What Can I Do With 300g Of Soy Flour?

This is the sort of question you are going to wish you had never asked.

Nowadays, yoghurt starter is obtained from a "live" yoghurt that still has living cultures in it: the bacteria that cause the fermentation process that makes milk into a wet solid.    Use a little left over from the last batch of yoghurt to get the next batch to set.  The relevant bacteria are mostly / usually: Streptococcus thermophilum and Lactobacillus bulgaricus.

So what you really want to know is, how did someone get hold of those bacteria in the first place?

Well, the people who started "making" yoghurt didn't know about bacteria.  They only got clued up about using old yoghurt to ferment new milk into being yogurt.  And nobody can be 100% sure how yoghurt was first made... but PROBABLY... It went like this.

I'm a poor shepherd on a mountain side with some sheep, 1000 years ago.  I'm either Turkish or Bulgarian (they both claim this legend).  I don't pasteurise my milk or sterilise anything because I don't know about germs.  If I get some spare milk from my sheep I put it in a goat-skin bottle.  Which I've never cleaned properly because I don't know about germs.  Although I might have coated the inside of the leather with goo from goat intestines to enhance its leak-proof-ness.  During a hot spell I forget about some milk in the goatskin for a few days... when I go to drink it it's soured, tasty but thick.  After a while I get the hang of this and just leave unpasteurised milk in a rotting goat skin, hoping it will make yoghurt.  Later I realise any container will do, as long as I add some old yoghurt and the milk is kept warm.

The bacteria in the goat skin came from the goat's intestines, originally.  That's okay, because I think of them as a delicacy, anyway.  Perhaps I wasn't very clean when I slaughtered the goat to eat it and use its skin to make liquid carriers.  Or maybe it was backwash into my leather bottle, after I had eaten some goat meat, that put the bacteria there.

Yummy!  Eh?  See, I said you didn't want to know.