Here [1] is a free 2011 review article on the science of sourdough, to which Ian(ArsP) posted a link in his blog [2] today. Nice collection of information for those curious about the biology and chemistry of natural levains.
Tom
Links[1] http://onlinelibrary.wiley.com/doi/10.1111/j.1541-4337.2011.00148.x/pdf [2] http://arspistorica.blogspot.com