Top-fermented and bottom-fermented: does it matter?
Yeast is the single most important ingredient to influence the resulting style of beer. Use bottom-fermenting yeast, and you end up with a lager-style beer. Use top-fermenting yeast and you end up with an ale. Lagers and ales are the two main categories of beers.
Basically, with top-fermenting yeast, the yeast rises to the top of the fermentation tank and forms a crust, sealing in the flavour. The resulting ales have correspondingly rich and varied characters. They include hop-rich bitters, dark stouts, and the high-octane Trappist beers of Belgium.
Just about all beer was top-fermented until the early 19th century, when a clearer, more polished kind of beer became popular. Developed in Germany and Austria, it was called lagerbier, because an important part of the process was to store it (German: lagern) for a period of time. Lager was originally brown or amber, but golden lager was produced in the Czech town of Pilsen (Plzen) in 1842, setting a trend for Pils. This style of clear, light, bottom-fermented beer is now the most popular kind of beer in the world, and is given a bad name by the gassy and insipid brews of many of the biggest brands. However, fine or 'premium' lager is still made, especially in Germany and the Czech Republic.
