Over time, brewers learned to control brewing, selecting purer strains, and learning about controlled fermentation schedules and seasonal cycles. In parts of Germany especially, bottom-fermentation and cool storage was becoming the norm some 500 years ago. But British, Belgian, Bohemian and Northern German brewers were still making top-fermented beers, mostly at the mercy of the season, often sour or musty, or both. Patient aging became de rigueur in Belgium and Britain, where aged and young beer was blended. In Britain, “stale” was not only the name of the aged stock ale, but also described its desirable tart and horsey notes. Much of this came from lengthy aging in wooden casks, from several months to several years. Oak (particularly French) was and still is the preferred cask material, being abundant, easy to work with and sturdy, while bestowing its own personality to the aging beer. Vanillin and tannin are the two most recognizable nuances imparted naturally by oak. It is also quite porous, providing microscopic grottos for persuasive organisms, and a permeable membrane, allowing oxygen, a nutrient for Brettanomyces and Acetobacter, to contact the beer. Only used wine casks are employed in the making of Flanders beers. New oak is overpowering, but aging brings almost magical distinction to the wood. Older oak looses some of its woody character, but becomes more hospitable to microorganisms. Coopers are as important for their barrel building and maintenance skill as are the brewers. The barrels are scraped regularly to remove pore-clogging material, and are periodically disassembled and reassembled. Rodenbach, the grandfather of the West Flanders red style, has many casks over 100 years old, and some that date to 1868. Wood aging makes for convenient sampling and hence, blending, during the process, an important aspect of brewing in the 18th and 19th centuries. The British were especially known for this then, and the Belgians still for some styles, Flanders ale and lambic among them. There was much brewing cross-pollination between the two countries during that era. The common Brettanomyces (British fungus) brewing strains were actually isolated from casks used to age British stock ales in the 19th century.
Though the Flanders styles are considered anachronistic, its brewers have modernized over the past few decades while retaining respectful attachment to heritage. Spontaneous fermentation has been replaced by mixed culture inoculation. Coolships, the large, shallow open fermenters that offer the best surface area for spontaneous fermentation, have mostly been replaced. Rodenbach introduces Saccharomyces cerevisiae, Lactobacillus delbrueckii and Pediococcus parvulus to their wort, allegedly culled from their old coolships. S. cerevisiae takes care of the initial ordinary fermentation of sugars during primary fermentation, with L. delbrueckii and P. parvulus taking their successive turns during secondary fermentation to produce souring, lactic products. Tertiary fermentation is done in their famous oaken barrels (foeders), where several Brettanomyces species finish the job, a duration of 18 months to 3 years. The voracious Brettanomyces is capable of fermenting sugars, including dextrine, which Saccharomyces cannot, resulting in ultra-high attenuation. This lengthy maturation and multi-organism fermentation coupled with the oaky nuances is, in the end, sublime. When mature, Rodenbach blends its young and old beer in two main products for bottling. The regular Rodenbach is three-fourths young and one-fourth aged beer, while the more assertive Gran Cru is two-thirds aged and one-third young beer. The aging beer, known as “foederbier,” can be sampled at selected times in its unblended, pristine condition.