The fascinating reason behind Swiss cheese's holes

Do you have a question about cheese? The Curious Kids column is here for children of all ages. If you want to have one of our experts answer your question, send it to [email protected].

What causes holes in Swiss cheese? – Owen F., age 13, Belmont, Massachusetts

Cheese-making is an artful science and there are thousands of varieties of cheese. Nutritional values, flavors, colors and textures all differ depending on the animal milk source. While cow, goat, and sheep milk create some of the most popular cheeses, others like camel milk, water buffalo milk, and even moose milk are used to concoct new tastes.

Want to make cheese? Milk is curdled by adding bacteria that cause a chemical reaction to shape solid curds and liquid whey. In general, the whey is drained out and dried into a powder. Cheese texture and flavor depend on various factors such as the amount and type of bacteria, the salting method, cheese temperature and of course, the aging process, which can last for years.

Swiss cheese, like many other cheeses, is made from cow's milk and bacteria that changes the milk into solid curds. But what's the story behind the "eyes" or the holes in Swiss cheese? "Blind" batches are defined by Swiss cheese makers as batches that lack eyes. These holes are caused by additional bacteria present in Swiss cheese, specifically Propionibacterium freudenrichii subspecies shermanii, also known as P. shermanii. This bacteria emits a gas called carbon dioxide under specific cheese making conditions, causing round openings. When the cheese cools down, the holes stay in place creating eyes. Think of a bubble in chewing gum that is forced into a circle by air pressure. If the pressure is released, the bubble pops. Swiss cheese is made at a warm temperature (~70℉), making it soft and malleable, so the gases emitted by the bacteria shape openings rather than bubbles.

It takes around four weeks at 70℉ for Swiss cheese eyes to form and six weeks to make Swiss cheese. Swiss cheese is finally aged for two months before it's sold.

The art of producing Swiss cheese dates back to the 15th century in Switzerland, where it is known as "emmental" or "emmentaller." However, other countries have created their versions of this savory delight. France boasts of its "Gruyere," Italy offers the world its "Fontina," and the US has developed its unique adaptation known as Baby Swiss, which has smaller eyes than the original.

Gouda cheese, originated in the Netherlands, is unique, with its miniature gas formation producing tiny eyes. In most cases, cheesemakers try to prevent the formation of gas because gas formation leads to unsightly crevices, cracks, and splits in harder cheeses instead of the nice round eyes desired. Fontina cheese also has eyes, but cheesemakers discourage gas formation.

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