The holes in Swiss cheese are formed by trapped gas
How the Holes Develop
The familiar holes in Swiss-style cheese are bubbles frozen in place. During ripening, helpful bacteria consume lactic acid and release carbon dioxide. Because the cheese is firm and enclosed by a rind, some of that gas cannot escape. Instead, it collects inside the cheese, pushing the flexible curd outward and creating rounded cavities.
Cheesemakers call these holes “eyes.” They are especially associated with Emmental, the traditional Swiss cheese that inspired many of the varieties labeled “Swiss cheese” in the United States.
From Bacteria to Bubbles
Eye formation begins when propionic acid bacteria become active during warm ripening. The carbon dioxide they produce moves through the cheese until it gathers around tiny starting points, or nuclei. According to a review in the Journal of Dairy Science, gas production, cheese structure, ripening conditions, and the presence of nuclei all influence the final holes.
Microscopic particles, traditionally including traces of hay dust from dairy farming, can provide places where gas bubbles begin forming. Cleaner modern milking methods have helped explain why some cheeses developed fewer or smaller eyes than their traditionally produced counterparts.
Size, Shape, and Flavor
Temperature, acidity, aging time, moisture, and bacterial activity affect the number and size of the holes. If too little gas develops, the cheese may be “blind,” meaning it has few visible eyes. Excessive or poorly timed gas production can create cracks instead of smooth cavities.
The bacteria involved also help produce the sweet, nutty flavor associated with Swiss-style cheese. This means the fermentation process shapes both its appearance and taste.
Unlike the deliberately manufactured holes that help crackers bake evenly, Swiss cheese eyes form naturally from trapped gas. Each hole is therefore a visible record of the biological and chemical changes that occurred while the cheese matured.