Eyes are the round holes that are a characteristic feature of Swiss-type cheese (e.g. Emmentaler cheese) and some Dutch-type cheeses. The eyes are bubbles of carbon dioxide gas. The gas is produced by various species of bacteria in the cheese.
In Swiss-type cheeses, the eyes form as a result of the activity of propionic acid bacteria (propionibacteria), notably Propionibacterium freudenreichii subsp. shermanii. These bacteria transform lactic acid into propionic acid and carbon dioxide, according to the formula:
The CO2 so produced accumulates at weak points in the curd, where it forms the bubbles that become the cheese's eyes. Not all CO2 is so trapped: in an 80 kg (180 lb) cheese, about 20 L CO2 remain in the eyes, while 60 L remain dissolved in the cheese mass and 40 L are lost from the cheese.
- Polychroniadou, A. (2001). Eyes in cheese: a concise review. Milchwissenschaft 56, 74-77.
- Fox, P.F. (ed.). Cheese: Chemistry, Physics, and Microbiology, Volume 1: General Aspects. Academic Press. ISBN 978-0-12-263652-3.
- McSweeney, Paul L.H.; Fox, Patrick F. (2004). "Metabolism of Residual Lactose and of Lactate and Citrate". Cheese: Chemistry, Physics and Microbiology. 2. Elsevier. pp. 366–367. doi:10.1016/S1874-558X(04)80074-5. ISBN 978-0-12-263653-0.
- Clark, William (1917). "On the Formation of "Eyes" in Emmental Cheese". Journal of Dairy Science. 1 (2): 91–113. doi:10.3168/jds.S0022-0302(17)94362-0.
- P.L.H. McSweeney, Biochemistry of Cheese Ripening: Introduction and Overview, in: Fox, p. 349
- "Propionibacterium freudenreichii ssp shermanii ATCC9614: A bacterium used in the production of Emmental". Genoscope. 16 January 2008. Archived from the original on 26 August 2016. Retrieved 23 October 2010.
- T. Beresford, A. Williams; The Microbiology of Cheese Ripening, in: Fox, p. 303