Now its hard to say how exactly, because we dont have written records, but it was most likely accidental change. Someone tried to store milk in something and due to the existence of ideal bacterie the milk turned to yoghurt. There is a good chance it was somewhere in Turkey, after all the word yoghurt itself is of Turkic origin.
The first yoghurt was infected with bacteria either from the air or any of the surfaces it came into contact with. And it was not pasteurized like modern yoghurt production requires so the bacteria were not killed. It is still possible to make yoghurt the same way but it will take a lot longer to make it the first time due to the low number of initial bacteria, and you have no control over which bacteria infects the yoghurt so you have no control over the taste, or even if it is drinkable.
It is much more popular to do this when making bread. This is how a sourdough starter is made. But in this case there is a much higher chance of success then with yoghurt. You also have naturally fermented beer and wine which are also made the same way but typically require specific conditions for a high chance of success.
Any milk that is left in the open at room temperature will eventually become home to bacteria. Most of the time, this is bad because those bacteria either make the milk taste bad, can make humans sick, or both. However, some kinds of bacteria are beneficial. Not only do they give the milk a pleasant tang and a thicker texture, they also out-compete the bacteria we don’t want to have. That’s how you get yogurt.
So the first yogurt was made pretty much by chance. For most of human history, we had no good way of keeping milk cold or otherwise protected from bacteria (nor a sense of what bacteria even is!) Mostly that meant we tried to consume milk before any bacteria got to it, but if some was left behind, we would occasionally get lucky and end up with a colony of yogurt bacteria rather than the nasty kinds. Eventually, we figured out that proactively mixing some old yogurt into milk that we couldn’t drink right now was a much more consistent way of “inoculating” the milk with our preferred bacteria. That still works today, though now you can also buy the bacteria on its own and add that.
The first yogurt was likely made by accident.
One possible example: People used to store milk in containers made from animal stomachs. The natural bacteria in these stomachs reacted with the milk, causing it to ferment and thicken, turning it into yogurt.
Over time, people realized that this thickened milk tasted good and was safe to eat. They then began making it on purpose by adding a bit of the already-made yogurt (which had the needed bacteria) to fresh milk, and then letting it sit for some time to ferment.
It was likely created by accident as a by product of people storing dairy, similar to cheeses.
Basically, the bacteria that “produce” yogurt or cheese are naturally occurring and will almost always contaminate dairy as it’s being harvested and stored. Nowadays we often pasteurized dairy to kill all this bacteria and more so our dairy doesn’t turn into something else.
But back in the day, any stored dairy would have hundreds of different bacteria in it, and it was largely a roll of the dice which strain would dominate and how it would affect the dairy. It’s an arms race between those little microbes. If you’re lucky, the strain that wins turns your dairy into something tasty. When you’re adding some existing yogurt, you’re skipping that game of chance and just throwing in a glob of one of those good bacteria. It just makes it faster and more certain that the desired strain is the winner in your next batch.
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