How are they created? holes in the cheese – or in technical jargon eyes – when cutting products such as Swiss Emmentaler or Gruyere? It all starts from milka mixture made up of water, proteins, fats, sugars and mineral salts. Its transformation into cheese follows various phases, but we are interested in the so-called maturationin which distinctive characteristics such as flavour, texture, color and the famous holes develop. Responsible for the formation of holes in the cheese is the fermentation propionica set of chemical reactions by specific bacteria that metabolize lactic acid, releasing, among other molecules, also carbon dioxide, the expansion of which leads to the formation of typical holes.
Holes in cheese are created by specific bacteria
Responsible for fermentation are microorganisms that are added to milk, the best known is Proponibacterium freudenreichii. The main task of this bacterium is to metabolize lactic acid through a process known as propionic fermentationwhich we can schematize as follows:
3 C3H6OR3 (lactic acid) → 2 C3H6OR2 (propionic acid) + C2H4OR2 (acetic acid) + CO2 (carbon dioxide) + H2O (water)
Propionic acid is the most characteristic of the process and produced in greater quantities, giving it its name. It is a three-carbon organic acid responsible for the sweet and nutty aromas.
Acetic acid, with two carbon atoms, contributes to more “sour” aromatic notes and slightly spicy, useful for balancing the sweetness of the propionic acid.
The third product of fermentation is one of the most famous molecules in the world: carbon dioxide (CO2). At room temperature and pressure it is a gas: released gradually by bacteria, it accumulates in small quantities bubbles inside the cheese paste, which is still soft and plastic in the initial maturation phase.

As the fermentation continues, the bubbles expand and begin to connect with each other, forming canaliculi which can also rise outwards, allowing the gas to disperse into the environment.
It is precisely these bubbles and these channels which, once the cheese has been cut, give life to the characteristic holes, which in technical terms are called eyes. Propionic bacteria such as Proponibacterium freudenreichii they are therefore responsible for the training of large and well-distributed holesconsidered a distinctive characteristic of this cheese
Why do Emmentaler and Gruyere have holes and Parmesan doesn’t?
Fermentation is a process common to all cheeses, but why do some have holes while others don’t? The explanation lies in two main factors: the type of bacteria used and the structure of the pasta of the cheese.
In the case of Parmigiano Reggiano or Grana Padano, for example, the bacterial flora used is not propionici.e. it does not produce CO2 in significant quantities during fermentation. In fact, lactic acid bacteria (LAB) are used. Furthermore, Parmigiano Reggiano is a hard cheese: during maturation, which takes place over long periods with controlled temperature and humidity, the crystallization of proteins and fats and this helps to form a compact and grainy structure.

Are all fermentation bacteria the same?
Holes in cheese, once seen as inevitable outcomes of the production process, have now become a essential element to define the identity of the product and increase its commercial value.
The choice of bacteria not only influences the visual appearance, but also the organoleptic and structural characteristics of the cheese. Furthermore, not all microorganisms chosen for fermentation bring benefits: a study published inInternational journal of food microbiology demonstrated how bacteria of the genus Clostridium may introduce significant defects.
It turned out that microorganisms such as C. tyrobutyricum are responsible for butyric fermentationin which lactic acid is metabolized to butyric acid (which can cause rancid flavors in cheese), carbon dioxide, and other byproducts. Furthermore, compared to other bacteria, C. tyrobutyricum he is the one most involved in the so-called late inflationan excessive accumulation of carbon dioxide in the cheese paste which often leads to deformation of the food, with lifting and breaking of the rind.
Sources
David E. Metzler, Carol M. Metzler, Biochemistry: The Chemical Reactions of Living Cells, vol. 2, Academic Press, 2003, p. 970
“Contribution of C. beijerinckii and C. sporogenes in association with C. tyrobutyricum to the butyric fermentation in Emmental type cheese”. Anne-Gaëlle Le Bourhis, Joël Doré, Jean-Philippe Carlier, Jean-François Chamba, Michel-Robert Popoff, Jean-Luc Tholozan
Giraffe, G. (2021). The Microbiota of Grana Padano Cheese. AReview. Foods, 10(11), 2632
Tansman, G. F., Kindstedt, P. S., & Hughes, J. M. (2015). Crystal fingerprinting: elucidating the crystals of Cheddar, Parmigiano-Reggiano, Gouda, and soft washed-rind cheeses using powder x-ray diffractometry. Dairy science & technology, 95, 651–664
Kindstedt, P. S., & Polowsky, P. J. (2021). Invited review: Crystals in cheese: More than a curiosity. Journal of dairy science, 104(2), 1233–1250