swisscheeseIt’s known for its sweet and nutty flavor, its buttery yellow color and its distinctive holes, but over the years, Swiss cheese’s distinctive “eyes” have been disappearing. Scientists have debunked a popular theory and discovered the real secret behind Swiss cheese’s holes —  hay particles in milk.

Though urban legend has it that mice eat holes into Swiss cheese varieties like Emmentaler and Appenzeller, it’s been theorized since 1917 that bacterial growth creates gases that make holes in the cheese as it ferments and ages. In fact, cheesemakers have identified three types of holes in Swiss cheese: Nissler holes (the smallest), “eyes,” (the medium-sized and most abundant holes) and “large blow holes” (which can make blocks of cheese undesirable because they make slicing difficult).

A new study by Swiss researchers at Agroscope has poked holes in that original theory, revealing that the eyes are caused by something different: tiny bits of hay that cling to the side of buckets used during milking. They claim that the hay particles stay in the milk and eventually cause the holes. As the traditional milk bucket method disappears, Swiss cheese varieties are slowly losing their holes.