The 21st century has not been kind to the trademark texture of Emmental cheese. To Americans, that’s Swiss cheese: the stuff with all the holes in. But the holes have been vanishing and the cheese becoming smoother over time. Scientists determined to find out why. The answer? Modern cheese is just too clean.

The AP reports that a Swiss government-funded agricultural institute delved into the mystery of the nation’s most famous cheese and found that holes need hay. Or, more specifically, that “microscopically small hay particles” that make their way into the milk are responsible for the holes when that milk becomes cheese.

When a dairy farm is all manual labor, with people doing their best to keep rooms clean but using pre-industrial tech, some of those airborne particles will make their way into milk and there’s nothing you can do about it. But the transition from traditional milking methods into fully-automated industrial systems means there’s less stuff in the air, and that means fewer holes in the cheese.

The solution? More hay. “In a series of tests,” the AP reports, “scientists added different amounts of hay dust to the milk and discovered it allowed them to regulate the number of holes.”

Mystery of disappearing holes in Swiss cheese solved [Associated Press]