Why Does Swiss Cheese Have Holes?

Instructor: Stephanie Gorski

Steph has a PhD in Entomology and teaches college biology and ecology.

In this lesson we will learn a little bit about the process of cheesemaking. We will discuss the three types of bacteria common to Swiss cheese, and how bacteria contribute to the holes in Swiss cheese.

Swiss Cheese Pairings

Suppose you're looking for a snack to go with a stout or Doppelbock beer, or a red wine like Burgundy. Maybe you'd like some cheese to pair with a slab of venison, or a chocolate dessert. You might consider the slightly nutty, hard, delicious Swiss cheese. Besides its distinctive flavor, you have no doubt noticed that Swiss cheese has peculiar, irregular holes. Why is that?

Swiss cheese
Swiss cheese

How Cheese is Made

Cheese is fermented milk, most commonly cow, goat, sheep, or buffalo milk. If you ever visit a cheese making facility, you can see milk being curdled using a starter culture of bacteria. It's soothing to watch cheesemakers cut or stir the freshly coagulated milk to expel the liquid part of milk, called whey, from the solid part, curd.

Draining whey in cheesemaking
Draining whey in cheesemaking

Traditionally, rennet, an enzyme mixture from calves' stomachs, is used to destabilize globules in the milk called casein micelles. But if you don't like the thought of using enzymes from calves, don't worry. Most modern cheese is made with synthetic rennet produced by genetically modified bacteria.

After removing most of the whey, a cheesemaker will then shape the cheese and allow it to age. During this aging process, bacteria will break down molecules in the cheese, altering its flavor and texture. Sugars will be broken down into acids, causing the acidic flavor of cheese. Proteins will first be broken down into smaller molecules called peptides, and eventually into even smaller amino acids.

Bacteria in Cheese

All cheeses have some bacteria involved in the aging process. A common bacterial species used in many cheeses, including Swiss, is Streptococcus thermophilis. S. thermophilis eats lactose, the sugar found in milk, and excretes lactic acid as a waste product.

Swiss cheese, however, is unique in its blend of three different bacterial species. In addition to Streptococcus, Swiss cheese also includes strains of bacteria from the Lactobacillus genus. The most commonly known Lactobacillus species is L. helveticus, but other species, such as L. casei and L. bulgaricus are also known in Swiss cheese. Lactobacillus is important to Swiss cheese because Lactobacillus gives Swiss cheese its texture, sharpness, and nutty flavor. Like S. thermophilis, Lactobacillus excretes lactic acid as a waste product.

Both of these cultures are necessary for a successful Swiss, but the star of Swiss cheese has got to be Propionibacterium shermanii. P. shermanii imparts much of Swiss cheese's distinctive flavor, and it's also responsible for those holes.

How do the Holes Get in Cheese?

P. shermanii has a very convenient lifestyle when Lactobacillus and S. thermophilis are around. That's because P. shermanii eats lactic acid; the waste product of Lactobacillus and S. thermophilis. It then excretes three things as waste products: acetate, propionic acid, and carbon dioxide.

Carbon dioxide is a gas, and it gets stuck in the wheel of maturing Swiss cheese. This carbon dioxide makes bubbles, which appear as holes or 'eyes.'

Blind Cheese

Sometime around the 1990s, blind cheese, or cheese without holes, started becoming more common. No one is totally sure why, but we have some ideas. It looks like part of the problem might be that modern cheese is simply too clean. We don't know exactly why Swiss cheese holes form where they do, but it seems they form around some nucleus, the way a pearl forms around a grain of sand.

Scientists first came up with the idea that hay dust was responsible for Swiss cheese eyes when they noticed that the eyes were seasonal. In the winter, most cows are in the barn eating hay, and Swiss cheese almost always has holes. In the summer, many cows are in the field eating grass, and the number of blind cheeses goes up.

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