We've mused on mysteries before (remember posts on why blanched vegetables turn brown, why meat should rest before you serve it, and the crunchy bits in cheese?) and we're doing it again in this week's Cheesemonger report: From where oh where do those holes in your cheese come?
Make all the jokes you want, but it's true: gassy bacteria are what's behind the holes in cheese. To make cheese-- in this case most likely a Swiss like Emmenthaler or an aged mountain cheese like Gruyere-- one must precipitate the separation of curd and whey. To begin this process, the cheesemaker will introduce a starter culture to the milk at the very beginning of cheesemaking.
Starter cultures contain bacteria that, once added to the vat, will consume lactic acid (a byproduct of the lactose in the milk) and, most significantly, exude bubbles of carbon dioxide gas, which, trapped within the suffocatingly restrictive rind of a Swiss cheese, slowly form pockets of air bubbles that become the "eyes." The key, eye-producing bacteria to keep in mind for friend-impressing purposes is "Propionibacter shermani," considered the chief of the three types of bacteria used to make Swiss style cheese. (The others are Streptococcus thermophilus and Lactobacillus.)
Think of it working in many ways like the bubbles that form because they have no where else to go after a secondary fermentation within a bottle of sparkling wine.
Cheesemakers can determine the size of the holes by changing any number of variables: the acidity of the milk, the temperature to which he or she brings the milk, and the aging time of the wheel. And, due to the following little known fact, it's a good thing they know how to control the size of those holes. The USDA has recently started restricting the hole size of American Swiss (a contradiction if we've ever heard one).
They've cut down the diameter by more than half because new industrial and and deli equipment used for slicing cheese got caught on larger holes.
The larger the eyes in a Swiss, the more developed and pronounced the flavors will be, because longer aging leads to greater development of flavor-producing bacteria and enzymes and larger holes. Interestingly enough, this explains in some ways why American versions, with their smaller air pockets (but oh-so-sliceable) tend to be young, buttery, and mild, while the Old World classics remains assertively nutty, rich, and nuanced by comparison:
Perhaps the USDA had production and efficiency in mind rather than historical preservation? This all begs a chicken and egg question: What came first, the age-old cheese tradition or the cheese slicer?