Classification based on the properties of brewer's yeast

Based on the properties of beer yeast, beer is classified into bottom-fermented beer and top-fermented beer.  

      Each type of yeast produces alcohol and a range of fermentation by-products during fermentation, but the fermentation by-products vary depending on the yeast strain used in production. Therefore, the taste and aroma of these two types of beer differ significantly.

      The classification of beer styles in the modern craft brewing industry is primarily based on the type of beer yeast. Brewers in the United States and most other regions generally refer to beers using top-fermenting (ale) yeast as Ale beers, and those using bottom-fermenting (lager) yeast as Lager beers. According to the fermentation method, many classification systems also allow for a third category, known as Spontaneous Fermentation beers. In Belgium, there is also a beer type known as Combined Fermentation beers, such as sour beers and beers from the South Flanders region. However, the more widely used term is wild yeast, which refers to the use of wild yeast or even non-yeast types such as bacteria for fermentation. In fact, wild yeast is not equivalent to spontaneous fermentation; the former mostly involves the artificial inoculation of the required fermentation strains, while the latter directly uses strains from nature (such as those in the air of Brussels for brewing Belgian Rabbek).

      In Germany and other traditional brewing regions, the terms top-fermented and bottom-fermented are commonly used to distinguish between beers. Germans habitually regard ale as a specific type of English beer and lager as a method of storing beer (note: "Lager" means "storage" in German). Therefore, Germans consider Kölsch to be a top-fermented lager, not an ale.

      In certain historical stages, British brewers would consider Ale, Porter, and Stout as different types of beer. In some historical contexts, beer had to be hopped (or used more hops than Ale), while Ale could be unhopped.