Lechem started a thread about goddisgoode or barm, the yeast that medieval brewers used to produce ale - in England and beer in Europe. This led to discussion about the difference between modern sourdoughs and yeast breads along with the prospect of using barm from brewing beer for baking bread. And there seems to be a consensus that early bakers regularly obtained barm from brewers for their baking and that households that did not do their own brewing would also obtain barm from brewers for their baking needs. So I wandered off into the swirling mists of googleland trying to find out more about barm. It seems that the yeast we so conveniently find on the shelf in the store in almost all its forms was isolated from the yeasts brewers were using and commercialized. I found some interesting information in an essay on the Economics of Medieval English Brewing, http://www.polysyllabic.com/?q=medieval/brewing that indicated the relationship between brewers and bakers was a two-way street with small scale brewers, unable to keep the process continually underway, would often have to obtain yeast from a baker. I'm wondering how much the yeasts used in the two processes differ and, if either the brewers or bakers made an effort to maintain their own supply of barm, how that would differ from our sourdough cultures. dabrownman says his NMNF rye based starter grows more sour over time but it is kept under refrigeration, a technology unavailable 'way back when'. Some of the country folk, 'ye olde yokels', would certainly have had to maintain a starter because the nearest brewer wasn't all that near. Any thoughts, SWAG's, certainties on the subject?