Why Does Swiss Cheese Have Holes?

What makes the holes in Swiss cheese?

Cheesemakers call the holes "eyes", and a cheese without them is "blind".^[5][7] U.S. Department of Agriculture grade standards treat the words "Swiss" and "Emmentaler" as interchangeable and define the cheese as having holes, or eyes, developed throughout by microbiological activity.^[7]

The gas that fills the eyes is carbon dioxide released by bacteria.^[1][5] Swiss cheese is made with two kinds of bacterial culture, one that produces lactic acid and one that produces propionic acid.^[6] The eye-forming culture is Propionibacterium freudenreichii.^[3][4] These bacteria ferment lactic acid and produce propionic acid, acetic acid, water and carbon dioxide.^[4]

Only part of that gas ends up in the eyes: some stays dissolved in the cheese, and some escapes from it.^[3] One estimate cited by Agroscope researchers for a typical Swiss Emmentaler puts the share that reaches the eyes at 15%.^[3]

When do the eyes form during ripening?

The eyes form in the first weeks after the cheese is made, during a warm stage of ripening.^[4][6] In the University of Guelph's cheese-making e-book, the cheese is stored at 10°C for 8–10 days and then moved to a warm room at 23°C, where eye development takes 2–3 weeks.^[4] A cooler at 2–5°C then stops eye development and firms the cheese.^[4]

Other sources give slightly different figures.^[5][6] The U.S. Food and Drug Administration's standard for Swiss cheese describes holding the cheese at about 50° to 60°F for 5 to 10 days, then at about 75°F until it is approximately 30 days old or until the eyes form.^[6] Stephanie Clark, a professor of food science at Iowa State University, writes that the eyes take about four weeks to form at 70°F.^[5]

Warmth matters because warm cheese is soft and malleable, so the gas can push out a round opening.^[5] When the cheese is cooled to around 40°F, the hole stays in place.^[5] The pace of gas production matters too: gas made too quickly gives a cheese with many small eyes, and little or no gas gives a blind one.^[4]

What did Agroscope find out about hay particles in 2015?

Bacteria explain the gas, but not where each hole starts.^[1] On 28 May 2015, Agroscope, part of Switzerland's Federal Department of Economic Affairs, Education and Research, announced an answer from a study carried out with the materials research centre EMPA: microscopic particles of hay in the milk act as starting points for the eyes.^[1] The hay particles contain tiny air bubbles, and the carbon dioxide made by the bacteria enlarges those bubbles into visible holes.^[1] The particles decide where the holes form and how many there are; without them hardly any holes form, because the gas largely escapes from the cheese.^[1]

The researchers recorded the holes over a 130-day ripening period with X-ray computed tomography.^[1] By changing the amount of hay particles they added, they could control the number of eyes almost at will.^[1] About 5–10 milligrams of hay particles per 1,000 kilograms of milk is enough for typical eye formation.^[1] The study was peer-reviewed and published in the International Dairy Journal.^[1]

Agroscope later corrected news reports that hay alone makes the holes: both the plant particles and gas-producing bacteria are needed.^[1] In March 2026, Agroscope researchers described a follow-up test with synthetic capillaries added to the curd, in which X-ray analysis showed at least one capillary inside each eye.^[2] They concluded that microscopic air bubbles are the most likely starting points for eyes.^[2]

Why did Swiss cheese start to have fewer holes?

Swiss Emmental once tended to have too many holes in winter, but in the 10–15 years before the 2015 announcement, cheeses increasingly had too few, a problem described as a "hole shortage".^[1] Agroscope linked the shortage to improved milking technology and ever cleaner milk.^[1] Over a few decades, traditional milking into open pails in the barn was replaced by closed milking systems.^[1] The change lowered the risk of unwanted microbes, but it also cut the number of microscopic hay particles reaching the milk, leaving fewer starting points for eyes.^[1] Agroscope researchers add that separating feeding from milking in loose-housing barns also left milk short of natural microparticles such as hay powder.^[2]

As of 2025, makers of Emmentaler PDO may add natural hay powder to the curd to help the eyes form.^[2]

Which cheeses have holes and which do not?

Swiss cheese was first made in Switzerland in the fifteenth century, and there it is known as Emmental or Emmentaler.^[4][5] Eyes are also typical of many semi-hard cheeses, such as Appenzeller, Swiss Tilsiter and Gouda.^[3] In Appenzeller the carbon dioxide comes from a different process, citric acid fermentation by the bacterium Lacticaseibacillus paracasei.^[3]

Agroscope says Gruyère PDO and Sbrinz PDO have no holes because the lactic acid bacteria in them form very little carbon dioxide.^[1] Measurements of ripened cheeses cited by Agroscope researchers show how much less gas a cheese without eyes holds.^[3]

• Swiss Emmentaler AOP (eyes)29
• Appenzeller (eyes)17
• Le Gruyère AOP (no eyes)6

Source: International Dairy Journal (Elsevier), open-access copy in the Agroscope institutional repository [3]

In the United States, a modified version called Baby Swiss tends to have smaller eyes.^[5] With most cheeses, makers try to keep gas from forming, because in harder cheeses it causes cracks and splits instead of round eyes.^[5]

How big should the holes in Swiss cheese be?

The U.S. Department of Agriculture's standards sort Swiss cheese into grades A, B and C.^[7] In Grade A cheese the eyes must be well developed, round or slightly oval, and relatively uniform, and the majority must be 3/8 to 13/16 inch in diameter.^[7]

These eye sizes come from a revision that took effect on February 22, 2001.^[7][8] It widened the range of eye sizes allowed in Grade A from 3/16 inch to 7/16 inch, after two Wisconsin trade associations proposed allowing smaller eyes.^[8] Switzerland's Federal Office for Agriculture raised concerns, pointing to a 1967 Codex Alimentarius standard that put acceptable eye size at between 1 and 3 cm.^[8] The U.S. standards are voluntary.^[8] The University of Guelph's e-book gives its own target: eyes 1.9–2.5 cm in diameter, spaced 2.5–7.6 cm apart.^[4]