Eyes (cheese)

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.

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

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