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Emmental is one of Switzerland 's best - known cheeses , and its round holes are famous . Cheese makers call these holes eyes . Yet in the ten to fifteen years up to 2015 , Swiss cheese makers found fewer and fewer holes in their Emmental . People linked the shortage to modern milking and ever cleaner milk , but nobody could explain how clean milk could take holes away . Scientists have wondered about this for a long time . In 1917 an American , William Clark , published a long article about the holes in Emmental . Gas from bacteria was already the main idea , but it was not yet proven . Today we know that propionic acid bacteria in Emmental feed on lactic acid and give off carbon dioxide . In 2015 researchers at Agroscope , a Swiss federal research centre , and the lab EMPA solved the rest of the puzzle . They followed cheeses through 130 days of ripening and used X - ray scans to watch the holes form . By adding tiny particles of hay to the milk , they could control the number of holes almost at will . Microscopic bits of hay carry tiny air bubbles . The carbon dioxide from the bacteria collects in these bubbles and makes them larger , until they become visible eyes . Without such starting points , most of the gas escapes from the cheese . Now the missing holes made sense . In the past , cows were milked in barns into open pails , and a little hay dust fell in . Closed milking machines keep the milk cleaner and safer , but they also keep the hay out . Agroscope says only 5 to 10 milligrams of hay per 1,000 kilograms of milk are enough , so the milk stays clean . So every eye in a slice of Emmental needs two things : gas from bacteria and a tiny place to start . Modern milk was simply too clean to give the gas one . 
