How do holes form in cheese with holes?

Those who love cheese love it because of what is eaten, not because of what is not eaten. That is why I did not expect to learn very interesting things relative to the holes we find in some cheeses, such as the very famous Emmental. After all, these holes contain nothing. Yet, I discovered things I never imagined!

Most of the most curious information I now tell you I found in a scientific article by Dominik Guggisberg and others, published in 2015 in a scientific journal in the field called International Dairy Journal (which, between you and me, seems like a wasted opportunity: I would have titled it Dairy News!)1But let’s go step by step and start with the basics.

The process of propionic fermentation

The holes that have madeEmmental famous, and that characterize many other cheeses, are called in technical jargon eyes . These cavities in the cheese paste are in most cases round in shape and are generated by the accumulation of gases produced by certain types of microbial fermentation. We have already talked a bit about fermentations in this article.

In this case, the fermentation involved is termed propionic, because starting from sugars or lactic acid it comes to produce propionic acid, the smallest of the fatty acids. This compound has a strong pungent smell, like rancid butter or cheese. Think of it as one of the substances that also gives sweat its characteristic, often unpleasant aroma. Even on our skin, in fact, bacterial strains that do propionic fermentation exactly like those used to make the famous Swiss cheeses can live.

In addition to producing the fatty acid that gives the cheese its distinctive smell and aroma, fermentation done by Propionibacterium freudenreichii and other similar bacteria releases large amounts of carbon dioxide. Some of this gas may be released from the cheese while it is still soft and lost during the early stages of aging, but it may also remain dissolved in the cheese paste, or it may form the eyes, collecting at specific points in the form of bubbles that gradually enlarge.

How to create homogeneous eyes in cheeses? The study

Thus, we discovered who is responsible for the formation of eyes:carbon dioxide produced by propionic bacteria. What more can there be to discover? What questions prompted Swiss scientists to invest time and resources in experimental research on Emmental?

Producing a Swiss cheese that has ‘standard’ eyes in number, size and distribution is not easy. Quality control for the formation of holes in the cheese is still conducted in the traditional way, either by listening to the sound the wheel makes when it is hit with a special hammer or by observing the appearance of a small cylinder-sample, extracted with a small ‘core’. However, these methods are highly dependent on the capabilities of the individual operator and are neither precise nor quantitative.

If you start from different raw materials, then, ensuring thehomogeneity of the cheeses is even more complicated; this is the case of producers who centrifuge or micro-filter their milk, to remove pathogenic bacteria and spores that might make the final product less safe; this practice changes some of the characteristics of the milk and its micro-environment sometimes in unpredictable ways, so this can be reflected in a cheese that one time comes one way and the next time another. This diversity between different batches is not acceptable to consumers and distributors, especially for cheeses whose production is highly regulated by PDO production guidelines.

So what are the factors that influence the production of eyes in cheese? Are they in any way controllable by producers? Here are the questions that many scientific studies have tried to answer. The study I mentioned earlier, led by Dr. Guggisberg of the Agroscope Institute for Food Science in Bern, focused on how plant-derived microparticles influence the formation of eyes in Emmental. As early as 1917, William Clark hypothesized that there were “favorable points” within the cheese paste where the gases that give rise to the eyes accumulate.2

Over the years, various studies have gradually come to identify these ‘favorable spots’ in plant microparticles, a kind of “hay dust” that remains suspended in the air of the stables and can end up inside the freshly milked milk and, therefore, in the cheese made from it.

Dr. Guggisberg and his collaborators wanted to test how much this hay powder affects eye production in Emmental cheese. To do this, they started with microfiltered milk, thus devoid of any kind of solid particles, and added hay powder themselves in varying and well-defined amounts: from 0 to 4 milligrams per 90 liters, resulting in eight different experimental forms of Emmental.

To precisely and quantitatively track the development of gas bubbles over time, without having to cut the cheese into pieces, scientists subjected Emmental wheels to a CT scan, a computed tomography scan that, using X-rays, virtually “slices” the cheese recreating an image of it section by section on the computer. Basically, they have taken more x-rays these cheeses in just over 4 months than I have in my lifetime. Crazy!

What didAgroscope scientists discover from this experiment? They found a direct correlation between the amount of hay powder added to the milk and the number of eyes within the resulting cheese. But this happens only up to values of 1 mg per 90 L of milk. At higher values, a “saturation” effect is observed, i.e., the number of eyes stops increasing despite the increase in added powder. Removal of all particles by filtration, on the other hand, produces a cheese that is almost entirely free of eyes. Not only that, if they cannot ‘vent’ by forming holes in the paste, the gases released during fermentation can create cracks and fissures in the cheese, ruining the shape and leading to a product that does not meet standards for sale.

Come creare occhiature omogenee nei formaggi? Lo studio
Fig. 1 Visualizzazione tomografica ai raggi X delle occhiature prodotte nelle 8 diverse forme di Emmental ottenute aggiungendo quantità variabili di polvere di fieno (0-4 mg) al 45° giorno di stagionatura.
occhiature buchi formaggi durante stagionatura
Fig. 2 Sezione delle 8 diverse forme al termine della stagionatura (130 giorni)


This study, the first to demonstrate precisely that the Eyeballing can be induced by triggers such as hay powder, also highlights the contrast to which producers of cheeses with eyes are exposed, torn between the need to ensure increasingly hygienic conditions for milk collection and the need to ‘contaminate’ the milk with microparticles essential for the formation of holes in their PDO products. Quite a dilemma indeed. Perhaps solvable by considering the addition of hay powder after the fact, as carried out byAgroscope scientists for their study.

Speaking of dilemmas, I leave you with this paradox about cheese with holes in it, with the task of finding its meaning:

The more cheese, the more holes;

the more holes there are, the less cheese there is;

So the more cheese there is, the less cheese there is.


1. Dominik Guggisberg et al, Mechanism and control of the eye formation in cheese, 2005, International Dairy Journal, 47, 118-127

2. W.M. Clark, 2017, On the formation of “eyes” in Emmental cheese, Journal of Dairy Science, 1, 91-113

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