Hole-y Truth: Why do cheeses have holes?

Have you ever wondered how and why some cheeses get holes in them? If your answer is positive, you’re definitely not alone. As a cheese professional I have been asked this question an endless number of times and normally my response in layman’s terms would sound something like this: “There is a special type of good bacteria [Propionibacterium freudenreichii ssp. shermanii] present in milk that gives off CO₂ gas and under certain maturing conditions, such as a warmer cheese cellar temperature, this gas makes the body of cheese expand therefore creating holes within it.”

Upon watching Food Unwrapped on Channel 4 the other day however, I was staggered to learn that some people in Switzerland believe that in order to produce CO₂ gas when making Emmentaler these bacteria absolutely required particles of hay to make their way into the milk and only then these bacteria are able to feed and give off CO₂…


That theory went completely against my understanding of cheese making and defied basic logic. Are these people suggesting that ALL cheese with holes in them have hay particles floating around in milking parlours (for instance Bethmale, Pannerone, Asiago, Ogleshield, Durrus, etc.)? Or are they suggesting that cheese makers can add this “essential” ingredient if hay is nowhere to be seen during the milking process (as would be the case with cheeses of supermarket quality)?

I was confused and dismayed, therefore I simply had to solve this “mystery” and set the record straight on this subject to be able to sleep that night so I marched over to my cheese book shelves and picked up a few titles.


Here’s what I found:

1. Swiss Cheese (D.Flammer, F.Scheffold): I read in this book that apparently back in the day in Emmentaler “holes were bigger than those of the Gruyère, this was due mainly to poor hygiene in the stables or some rogue bacterial growth during hot summers. In those days, most of the bacteria responsible for making the holes got into the milk through traces of cow’s faeces” (p.138)

JEEZ… Aren’t I glad we don’t live in “those days” anymore! But is it possibly where the Swiss expert on Food Unwrapped was getting his information from (an old wive’s tale of olden days)? Lucky for us, humanity has made significant leaps forward and we have now scientific research that has been done on the subject.

As we move forward in time the same book offers the following explanation for holes in the modern Emmentaler: “The distinctive regular-sized holes, however, were caused by the heating systems, which the Sulzer Company if Winterthur began to install in the dairies from the second half of the 19th century. These systems heated the fermenting cellars in a regular fashion and to a temperature that enabled the propionic bacteria present in the cheese to proliferate and create air bubbles. Of course, there had always been holes in this type of cheese, but it was not until the heating systems were installed that they could be created in the even and controlled manner that made the Emmentaler famous.” (p.140)

2. Cheese: Slices of Swiss Culture (S. Style): In her book Ms. Style recounts about the time spent at a farm where she helped to make Emmentaler cheese. This is what she learned from this particular producer: “The specific Emmentaler culture added at the beginning contains a particular bacteria (otherwise known as the “holemaker”), which gobbles up and converts the cheese’s lactic acid to produce carbon dioxide bubbles, which are the holes”.  (p. 16)

Now, for those who have made it thus far, I offer two quotes from cheese books that contain a more scientific explanation for the cheese holes.

3. The Science of Cheese (M.H. Tunick): “The eyes in Swiss-type cheeses are created when Propionibacterium freudenreichii subspecies shermanii metabolizes lactic and propionic acid. […] Extra oxygen and hydrogen atoms are also produced, and these go toward an enzyme that transports energy within the bacterial cells. The CO₂ gas accumulates at the weaker spots in the cheese matrix, forming the bubbles that we call “holes” or “eyes”. The cheese is made with a relatively low NaCl [sodium chloride] level, which allows the bacteria to survive.” (p.140)

4. American Farmstead Cheese: The Complete Guide to Making and Selling Artisan Cheese (P.Kindstedt): This highly technical book takes a close look at the importance of pH and salt content in relation to hole production. “Let’s first consider Emmental or Swiss-type cheese, which relies upon the bacterium Propionibacterium freudenreichii ssp. shermanii to produce carbon dioxide (necessary to form the eyes) and propionic and acetic acids (necessary for typical Swiss-like flavor). This organism grows best at about pH 6.0 or higher and decreases steeply in activity with decreasing pH at values below 5.5.” (p.126)

Regarding the salt content it says “This organism [Propionibacterium freudenreichii ssp. shermanii] is strongly inhibited at brine concentrations greater than about 2.0 to 2.5 percent salt, which typically correspond to salt contents in Swiss-type cheeses of around 0.8 to 1.0 percent. […] Cheese with greater than 1.0 percent salt content is likely to develop few if any eyes and to lack typical Swiss flavor, due to strong salt inhibition of Propionibacterium.” (p.130)

To sum up, it looks clear as day to me that holes in cheese are the result of a chemical process that (at least theses days) does not involve the presence of any foreign bodies (such as traces of hay or cow’s faeces). Instead, it relies on the following factors:

  • Propionibacterium freudenreichii subspecies shermanii must be present in milk

  • Cheese has to be made at a relatively low sodium chloride level

  • CO₂ gas forms as a result of metabolism of lactic and propionic acid

  • During the ageing process cheese has to be kept at a relatively higher temperature

I rest my case. Good night.

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