Why does Swiss cheese have holes?

Swiss cheese is known for its distinctive holes, a feature that has puzzled and fascinated people for generations. These holes, technically called "eyes," are formed during the cheese-making process due to the activity of bacteria. Specifically, a bacterium called Propionibacterium freudenreichii plays a crucial role. This bacterium consumes lactic acid in the cheese and releases carbon dioxide gas, which becomes trapped in the curd, forming bubbles that create the characteristic holes. The size and number of holes can vary depending on the cheese-making process, environmental conditions, and adjustments in bacterial cultures.

For decades, scientists believed that the holes were simply a byproduct of bacterial fermentation, but recent research has added another layer to the explanation. Studies show that tiny hay particles in the milk influence hole formation. When modern dairy processes became more sterile, fewer hay particles made their way into the cheese, leading to a decrease in the number and size of holes in Swiss cheese. This discovery has allowed cheesemakers to fine-tune production by adjusting the milk filtration process, controlling the size and consistency of the holes.

Swiss cheese production is a delicate balance of bacteria, temperature, and aging. Traditionally, cheese wheels are aged in temperature-controlled rooms for weeks or months to allow the bacteria to fully develop. Cheesemakers can alter hole size by adjusting temperature and acidity levels. “It’s a fascinating example of microbiology at work,” says one dairy scientist, explaining how cheese chemistry is influenced by even minor environmental changes.

The presence of holes in Swiss cheese also affects its texture and flavor. Larger holes typically mean a milder taste, while cheeses with smaller holes tend to have a denser texture and more intense flavor. This is why variations like Emmental and Gruyère, both Swiss-style cheeses, have different hole patterns and taste profiles. The famous cheese-making regions in Switzerland and the U.S. have perfected the process, using scientific advancements to maintain consistency while preserving traditional methods.

AP News, The Wall Street Journal, Reuters, The New York Times