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SciShow explains a) why Swiss cheese has holes, b) what Swiss cheese is called in Switzerland and c) what vested interest the U.S. government has in said cheese holes. Seriously, people.
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Quick Questions [QQs]: "Why does Swiss cheese have holes?"

Other than the smell of your feet, which we've covered already, this is probably the most popular cheese-related question on the internet. And the answer to why Swiss cheese has holes is actually pretty simple. All cheese is the product of fermentation, a type of respiration that some microorganisms use to feed in the absence of oxygen. There food in this case is lactose, the sugar found in milk. And to make cheese all you really need to do is warm up some milk, toss in some bacteria, wait for it to thicken into chunky curds, and then give the whole thing a brine bath to make it form a protective rind. Then you just let the bacteria do their thing somewhere safe like a cheese cave, then a few months or years later, boom. You've got sandwiches.

Now the holes have to do with what kind of bacteria you have in your cheese. Some cheeses are made with homofermentative bacteria. They tend to produce one main byproduct when they respire, in this case lactic acid, and in addition to certain cheeses like cheddar they're used to make pickles, sauerkraut, yogurt, and other tart-tasting things. But there are also heterofermentative bacteria, which create a whole bunch of byproducts including carbon dioxide. These bacteria, like P. shermani, are what are used to make Swiss cheese. So as the bacteria are in their little cave or whatever eating lactose and excreting carbon dioxide, the gas gets trapped inside the cheese rind and forms bubbles. If you're making a soft cheese these gas bubbles can move through the cheese and leak out the rind. But Swiss cheese is more dense, with a thick rind, and the gas can't escape, so these bubbles form air pockets which eventually leave behind the cheese's signature holes.

Cheese makers refer to these holes as eyes and they can control the size of them by adjusting the cheese's acidity, temperature, and aging time, all of which influence the byproducts of the bacteria. So, for example, milder baby Swiss has smaller holes because it doesn't age as long. And here's a fun fact, in the US the Food and Drug Administration actually regulates the size of Swiss cheese holes. Holes must be between 3/8 of an inch and 13/16 of an inch in diameter so that they don't gum up the commercial cheese slicers, who knew?

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