If you hate Swiss cheese, here's your vindication. It's got hay in it. Those holes have to come from somewhere, and researchers from Switzerland found that hay particles from the cow's food are needed to form the center of the Swiss cheese holes.

The hay helps to trap the air inside, which allows carbon dioxide to flow into the cheese. So the more hay, the more holes. But just throwing hay into a bucket of milk won't make Swiss cheese on its own. You need to ferment the milk to form lactate, which curdles the milk. 

Bacteria found in the digestive system of cows, called propionibacteria, ferments milk's lactose and releases carbon dioxide, which mixes with nitrogen from the air. This gassy mixture diffuses through the cheese. And the diffused gas then starts to form bubbles around the hay particles. And those bubbles then become the traditional Swiss cheese holes. 

So if you try to make the milk hay and bacteria-free, well Swiss cheese just becomes less holey. Now you might be thinking, "Hay? Bacteria? This cannot be good for you to eat." But actually, propionibacteria are generally recognized as safe. So feel free to keep munching on that holey cheese.