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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.
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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. These bacteria transform lactic acid into propionic acid and carbon dioxide, according to the formula:
3 Lactate → 2 Propionate + Acetate + CO2 + H2OThe 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.
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.