Swiss cheese is well-known for its distinctive holes as well as its earthy aroma.

Now, for the first time in a century, researchers have discovered what makes the holes in this tasty dairy treat. Surprisingly, the nature of the holes is dependent on the amount of contaminant present in the buckets used to collect milk.

Scientists used to believe that the holes were created through the release of carbon dioxide by bacteria within the body of the food product, an idea first proposed by American scientist William Clark in 1917. Instead, the process appears to be driven by minute traces of a material found on every farm.

"[M]icroscopically small hay particles" fall into buckets, and the tiny vacuoles they create grow in size as the cheese matures, researchers concluded.

Hay dust was added in small amounts to samples of milk during the study. This raw material was then processed into cheese over a period of 130 days. Investigators determined that by varying the amount of hay dust in the milk, they were able to control the number of holes created in the final product.

Researchers believe the formation of the holes by hay could explain why fewer holes have appeared in Swiss cheese produced during the last decade and a half. As farming technology advances, fewer particles of hay have been falling into milk buckets, investigators theorize. Milk produced from cows connected to closed, automated systems is not exposed to hay dust in the air, meaning fewer holes are present in cheese.

Agroscope is a Swiss agricultural agency that conducted an investigation of the conditions leading to the development of holes in some forms of Swiss cheese, including Appenzell and Emmental.

Milking machines "completely did away with the presence of tiny hay particles in the milk," Regis Nyffeler, Agroscope spokesman, said.

In the United States, several varieties of cheese are referred to as Swiss. Most of these brands are made in North America or Norway, where Jarlsberg is produced. The two most popular varieties are Lacy and Baby Swiss, developed in Ohio during the mid-1960s.

Traditional Swiss cheese is treated with three types of bacteria — Lactobacillus, Streptococcus salivarius and Propionibacterium.

Holes in Swiss cheese are known as "yes" within the dairy industry, while cheese-makers refer to any cheese without holes as "blind."

In addition to the century-old notion that bacteria resulted in holes in Swiss cheese, a popular myth held that mice ate through the cheese, creating the distinctive characteristic.

Photo: Smabs Sputzer | Flickr