In general, when someone uses the word yeast, they are referring to the type of yeasts used to make beer, bread, biofuel, etc. This also usually implies a particular species of yeast, Saccharomyces cerevisiae, which is the species name for ale yeast. The other species widely used in the production of beer, but not typically used in other industries, is lager yeast, Saccharomyces pastorianus. There are other species of wild yeast, like Brettanomyces, that are utilized in certain craft beer styles (lambics, wild ales) but they are generally considered spoilers in traditional beer making.
So, when it comes down to it, an ale yeast makes an ale and lager yeast makes a lager. Within each of the species, there are many subspecies of yeast that have been collected from breweries all over the world. Before we had the ability to store yeast indefinitely using cryogenics, we had to use the same yeast culture over and over again, year after year. The yeasts used in different breweries would slowly evolve to the conditions it was subjected to in each of those places. If a brewery was making lots of high alcohol abbey ales, the yeast would mutate differently than if a brewery was making lower alcohol German ales. So, for example, if we wanted to make a German Hefeweizen, we would buy a particular ale yeast that was sourced from a brewery in Germany. That yeast is unique as it mutated to produce a specific compound that smells like ripe bananas, a crucial part of the flavour and aroma of Hefeweizens. Without that yeast, a Hefeweizen wouldn’t be a Hefeweizen.
Since beers were traditionally very similar in a region, there are some generalities that can be made about the flavour profiles generated by the yeast in a particular area: Belgian ales are typically spicy/phenolic, UK ales are fruitier, and American and German ales tend to ferment fairly neutral. The last point is not a coincidence since some of the first brewers in North America were German and they brought their yeast with them.
Sometimes the terms ‘bottom’ or ‘top’ fermenting is used to describe lager and ale yeast, respectively. Top fermenting refers to the large, foamy, yeasty head seen on top of an active ale fermentation. Bottom fermenting refers to lager yeast and how usually there is very little head on top. Since the fermentation is relatively slow, it doesn’t generate a huge foamy head like ale yeast does. These terms aren’t often used any more. They are a bit of a misnomer. The yeast that is either on the bottom or top of a fermentation isn’t the yeast that is fermenting the beer. The yeast that’s in the beer itself, in contact with the sugars, is fermenting the beer.
Alley Kat Brewing Company