Pre-industrial yeast
<http://thelibrary.org/lochist/periodicals/bittersweet/sp79i.htm>
hasinformation about making yeast cakes from hops. I've heard that
the term leavening used in the Bible referred to actual bread saved
back and left to go moldy before being used somehow. Some sort of
starter would make more sense to me, as a baker, and would still fill
the place in the parable as something inedible and undesirable that's
necessary for bread making. But I'd never thought about people making
yeast for bread separately from a sourdough type starter. But
evidently they did. Thanks to Seth for promoting my early morning
Google session!