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

Carbon dioxide

Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...

 gas that is produced by bacteria

Bacteria

Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...

 in the cheese.

Historically (Middle Ages), the holes were considered a sign of imperfection and cheese makers would try to avoid them.

Swiss cheese

In Swiss-type cheeses, the eyes form as a result of the activity of propionic acid bacteria

Bacteria

Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...

 (propionibacteria), notably Propionibacterium freudenreichii

Propionibacterium freudenreichii

Propionibacterium freudenreichii is a Gram-positive, nonmotile bacterium that plays an important role in the creation of Emmental cheese, and to some extent, leerdammer. Its concentration in Swiss-type cheeses is higher than in any other cheese. Propionibacteria are commonly found in milk and dairy...

subsp. shermanii. These bacteria transform lactic acid

Lactic acid

Lactic acid, also known as milk acid, is a chemical compound that plays a role in various biochemical processes and was first isolated in 1780 by the Swedish chemist Carl Wilhelm Scheele. Lactic acid is a carboxylic acid with the chemical formula C3H6O3...

 into propionic acid

Propionic acid

Propanoic acid is a naturally occurring carboxylic acid with chemical formula CH3CH2COOH. It is a clear liquid with a pungent odor...

 and carbon dioxide, according to the formula:

3 Lactate

Lactate

Lactate may refer to:*The act of lactation*The conjugate base of lactic acid...

 → 2 Propionate

Propionate

The propanoate or propionate ion is C2H5COO− .A propanoic or propionic compound is a salt or ester of propanoic acid....

 + Acetate

Acetate

An acetate is a derivative of acetic acid. This term includes salts and esters, as well as the anion found in solution. Most of the approximately 5 billion kilograms of acetic acid produced annually in industry are used in the production of acetates, which usually take the form of polymers. In...

 + CO₂

Carbon dioxide

Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...

 + H₂O

Water

Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

The CO₂ so produced accumulates at weak points in the curd

Curd

Curds are a dairy product obtained by curdling milk with rennet or an edible acidic substance such as lemon juice or vinegar, and then draining off the liquid portion. The increased acidity causes the milk proteins to tangle into solid masses, or curds. The remaining liquid, which contains only...

, where it forms the bubbles that become the cheese's eyes. Not all CO₂ is so trapped: in a 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.

Dutch cheese

In Dutch-type cheeses, the CO₂ that forms the eyes results from the metabolisation of citrate

Citrate

A citrate can refer either to the conjugate base of citric acid, , or to the esters of citric acid. An example of the former, a salt is trisodium citrate; an ester is triethyl citrate.-Other citric acid ions:...

 by citrate-positive ("Cit+") strains of lactococci.