If the germ theory is relatively new, how do they think fermentation was happening (like wine, ale, yogurt etc.) thousands of years ago?
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It’s really hard to explain history of science topics like you’re five, but here’s my best go.
If you want some historical sources of thought on this topic, check out [book IV](http://classics.mit.edu/Aristotle/meteorology.4.iv.html) of Aristotle’s Meteorology (despite the title, the fourth book is actually not about meteorology at all). Aristotle discusses at length the way that different terrestrial bodies can change and be changed. He didn’t have a theory of fermentation per se, but it and a number of other processes (digestion, cooking, smelting) are considered to be part of the same family of processes. You’ve got to think about things in terms of bodies with certain properties having their properties changed, not in terms of atoms. Fermentation is just another way by which people can change the properties of certain bodies.
Much later, the great alchemist Paracelsus also writes about fermentation as an analogue for the elixir of life. For him, yeast and other agents of fermentation just have a special preservative property to fix things in their current state. This kind of thinking is common in alchemical and non-alchemical texts alike – all the things in nature just have special god-given properties, and it’s up to the clever alchemists to find them. Paracelsus says:
>”We call this preservative an elixir, as if it were yeast, with which bread is fermented and digested by the body. Its virtue is to preserve the body in that state wherein it finds it, and in that same vigour and essence. Since this is the nature of preservatives, namely, that they defend from corruption, not in any way by purifying, but simply by preserving. The fact that they also take away diseases is due to the subtlety which they possess. So, then, they do not only preserve, but they also conserve. They have a double labour and duty, that is to say, to prevent diseases and to keep the essence itself in its proper condition”
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>- Paracelsus, *The Archidoxies of Theophrastus Paracelsus*, Book 8, *trans*. Waite
Essentially, some stuff just has the power to act on other stuff in a way that preserves it. This isn’t all that weird on an ancient or early modern worldview. Some rocks just have the power to attract iron! Some plants have the power to induce vomiting! This kind of magical essentialist thinking is a major feature of western science up into the 17th century. It may seem like a bit of a cop-out explanation to us, but it worked pretty well for them.
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