<sod>  There are four main families of beer styles determined by the variety of yeast used in their brewing.
 
 ===Ale (top fermenting yeasts)===
[[Ale]] yeasts ferment at warmer temperatures of 65 to 75 °F (18 to 24 °C) and tend to collect on the surface of the fermenting beer.  Because of this they are often referred to as Top Fermenting yeast.  Ales are generally ready to drink within three weeks after the beginning of fermentation.  Ales range in color from very pale to black opaque.  England is best known for its variety of Ales.
 
 ===Lager (bottom fermenting yeasts)===
 While the nature of yeast was not fully understood until [[Emil Hansen]] of the [[Carlsberg brewery]] in [[Denmark]] isolated a single yeast cell in the 1800s, brewers in [[Bavaria]] had for centuries been selecting these cold-fermenting Lager yeasts by storing or "Lagern" their beers in cold alpine [[cave]]s.  The process of natural selection meant that the wild yeasts that were most cold tolerant would be the ones that would remain actively fermenting in the beer that was stored in the caves. Some of these Bavarian yeasts were stolen and brought back to the Carlsberg brewery around the time that Hansen did his famous work.
 <eod> <sod> There are four main families of beer styles determined by the variety of yeast used in their brewing.
 
 ===Ale (top fermenting yeasts)===
[[Ale]] yeasts ferment at warmer temperatures between 15&deg;C and 20&deg;C (60&deg;F to 68&deg;F), and occasionally as high as 24&deg;C (75&deg;F).  Pure ale yeasts form a foam on the surface of the fermenting beer, though many British yeasts contain yeast strains that settle to the bottom.  Because of this they are often referred to as Top Fermenting yeast.  Ales are generally ready to drink within three weeks after the beginning of fermentation, though they benefit from additional storage of up to two months.  Ales range in color from very pale to black opaque.  England is best known for its variety of Ales.
 
 ===Lager (bottom fermenting yeasts)===
 While the nature of yeast was not fully understood until [[Emil Hansen]] of the [[Carlsberg brewery]] in [[Denmark]] isolated a single yeast cell in the 1800s, brewers in [[Bavaria]] had for centuries been selecting these cold-fermenting Lager yeasts by storing or "Lagern" their beers in cold alpine [[cave]]s.  The process of natural selection meant that the wild yeasts that were most cold tolerant would be the ones that would remain actively fermenting in the beer that was stored in the caves. Some of these Bavarian yeasts were stolen and brought back to the Carlsberg brewery around the time that Hansen did his famous work.
 <eod>