What is the difference between ale and lager?

Top fermentation and bottom fermentation refer to where the yeast sits while working: ale yeast collects at the surface, lager yeast sinks to the bottom of the vessel.

Ale is the collective name for top-fermented beers, lager for bottom-fermented ones. So the distinction does not come from colour, bitterness or alcohol content, but from the yeast and the way it behaves.

In a modern cylindro-conical fermenter that difference can hardly be seen with the naked eye any more. The classification has remained because the two yeast species require fundamentally different conditions.

Ale is fermented with Saccharomyces cerevisiae, lager with Saccharomyces pastorianus, a natural hybrid of cerevisiae with the cold-loving species Saccharomyces eubayanus.

That hybrid explains why lager yeast keeps working well at low temperatures: the eubayanus half brings cold tolerance. S. pastorianus is the most widely used yeast in breweries worldwide.

Researchers do not yet know exactly where and when that hybrid arose. A common idea is that S. eubayanus once ended up in an ordinary ale fermentation and crossed there with the cerevisiae present.

Ale yeast works best between 15 and 25 degrees Celsius, lager yeast ferments well between 8 and 12 degrees.

Warmth speeds up fermentation: an ale is therefore fermented more quickly. Cold slows the process down, so a lager needs more time before the sugars have been converted.

That temperature also determines which aroma compounds the yeast produces alongside alcohol. Brewers deliberately steer this by keeping the fermenter at a fixed temperature.

Lagering is the cold conditioning of beer after the main fermentation. The name comes from the German lagern, to store.

With lager beer, the main fermentation is followed by a second phase, carried out at 12 degrees in research, in which the beer matures and clears. Historically this took place in cool cellars, which explains why bottom fermentation developed in Central Europe.

Ales are usually conditioned for a shorter time and at a higher temperature. The difference in production time between the two groups thus comes both from the slower fermentation and from the longer cold conditioning.

Ales include pale ale, IPA, stout, porter, witbier, weizen, tripel and most Belgian abbey beers. Lagers include pils, helles, bock and schwarzbier.

So there are pale and dark beers in both groups. Schwarzbier is pitch-black and yet a lager; a tripel is golden blond and yet an ale.

If you want to see on the label which group it is, look for words such as bottom-fermented or top-fermented. Many Belgian and Dutch brewers simply state this on the label.

Yes. Pils is the best-known lager in the world; it is bottom-fermented and then cold-lagered.

The first pale lager of this type was brewed in 1842 in Plzen, in a new brewhouse the citizens of the town had had built. The Bavarian brewer Josef Groll is named as its first brewer.

German pils emerged in the early eighteen-seventies as an adaptation to German water and German hops. It is usually 4.4 to 5.2 per cent alcohol and 22 to 40 IBU; the Czech variant ranges from 4.2 to 5.8 per cent at 30 to 45 IBU.