What gas makes holes in Swiss cheese?

Here's how it works:

* Bacteria: The characteristic holes in Swiss cheese are created by bacteria called *Propionibacterium freudenreichii*. These bacteria are added to the milk during the cheesemaking process.

* Lactic Acid: During fermentation, the bacteria convert lactose (milk sugar) into lactic acid, giving the cheese its tangy flavor.

* Carbon Dioxide Production: As a byproduct of their metabolism, the bacteria produce carbon dioxide gas.

* Hole Formation: The carbon dioxide gets trapped within the cheese curd as it sets, forming the characteristic holes, known as "eyes".

The size and distribution of the holes depend on factors like the type of bacteria used, the temperature during fermentation, and the age of the cheese.