Swiss cheese is made by adding cultures of the bacteria S. thermophilus, Lactobacillus and P. shermani to warm (and, in the case of Swiss Emmentaler, unpasteurized) milk. These bacteria form curds in the milk, which are then pressed into wheel-shaped molds and soaked in a brine bath. The brine forms a thick rind around the cheese and the wheels are placed in ripening caves to mature.
As the cheese ages, the bacteria continue to eat away at it. P. shermani also consumes the lactic acid excreted by the other types of bacteria and, in turn, produces carbon dioxide and propionic acid. The latter helps produce Swiss cheese's characteristic flavor and the former, because of the density of the cheese and the rind, forms bubbles. The more the cheese ages, the bigger the bubbles get, and when the wheel is broken into more manageable pieces, the bubbles burst, leaving behind the holes.
These holes are called "eyes" by cheesemakers, and their size can be controlled by changing the acidity and aging time and temperature of the cheese. The size of the eyes might not seem worth worrying about, but it is fairly important. In 2000, the FDA ruled that the eyes in Grade-A Swiss sold in America had to be between 3/8 and 13/16 of an inch in diameter because newer cheese slicing equipment was tearing large-eyed Swiss cheese apart.
This question was first asked by my girlfriend and fellow Flosser, Erica, and again this week by reader Heather. If you've got a burning question that you'd like to see answered here, shoot me an email at flossymatt (at) gmail.com. Twitter users can also make nice with me and ask me questions there. Be sure to give me your name and location (and a link, if you want) so I can give you a little shout out.
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