Yeast
Yeast is a living organism, of which there are hundreds of species. Beer brewers usually use isolated or cultivated 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 beer yeast: top yeast, bottom yeast and wild yeast.
Bottom fermented
(Saccharomyces Pastorianus)
Also called Saccharomyces Carlsbergensis. This yeast prefers to work at lower temperatures, 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 can taste more malt and hop tones. That is because a lower yeast releases fewer flavours.
All bottom-fermented beers are called grouped under the style “lager” in the UK. The best-known and most widely consumed lager is a pilsner.
Top yeast
(Saccharomyces Cerevisiae)
This yeast prefers to work at higher temperatures, roughly between 15 and 25 ° C. There are many types of top yeast, each with its own unique characteristics. Top yeast produces more esters, a (often) desirable by-product of yeast that generates “fruity” aromas in beer. Esters are produced by the yeasts during the fermentation process and are influenced by three main elements: yeast characteristics, wort composition, and fermentation conditions. In the beer, esters are usually recognised as fruit, such as banana flavour in a Weizen or pear in a Belgian Tripel. Each variety of yeasts produces its own esters, which is why many brewers have their own unique yeast strain.
Top- fermented beers are called referred to as “Ale” in UK. IPA is perhaps the most popular.
Wild or spontaneous yeast
For a long time, this type of yeast was considered an undesired contaminant that brewers tried to keep away from their breweries. It was much later brewers started to cultivate it and use it to their advantage in beer production.
As a rule, this type of yeast is not actively added by the brewer to the beer. After cooking, the brewer pumps the wort into a cooling vessel (large, shallow bathtub), where it is exposed to the open air. The natural yeast cells from the air then come in contact with the wort and trigger the fermentation process. These are non-isolated yeasts.
Brettanomyces (Brett) is seen as a "wild" yeast, although these are often added in cultivated form. Orval is a well-known example in which this happens.
Brett is a notorious yeast and can be hard to manage, it is also capable of converting the non-fermentable sugars. Brett can take a long time to work its magic, but in the end the Brett will devour everything. In addition, it is a very strong yeast that is difficult to eradicate. This combination means that a Brett infection is often discovered very late in the process and is irreversible. That is why brewers who work with Brett keep this yeast strictly separate from the rest of the brewery.
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