How are the holes on Swiss cheese made?

The holes in Swiss cheese, known as "eyes," are formed during the fermentation process. Specifically, bacteria such as Propionibacterium freudenreichii consume lactic acid and release carbon dioxide gas, which creates bubbles within the cheese. As the cheese ages, these gas bubbles expand, resulting in the characteristic holes. The size and number of holes can vary depending on the specific cheese-making process and conditions.