The same bed of yeast may be cultured from batch to batch in an effort to modify the yeast to recognize and react to the ingredients of a particular recipe. In this rudimentary culturing process, the yeast evolves with each new batch, producing predictable qualities in the finished beverage. The medieval brewer facilitated this by scraping the foam layer from a currently fermenting batch or by setting aside the lees from the fermenter in order to use it to ‘start’ the next batch.
When flowers and other leafy herbs or fruits were added to the brew it almost certainly provided necessary yeasts; for yeast, as such, was not an ingredient of itself. The people of the Middle Ages knew that yeast was necessary but didn’t quite understand where it came from. The barm (yeast froth) was scraped off the top of the fermenting beer, saved and used to make bread or promote the next batch of brew.
Some brewers did, it seems, recognize the possible infection of their brews by airborne yeast, a situation first mentioned at Munich in 1551. The realization was slow in coming, however. As early as the mid-fourteenth century a Flemish recipe book mentions adding yeast to beer, and it seems likely that already by 1300 brewers were using some of the foam skimmed off the top of the fermenting beer of the last brew to start fermentation with the next one. By the sixteenth century, brewers commonly added yeast to wort from cultures which they kept separate and which they controlled and maintained. Regulations in Harlem in 1519 and 1550 leave no doubt that brewers added yeast once the wort was in the fermenting troughs.
The greatest example of culturing yeast from batch to batch comes to us from the Trappist breweries of Belgium. Of the modernly remaining ten Trappist abbeys that produce beer, two date to our period: The Brasserie d’Orval in the Gaume region of Belgium, and the Abbey of Notre Dame de Saint-Remy near Rochefort. The Rochefort beers have been brewed with the same strains of yeast being cultured forward batch-to-batch since 1595. Although the current Orval brewing facility only dates to 1931, the original brewery dates back to at least 1628, based on writings from the abbot at the time.
 Unger, pg. 152
 Saint Peters List
Fourneau, Auguste (2006). L’abbaye Notre-Dame de Saint-Remy à Rochefort. Histoire d’une communauté cistercienne en terre de Famenne, 2nd edition. Abbey of Saint-Remy.
Saint Peters List (2013). “The Ten Authentic Trappist Monk Beers”. History / Life & Leisure. Retrieved 11 February 2015, from http://www.stpeterslist.com/197/the-7-authentic-trappist-monk-beers.
Unger, Richard W. (2007). Beer in the Middle Ages and the Renaissance. University of Philadelphia: Pennsylvania Press.
Westerson, Jerry (April 1998). “Searching for Medieval Ale”. Brew Your Own.