Why Does Swiss Cheese Have Holes?

When you were a kid, were you ever disturbed by the old wives' tale about mice chewing the holes through Swiss cheese? Well, now you can rest easy: According to modern science, hungry rodents have absolutely nothing to do with that slice of cheese on your sandwich.

Here's what really causes those holes.

What Is Swiss Cheese?

When we talk about "Swiss cheese," we're referring to several varieties of medium-hard cheese that resemble Emmental cheese, which originated in Emmental, Switzerland.

Most blocks of Swiss cheese are dotted with holes, also called "eyes." Swiss cheese varieties without eyes are known as "blind."

Why Does Swiss Cheese Have Holes?

In 1917, William Clark published a detailed explanation of how Swiss cheese holes were caused by carbon dioxide released by bacteria present in the milk.

Clark's idea was accepted as fact for almost 100 years — until a 2015 study by Agroscope, a Swiss agricultural institute, blew a hole right through his theory (pun definitely intended).

The eyes are actually caused by tiny bits of hay present in the milk, according to Agroscope researchers.

This theory explains why the holes have mysteriously become smaller — and sometimes nonexistent — in recent years. When cheese is made in barns using open buckets, it's likely that hay particles will find their way into the collected milk. It's those little bits of hay that cause a weakness in the structure of the curd, according to The Spruce Eats, allowing gas to form and create the holes.

"It's the disappearance of the traditional bucket" used during milking that caused the difference, said Agroscope spokesman Regis Nyffeler, adding that bits of hay fell into it and then eventually caused the holes.

Milk for cheese-making is now usually extracted using modern methods, which explains why we don't see nearly as many holes in our Swiss anymore.

Swiss Cheese Recipes

Hungry for Swiss? We've got you covered.

• Swiss Chicken Casserole
• Old Fashioned Mac and Cheese
• Classic Chicken Cordon Bleu
• Baked Ham and Cheese Sliders
• Broccoli Cheese Bake
• Asparagus Quiche