BERLIN (AP) — The mystery of Swiss cheese and its disappearing holes has been solved: The milk's too clean. 
 
 FILE - This undated file photo shows two slices of the famed Swiss Emmental cheese. The mystery of the disappearing holes in the famous Swiss cheese has been solved: the milk is too clean. A Swiss agricultural... (Associated Press) 
 
 A Swiss agricultural institute discovered that tiny specks of hay are responsible for the famous holes in cheeses like Emmentaler or Appenzeller. As milk matures into cheese these "microscopically small hay particles" help create the holes in the traditional Swiss cheese varieties. 
 
 The government-funded Agroscope institute said in a statement Thursday that the transition from age-old milking methods in barns to fully-automated, industrial milking systems had caused holes to decline during the last 15 years. 
 
 In a series of tests, scientists added different amounts of hay dust to the milk and discovered it allowed them to regulate the number of holes. ||||| Image copyright AP Image caption Hole again: and it is all thanks to tiny flecks of hay, say Swiss scientists 
 
 Scientists say they have discovered why Swiss cheese has holes in it: apparently, it is all down to how dirty buckets are when the milk is collected. 
 
 Contrary to what cartoons have suggested over the years, the holes are not made by mice eating their way through the cheese. 
 
 And nor are they produced by carbon dioxide released by bacteria, as popular scientific belief held. 
 
 Instead, a Swiss laboratory says they are created by flecks of hay. 
 
 Agroscope, a government agricultural institute, said "microscopically small hay particles" would fall in to buckets collecting milk, and develop into bigger holes as the cheese matures. 
 
 The process affects only some Swiss cheeses, such as Emmental and Appenzell. 
 
 Image copyright AFP 
 
 Agroscope said the findings explained why fewer holes had been appearing in Swiss cheeses over the last 15 years, since more modern milking methods made it less likely for hay to fall into containers. 
 
 Scientists reached their conclusions after adding small amounts of hay dust to milk and making it into cheese over 130 days. 
 
 The research has not been peer reviewed. 
 
 The theory that bacteria caused the holes has been around since 1917, after findings were published by an American scientist, William Clark. 
 
 The cheese industry calls holes in cheese "eyes". Any cheese without eyes is known as blind.
– Swiss scientists say they've solved a problem many people may not have noticed: the disappearing holes in Swiss cheese. Researchers at a government agricultural institute say that contrary to a century of scientific belief, the holes are caused by hay particles, not bacteria, and modern milking is just too clean, the BBC reports. The researchers say that "microscopically small" flecks of hay that fell into milk-collecting buckets in barns helped the holes to form, and they were able to alter the number of holes in cheeses like Emmental by changing the amount of hay dust they added to milk, reports the AP. Holes in Swiss cheese have been vanishing over the last 15 years, and the researchers say this is because modern industrial milking methods don't expose the milk to open barn environments, let alone dirty buckets, Swissinfo reports. The hole question has been around for a while: American scientist William Clark published research in 1917 stating that bacteria caused the holes to form, reports the BBC, which notes that the latest Swiss research hasn't been peer-reviewed, so other scientists haven't had a chance to find holes in it yet. (Another scientific food find: why popcorn goes "pop.")