Although yeast as a microbe was not known until the early 19th century, yeast itself was known earlier than the days of the Roman empire. Recognized as both the froth that rose to the top of the fermenting vat and the sludge that fell to the bottom, its cause may not have been well known but its effect was certainly common knowledge among both brewers and bakers.
The traditions of brewing were carried on and thrived under the Roman Catholic Church. Nearly every major monastery in medieval Europe contained a brewery that served not only the monks but also pilgrims and the surrounding villages (perhaps as an inducement for attending mass). One large monastery in Switzerland had three breweries, each adjacent to a bakery. Brewing and baking, in fact, were closely related activities in ancient and medieval times.
The presence of a fermenting agent was known in France in the 13th century. In a comprehensive law enacted by Louis IX in 1268, he laid out numerous strictures to ensure the quality of fermented beverages, including the sale of yeast barm. “No beer yeast shall be hawked about the streets, but shall be all sold in the brew-houses to bakers and pastry cooks, and to no others. Beer yeast brought by foreigners shall be inspected by a jury before it is exposed to sale.”
Spontaneous fermentation was undoubtedly the result of natural yeasts found in the atmosphere, or exigent on the leaves and skins of various ingredients added to provide flavor. Dominant strains of wild yeast in any locale would have been the primary means by which yeast made it into the fermenting vessel. Wild yeasts by their very nature are aggressive, highly tolerant of a hostile brewing environment (able to survive in more extreme temperatures or in a high-alcohol beverage), and more likely to produce what we would today call ‘off flavors’.
Spontaneous fermentation—what I am hereby referring to as nothing short of “immaculate”—is an age-old practice, first by accident and then by intention, that truly puts Mother Nature at the helm of brewing magic. This type of fermentation takes place when ales are fermented with wild yeasts—from an open window, for instance, or already residing in a barrel…
Before the advent of refrigeration and advances in the science of fermentation in the mid-nineteenth century, almost all beer was, to varying degrees, sour. The culprits were pre-modern sanitation and poorly understood, often naturally occurring bacteria including Lactobacillus and Pediococcus, as well as Brettanomyces yeasts, which can contribute a hint of tartness and characteristic ‘funky’ flavors and aromas, sometimes compared to leather, smoke, and ‘horse blanket.’
Relying on airborne yeasts, brewers virtually never got a distinctly high or low fermentation variety but rather something mixed. The method worked but was haphazard and raised the risk of infection from unwanted yeast strains which could ruin the beer.
 Salem, pg. 19
 Unger, pg. 152
DeBenedetti, Christian (26 July 2013). “A Brief History of Sour Beer”. The New Yorker.
Herz, Julia (n.d.) “Spontaneous Fermentation: Science, Not Sorcery”. Craft Beer Muses. Retrieved 17 January 2015, from http://www.craftbeer.com/craft-beer-muses/immaculate-fermentation-science-not-sorcery.
Kiefer, David M. (2001). “Brewing: A Legacy of Ancient Times”. Today’s Chemist at Work. American Chemical Society.
Meade, Ronan (n.d.) “Yeast Research Project Documentation”. Retrieved 15 January 2015, from http://mbhp.forgottensea.org/dyrp.html.
Salem, Frederick W. (1880). Beer: Its History and Its Economic Value as a National Beverage. Hartford: F. W. Salem and Co.
Unger, Richard W. (2007). Beer in the Middle Ages and the Renaissance. University of Philadelphia: Pennsylvania Press.