<Just how did fermentation work to purify bacteria-laden water in those
days of yore?>

This one I can field at least in part.  First off the notion that a tiny
bit of bacteria can spoil a batch is a bit of a misnomer (though
cleanliness is still important for many flavor considerations).  If you
pitch healthy active yeast, they can generally out-compete pretty much any
lurking bacteria within reason.  If you save yeast and want to maintain a
pure culture this is a different story and small repeated contamination
will compound over time.

Secondly I guess it should be noted that modern typical single yeast strain
fermentation is new to brewing.  Traditional beers from "back in the day"
would likely have been spontaneously fermented, or inoculated from a
previous batch.  Most beers would have had poorer attenuation and been
"sours", including both bacteria and yeasts in the mix.  These buggers
produce alcohol and acids that lower the pH, both of which inhibit growth
of "harmful" bacteria, and prevent other types of spoilage.  I imagine the
same benefit for bees use of fermented bee bread. (see look bee connection!)

Third, wort is often boiled before fermenting, killing bacteria in
contaminated water sources to begin with.

Fourth, define spoiled.  I have 55 gallons of beer in a barrel downstairs
that is spontaneously fermented, aka spoiled, no yeast pitched by me.  In
my somewhat limited experience the best beer often spends a good amount of
time smelling like rotting meat before it finishes aging.  Now it tastes
like black cherries and dark fruit with a nice light sour tang.

Jeremy
West Michigan

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