Yeast is a living organism and there are hundreds of species. Beer brewers usually use isolated or cultured yeasts, which means that all yeast cells are the same. This allows the brewer to best control the fermentation process.

There are roughly 3 types of brewer’s yeast: top yeast, bottom yeast and wild yeast.

Bottom yeast (Saccharomyces Pastorianus)

Also sometimes called Saccharomyces Carlsbergensis. This yeast prefers to do its job at lower temperatures, approximately between 5 and 12°C and sinks to the bottom of the yeast tank during the fermentation process. The result is a beer with a “cleaner” profile, in which you taste more malt and hop notes. That’s because bottom yeast releases fewer flavors.

All bottom fermented beers are called Lagers in English. The best-known type of lager is pilsner.

Top yeast (Saccharomyces Cerevisiae)

This yeast prefers to do its job at higher temperatures, roughly between 15 and 25°C. There are many types of top yeast, each with its own unique properties, of which the production of esters gives the biggest difference in taste. Top yeast produces more esters, a (often) desired by-product of yeast. In the beer usually recognizable as fruit, such as banana aroma in a weizen or pear in a Belgian triple. Each strain produces its own esters, which is why many brewers have their own unique yeast strain.

Top-fermented beers are called Ale in English. India Pale Ale, for example, is a beer that is by definition a top-fermented beer.

Wild or spontaneous yeast

As a rule, a brewer does not add this yeast to the beer itself. After boiling, the brewer pumps the wort into a cooling vessel (large, shallow bathtub), where it is exposed to the open air. The yeast cells from the air then end up in the wort and start the fermentation. So these are non-isolated yeasts.

Brettanomyces (Brett) is considered a “wild” yeast, although these are often added in cultivated form. Orval is a well-known example where this happens.

Brett is a notorious yeast, because it is also able to convert non-fermentable sugars. This takes a long time, but eventually Brett devours everything. In addition, it is a very tough yeast that is difficult to eradicate. This combination means that an infection with Brett is often discovered very late. That’s why brewers who work with Brett keep this yeast strictly separate from the rest of the brewery.