The bacterium responsible for fermenting yoghurt is called lactobacillus. It doesn’t only exist in yoghurt – it’s commonly found in the environment. If you left milk out it might naturally turn to yoghurt, and at many points in human history it did – totally naturally. However, there are competing organisms, like mold, yeasts or other bacteria, that might infect the milk first. So to be sure it’s the lactobacillus that takes hold, you import it from a previous batch. That way you always get yoghurt, and you get it as fast as possible.
A lot of food preparation methods we now now were discovered by accident. Creation of Camembert cheese for example is believed to have happened because some dude forgot his cheese in the cellar and it got infected by mold. Also sourdough bread is infected by bacteria that are in the air. The good bacteria tend to take over and make it safe to use/eat
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