Swiss cheese, or fromage Suisse, has holes in it due to the fermentation process used to create the cheese. Cheese is made by introducing bacteria to milk, which begins to curdle as the bacteria eat and produce lactic acid. Another byproduct of the digestion process is often carbon dioxide gas, which in some cheeses is trapped inside the rind, forming bubbles in the finished cheese product. Swiss cheese is the most well known for this, with some types having holes as large as walnuts.
A number of cheeses are marketed under the name of "Swiss" cheese. The true version is Swiss Emmental or Emmentaler, produced in a particular part of Switzerland. Emmental has a protected origin designation, which means that only cheese prepared in that area of Switzerland, and in a certain way, can be labeled with this name. It has the creamy texture and large holes most consumers associate with Swiss cheese.
Other holey cheese is made in various parts of the world and labeled as Swiss because it uses the same bacteria and a similar fermentation practice. Some dairies also specialize in the manufacture of Lorraine Swiss, also known as baby or lacy Swiss. It has much smaller holes than Emmentaler, because the cheese is not allowed to age as long. The longer the cheese cures, the larger the holes will be.
Swiss cheese is made by adding Propionibacter shermani and other cultures to milk after it has been warmed. If the cheese in question is Emmental, the milk is not pasteurized. In the United States, this cheese is usually made with pasteurized milk. The bacteria forms curds in the milk, which are pressed into heavy molds and briefly allowed to soak in a brine bath, forming a wheel of cheese. The brine bath forms a thick rind on the cheese, which is then placed into ripening caves to mature.
As cheese ages, the Propionibacter shermani bacteria inside continue to eat, producing carbon dioxide. In looser cheeses, the gas would slowly move through the cheese and escape through the rind. Swiss cheese, however, is densely packed and has a very thick rind, so the gas cannot escape, forming bubbles instead. A briefly aged Swiss will have smaller bubbles and a mild, buttery flavor. More mature products, such as Emmentaler, will have larger holes and a more assertive flavor.