The holes in Swiss cheese are not drilled into the wheel and they are not empty places where curd failed to form. They are the visible result of fermentation continuing inside an already formed cheese.
The first cultures make lactic acid
Early in cheesemaking, starter bacteria ferment lactose and acidify the curd. That lactic acid is not the end of the story in Swiss-style cheese.
Propionic bacteria make carbon dioxide
During ripening, bacteria such as Propionibacterium freudenreichii metabolize lactate into propionic acid, acetic acid and carbon dioxide. The acids contribute to the sweet, nutty flavor associated with Swiss-type cheese; the CO₂ creates the eyes.
How gas becomes a round hole
Carbon dioxide accumulates at nucleation points inside the cheese. If the protein matrix is closed and elastic enough, pressure stretches it instead of cracking it. In the warm ripening stage, those gas pockets enlarge. Cooling later helps the shape remain fixed.
Cheesemakers therefore call the openings eyes.
The recipe controls whether eyes form
Propionic bacteria alone are not enough. Salt, pH, moisture, temperature and the mechanical properties of the curd all influence whether CO₂ dissolves, escapes, forms neat eyes or causes splits.
The University of Wisconsin Center for Dairy Research notes that propionic bacteria are important for both the characteristic flavor and the eye formation of Swiss cheese.
Not every cheese hole has the same origin
Small openings in Gouda and other cheeses can involve different bacteria and substrates, including citrate fermentation. Irregular mechanical openings or cracks are also different from the smooth, rounded eyes formed as a normal part of Emmental-style cheese.
Do more holes mean better cheese?
No. Eye size and distribution are quality targets specific to certain varieties. A Parmesan wheel full of large eyes would be defective, while well-formed eyes are expected in Swiss cheese. Even there, excessive gas can produce splits and poor texture.
What the holes reveal about the cheesemaking process
The eyes in Swiss-type cheese are the visible result of microbiology and physics inside a curd that is still changing during ripening. Propionic bacteria use compounds produced earlier in fermentation and release carbon dioxide. The gas needs places where it can collect, and the cheese body needs enough elasticity for a bubble to expand rather than immediately escape or split the paste. That is why eye number, size, and shape depend on more than simply “having bacteria”: cultures, moisture, temperature, salt, curd structure, and ripening time all influence the result.
A round, well-distributed eye can be a desired part of a particular cheese style, while irregular openings, cracks, or mechanical gaps may have a different origin. Cheesemakers assess those features together with aroma, texture, and the behavior of the paste; counting holes alone does not measure quality. It also explains why cheddar, mozzarella, Parmesan, and many other cheeses are not expected to form the same pattern. Their cultures, curd handling, moisture, and maturation are different. The holes are therefore a consequence of a specific cheesemaking process, not a universal property of milk and not a simple sign that one cheese is “better” than another.
What gas makes holes in Swiss cheese? Mostly carbon dioxide produced by propionic bacteria during ripening.
Do all cheeses with holes use the same bacteria? No. Swiss-style eyes are strongly linked to propionic bacteria, while other cheeses can form small openings through different fermentations or mechanical effects.
Does more holes mean better Swiss cheese? No. Cheesemakers target a characteristic size and distribution; too much gas can cause splits and other defects.
Eye size and shape are not merely decorative features. They depend on how much gas is produced, how that gas can move through the curd, and how elastic the cheese remains while it ripens. A firmer paste restricts expansion; a more flexible one can allow rounder cavities to grow. Ripening temperature also affects microbial activity, so cheesemakers can influence eye development through the conditions they maintain. The pattern is therefore a record of fermentation and cheese structure rather than holes that have been mechanically added to the finished cheese.
The two-part explanation
You need both microbes that generate gas and a cheese body designed to trap it. Change either side of that equation and the cheese may have tiny eyes, no eyes or unwanted cracks instead.
Sources
- NCBI Bookshelf / ASM — Microbes Make the Cheese — Explains propionic fermentation, CO₂ and eye formation in Swiss-style cheese.
- University of Wisconsin Center for Dairy Research — What is Swiss cheese? — Industry-science overview of propionic bacteria, flavor and eye formation.
- International Journal of Food Microbiology — atypical eye formation in Swiss-type cheese — Research on microbial control of eye formation.