atwell, professor of grain science at the University of Minnesota. Spiderwebs of amylose then attach to the gluten network. As the crumb sheds moisture, these webs stiffen. Some bread manufacturers extend shelf life by adding enzymes that sever these amylose strands, or by way of additives that inhibit starch from interacting with gluten.
Staleness can also be a state of mind. In a 2004 study, Oxford University researchers asked people to eat Pringles while sitting in a soundproof booth wearing headphones that amplified their munching. When researchers cranked up the crunching, subjects rated the chips as fresher.
This article was originally published in the December 2015 issue of Popular Science.