Bottom-fermented beer started to appear in the Cologne region in the early 17th century and its popularity threatened the business interest of the brewers of Cologne, who only produced top-fermented ales. In response, the town council of Cologne in 1603 forced young brewers to swear an oath "that you prepare your beer, as of old, from good malt, good cereals, and good hops, well-boiled, and that you pitch it with top-yeast, and by no means with bottom yeast."
[2]:305 In 1676 and again in 1698, the council again tried to legislate against bottom-fermented beer by forbidding its sale within the city walls.
[2]:305 However, by 1750, Cologne brewers were competing against bottom-fermented beers by using a hybridized brewing process, first brewing their beer using top-fermenting yeast but then aging the beer in cold cellars like bottom-fermented beer.
[2]:306