Denise wrote:How did yogurt fermentation begin? How did the poeple know what to use for a culture? Where would they have found the culture to use the first time? Did it ever become contaminated, and if so, how did they get a fresh starter?
Denise, I'm not sure anyone knows all this for sure, and I certainly don't. But I suspect, as with most things, it was partly accidental discovery, part luck, and maybe part providence.
Before the advent of ceramics and vessel making, it was common to carry fluids in natural "vessels," such as hollow gourds, animal skins and even animal stomachs. Some animal stomachs would still be imbibed with rennet, causing milk stored in it to curdle. This is probably how the first cheese was "accidentally" made.
Raw milk carries its own cultures. These cultures vary from place to place, animal to animal. Once it was discovered that milk from a particular place naturally turned into what we now call yogurt or kefir, that culture was preserved. Because the aforementioned vessels (and un-glazed pottery) were porous, they would have remained inoculated with these cultures. Each subsequent batch of milk (or fruit juice or honey-water, etc.) poured into the vessel would begin to ferment in whatever fashion the culture provided.
"And no one pours new wine into old wineskins. If he does, the new wine will burst the skins, the wine will run out and the wineskins will be ruined."