What is the reason behind production of large holes in ’Swiss Cheese’ ?

Swiss‑cheese holes (sometimes called “eyes”) are formed during the ripening stage when gas bubbles become trapped in the cheese matrix.

The principal micro‑organism responsible for generating these bubbles is Propionibacterium sharmanii. This bacterium ferments the lactate that remains after the action of lactic‑acid bacteria.

The metabolic pathway can be summarized by the following overall reaction:

C_3{H}_5{O}_3\to C_2{H}_3{O}_2+C_{\mathrm{O}}₂+H2O

In words, propionibacteria convert three molecules of lactate into one molecule of propionic acid, one molecule of carbon dioxide (CO₂) and one molecule of water. The CO₂ produced is insoluble in the cheese matrix and accumulates as gas bubbles.

As the cheese continues to mature, the protein network gradually solidifies around these bubbles, locking them in place. The result is the characteristic large holes that give Swiss cheese its distinctive appearance.

Thus, the reason behind the production of large holes in “Swiss Cheese” is the copious generation of CO₂ by Propionibacterium sharmanii during cheese ripening.