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Look at a slice of Emmental . Emmental is a cheese from Switzerland . Cheese makers call them eyes . For ten or fifteen years , Emmental had fewer and fewer holes . People blamed the clean milk . But nobody knew how it worked . Bacteria are very small living things . In Emmental , some bacteria eat an acid in the young cheese . Then they make a gas , carbon dioxide . But the gas needs a place to start a bubble . In 2015 scientists at Agroscope found the answer : hay . Agroscope is a research centre in Switzerland . Tiny bits of hay have tiny air bubbles inside . The gas fills these bubbles , and they grow . Without hay , most of the gas leaves the cheese . The scientists made cheese with more hay and with less hay . They looked inside the cheese with X - rays for 130 days . More hay gave more holes . In the past , farmers milked cows in the barn into open buckets . Tiny bits of hay fell into the milk . Today closed machines milk the cows . The milk is very clean , and it has almost no hay . Look at your slice of Emmental again . Each eye starts at a tiny point . Very often , that point is a bit of hay . 
