Why does aging cheese make it taste different but aging milk makes it spoil?

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I don’t understand since to my knowledge they’re very similar compounds. Is something removed from the milk during the cheesemaking process that affects how it acts as it ages? Milk tends to go bad within a month but cheese can be aged for several years without becoming dangerous to consume.

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“Bad” species of microbes are what spoil milk, cheese, and other things. Cheeses can be preserved/aged with the right salt content, temperature, moisture content, and microbes. Cheesemakers control these conditions to create a hospitable environment for safe/favorable microbes to flourish and age the cheese. Cheese without these conditions would favor “undesirable” microbes, thus spoiling it. Not all cheeses can be aged and they’re not meant to be.

Since microbes shit where they eat, there comes a point where the favorable microbes exhaust all their resources and transform the cheese (by metabolizing it and releasing waste) into something they can’t thrive in anymore. By then a new favorable species comes along and further transforms the cheese or something unfavorable takes over, thus spoiling the cheese.

Food spoilage is all about the water activity.

Food needs a water activity of 0.9 to grow bacteria and a water activity above 0.7 to grow mold. Water activity is not directly the same as moisture content (for example marmalade has high moisture, but the high sugar content reduces the “free” water)

Milk has a very high water activity, and will spoil with bacteria, but hard cheese does not. Soft cheese is in-between, and is often prone to mold. Salt also reduces water activity, and cheese has a lot of salt.

Cheese is not just old milk. Milk goes through many steps in the process to turn it into cheese. Aging is just one step. First, bacteria are added to the milk to start producing lactic acid to acidify the milk. Unlike old milk that has gone bad, these are not random bacteria, but specific bacterial strains that are chosen on purpose because they don’t make us sick *and* they have the desired effects on the milk. This bacteria also has the effect of “crowding out” bad bacteria. Next, an enzyme called rennet is added to the milk to make it solidify. When the milk solidifies, it separates into curds and whey. Next, the curds and whey are separated, and the curds are cut into various sizes depending on the type of cheese you are making. Next, salt is added (salt helps preserve the cheese and inhibits bacterial growth), and then the cheese is shaped. Then finally, it is ripened (aged) in very specific conditions and things like temperature and humidity are constantly monitored to ensure ideal ripening. During the aging process, some cheese have mold added, some are brined, some are aged for a long time, and some for not long at all.

So you see, cheese is not just leaving out milk until it gets old. It’s a long process with many steps to get from milk to cheese, and several of the steps inhibit spoilage.

TLDR at the bottom. I am not a food scientist, but I do make a lot of fermented/aged food products including occasionally cheese. Cheese is basically an old school way to preserve milk.
Milk is super high in liquid, which allows it to spoil uniformly and creates a nice home for bacteria, it also contains no salt, a decent amount of sugar in the form of lactose and is an all around nice place to live if your a bacteria. If you take out the solids out and add salt you get cottage cheese, and if you press it you get paneer. These will spoil slower due to the water content being lower and the added salt, but you still get a pretty short shelf life. But basically the less water you leave in the cheese and the more salt you add, the longer it lasts until you end up with something like Parmesan that can stay good for years in its wheel form with the rind. That said there are also other things at play, for example cheese can be further preserved by introducing “good” mold/bacteria (like blue cheeses or “stinky” cheeses) that basically out compete any new mold or bacteria and add flavor without harming us. Or you can introduce an artificial rind with wax that will allow you to age a wheel of cheese that wouldn’t otherwise be suitable for long aging as is sometimes done with cheddar. Cheese making is old and pretty much every tradition that consumes milk has their form of it, so it’s a lot bigger than ELI5 but that’s the gist. The other way to preserve milk would be to intentionally introduce bacteria to outcompete the bad ones without removing the solids, that’s essentially what yogurt is.
TLDR More salt and less water is good for shelf life, people do other things to preserve it even further.

Forgot to mention cheeses that age for years do so in the wheel form and will begin to mold/spoil fairly quickly when cut, that’s because they are either waxed to form a rind, or because the outer layer forms a rind on its own because it does essentially spoil under controlled conditions and hardens. Either way it keeps the interior of the wheel under anaerobic conditions, which further prevents spoilage.

Aging milk and then shaking it makes it different. It is called Laben/l’ben. A nectar in North Africa.