Eyes are the round holes that are a characteristic feature of Swiss-type cheese (e.g. Emmentaler cheese) and some Dutch-type cheeses. They are bubbles of carbon dioxide gas that is produced by 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 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.
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- T. Beresford, A. Williams; The Microbiology of Cheese Ripening, in: Fox, p. 303
- P.L.H. McSweeney and P.R Fox; Metabolism of Residual Lactose and of Lactate and Citrate, in: Fox, p. 367