Eyes (cheese)

Eyes are the round holes that are a characteristic feature of Swiss-type cheese^[1] (e.g. Emmentaler cheese) and some Dutch-type cheeses. The eyes are formed by bubbles of carbon dioxide gas during the cheesemaking process. The gas is produced by various species of bacteria in the cheese.^[2]

Emmentaler with eyes

Swiss cheese

In Swiss-type cheeses, the eyes form as a result of the activity of propionic acid bacteria (propionibacteria), notably Propionibacterium freudenreichii subsp. shermanii.^[3][4] These bacteria transform lactic acid into propionic acid and carbon dioxide, according to the formula:

3 lactate → 2 propionate + acetate + CO₂ + H₂O^[5]

The CO₂ so produced accumulates at weak points in the curd, where it forms the bubbles that become the cheese's eyes.^[3] Not all CO₂ is so trapped: in an 80 kg (180 lb) cheese, about 20 L of CO₂ remain in the eyes, while 60 L remain dissolved in the cheese mass and 40 L are lost from the cheese.^[1]

In Swiss cheese production, the number and size of eyes declined in the 2000s. This was the result of increasing standards of hygiene, which reduced the number of dust particles in the milk around which gas bubbles form. In 2025, the Swiss Federal Administrative Court approved the addition of hay flower [de] powder (Flores graminis) to the milk during cheesemaking, to allow for eyes of the typical number and size to form.^[6][7]

Dutch cheese

In Dutch-type cheeses, the CO₂ that forms the eyes results from the metabolisation of citrate by citrate-positive ("Cit+") strains of lactococci.^[1]

Bibliography

• Polychroniadou, A. (2001). Eyes in cheese: a concise review. Milchwissenschaft 56, 74–77.

References

• Fox, P.F., ed. (13 October 2004). Cheese: Chemistry, Physics, and Microbiology, Volume 1: General Aspects. Academic Press. ISBN 978-0-12-263652-3.